Ultraviolet Signposts of Resonant Dynamics                                                       in the Starburst-Ringed S...
Subject headings: galaxies: evolution — galaxies: individual (M94), (NGC 4736) —galaxies: kinematics and dynamics — galaxi...
1.   Introduction                                                  The discovery of compact thermal & nonthermal ra-      ...
and spectroscopic observations and reductions are de-           IUE observations of standard stars (cf. Stecher et al.scri...
in The Digitized Sky Survey (STScI 1994)14.                              toelectric photometry of Munoz-Tunon et al. (1989...
Keel (1983) and discuss a new high-resolution spec-               3.2.   Inner Disk  trum, obtained with the 2.5-m Isaac N...
ated by median smoothing the R-band image over a              of 40◦ , based on the H I study of Mulder & van Driel13′′ × ...
than 35 km/s to avoid detection. Implications of this            In summary, the bi-symmetric FUV knots have B-propagation...
scale of <2.2 pc at the assumed distance. Although an          9.45 mag, or M(FUV) = −18.91 mag at the adoptedextremely co...
FUV emission. Stellar absorption at Hα is proba-               1996]), and close to the mean of M51’s wide-rangingbly less...
Based on the aperture mapping shown in Figure 3a                an issue). The off-nuclear spectrum shown here issome emiss...
(more or less equivalent to E(B − V ) = −0.3 mags,             emission beyond 60 arcsec radius – further corrobo-where th...
nances are respectively traced by the outer pseudor-                  to 1.5 M⊙ yr−1 — sufficient to build up the stellaring...
The results reported herein may have important               the Spacelab Office at NASA Headquarters underimplications with...
Dressler, A., & Shectman, S. A. 1987, AJ, 94, 899               Marcum, P. M, et al. 1997, in the Ultraviolet UniverseDuri...
STScI 1994, The Digitized Sky Survey (San Francisco:         Wu, C.-C., et al. 1991, The IUE Ultraviolet Spectral  ASP)   ...
Fig. 1.— A wide-field J-band image (extracted from               knots in the starburst ring.the Digital Sky Survey) shows ...
ties have been extrapolated and most likely representupper limits (see text). This was done to provide a(probable) lower l...
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)
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Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)

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Transcript of "Ultraviolet signposts of resonant dynamics in the starburst ringed sab galaxy, m94 (ngc 4736)"

  1. 1. Ultraviolet Signposts of Resonant Dynamics in the Starburst-Ringed Sab Galaxy, M94 (NGC 4736) William H. Waller12 , Michael N. Fanelli32 , William C. Keel4 , Ralph Bohlin5 , Nicholas R. Collins2 , Barry F. Madore6 , Pamela M. Marcum7 , Susan G. Neff8 , Robert W. O’Connell9 , Joel D. Offenberg2 , Morton S. Roberts10 , Andrew M. Smith8 , and Theodore P. Stecher8arXiv:astro-ph/0011101v2 13 Nov 2000 ABSTRACT The dynamic orchestration of starbirth activity in the starburst-ringed galaxy M94 (NGC 4736) is investigated using images from the Ultraviolet Imaging Telescope (FUV- band), Hubble Space Telescope (NUV-band), Kitt Peak 0.9-m telescope (Hα, R, and I bands), and Palomar 5-m telescope (B-band), along with spectra from the International Ultraviolet Explorer and Lick 1-m telescopes. The wide-field UIT image shows FUV emission from (a) an elongated nucleus, (b) a diffuse inner disk, where Hα is observed in absorption, (c) a bright inner ring of H II regions at the perimeter of the inner disk (R = 48′′ = 1.1 kpc), and (d) two 500-pc size knots of hot stars exterior to the ring on diametrically opposite sides of the nucleus (R = 130′′ = 2.9 kpc). The HST/FOC image resolves the NUV emission from the nuclear region into a bright core and a faint 20′′ -long “mini-bar” at a position angle of 30 deg. Optical and IUE spectroscopy of the nucleus and diffuse inner disk indicates a ∼107−8 yr-old stellar population from low-level starbirth activity blended with some LINER activity. Analysis of the Hα, FUV, NUV, B, R, and I-band emission along with other observed tracers of stars and gas in M94 indicates that most of the star formation is being orchestrated via ring-bar dynamics involving the nuclear mini-bar, inner ring, oval disk, and outer ring. The inner starburst ring and bi- symmetric knots at intermediate radius, in particular, argue for bar-mediated resonances as the primary drivers of evolution in M94 at the present epoch. Similar processes may be governing the evolution of the “core-dominated” galaxies that have been observed at high redshift. The gravitationally-lensed “Pretzel Galaxy” (0024+1654) at a redshift of ∼1.5 provides an important precedent in this regard. 1 Tufts University, Department of Physics and Astronomy, Medford, MA 02155 2 Raytheon ITSS Corporation, NASA Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 681, Greenbelt, MD 20771 3 University of North Texas, Department of Physics, Denton, TX 76203 4 University of Alabama, Department of Physics and Astronomy, P. O. Box 870324, Tuscaloosa, AL 35487-0324 5 STScI, Homewood Campus, Baltimore, MD 21218 6 Infrared Processing and Analysis Center, Caltech, M/S 100-22, 770 So. Wilson Ave., Pasadena, CA 91125 7 Texas Christian University, Department of Physics, Box 298840, Fort Worth, TX 76129 8 NASA Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 680, Greenbelt, MD 20771 9 University of Virginia, Department of Astronomy, P. O. Box 3818, Charlottesville, VA 22903 10 National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville, VA 22903–2475 1
  2. 2. Subject headings: galaxies: evolution — galaxies: individual (M94), (NGC 4736) —galaxies: kinematics and dynamics — galaxies: photometry — galaxies: spiral — ultra-violet emission 2
  3. 3. 1. Introduction The discovery of compact thermal & nonthermal ra- dio sources in the ring (Duric & Dittmar 1988) indi- Star-forming rings or “pseudorings” are common cates the presence of dense H II regions and young to early and intermediate-type spiral galaxies (cf. SNRs. The ring’s velocity field can be described by Athanassoula & Bosma 1985; Buta, Purcell, & Crocker a combination of circular rotation with velocities of 1995; Buta & Combes 1996), including our own Milky order 200 km/s and residual non-circular motions of Way galaxy (cf. Gusten & Mezger 1982; Clemens, order 15 km/s (Mulder 1995) to 25 km/s (Buta 1988), Sanders, & Scoville 1988; Waller 1990a). Such ring- depending on the adopted inclination and major axis like accumulations of gas and associated starbirth ac- position angle. tivity may have helped to build the inner parts of Interior to the ring, the bright bulge and inner many primeval disk galaxies (Friedli & Benz 1995), disk show twisted isophotes at red and near-IR wave- as exemplified by the recent discovery of a starburst- lengths, indicative of a weak bar-like distortion (Beck- ringed galaxy at z ∼ 1.5 (Colley, Tyson, & Turner man et al. 1991; Shaw et al. 1993; Mollenhoff et al. 1996; Tyson et al. 1997). The formation and mainte- 1995). FIR and CO observations interior to the ring nance of these starburst rings are often attributed to reveal a rich ISM with gas surface densities exceeding orbital resonances with rotating bar or “oval” asym- that of the ring (Smith & Harvey 1994; Garman & metries in the stellar disks (cf. Combes 1994, Byrd Young 1986; Gerin et al. 1991; Wong & Blitz 2000). et al. 1994; Combes et al. 1995; Buta & Combes 1996 and references therein). However, other dynami- Optical spectroscopy of the nuclear region yields cal mechanisms — including gravitational instabilities LINER-type emission lines along with absorption lines (Elmegreen 1992, 1994; Kenney & Jogee 1997), out- from the circumnuclear stellar population, consistent ward propagating star formation (Walker, Lebofsky, with an early main-sequence stellar turnoff (A4–A7) & Rieke 1988; Waller, Gurwell, & Tamura 1992), and and corresponding age of ∼500 Myr ( Pritchett 1977; even radially-driven pileups from nuclear outbursts Keel 1983; Taniguchi et al. 1996). Further support (Waller et al. 1992; Tenorio-Tagle et al. 1997) — for a young central population comes from NIR spec- may play significant roles in orchestrating some of the troscopy which shows deep CO absorption bands from starburst rings that are observed. 11 red giant and asymptotic giant branch stars of sim- ilar age (Walker et al. 1988). These authors have As the closest early-type spiral galaxy of low incli- proposed an outward propagating mode of star for- nation, M94 (NGC 4736) has received concentrated mation, whereby NGC 253, M82, M94, and M31 rep- attention from both observers and theorists. This resent increasingly evolved versions of the same star- (R)SA(r)ab-type galaxy (de Vaucouleurs et al. 1991) bursting sequence. Although the kinematics of the is noted for its inner ring of ongoing starburst activity ring show very little evidence for outward expand- (R ≈ 45′′ ), oval stellar distribution at intermediate ra- ing motions (contrary to prior claims of bulk expan- dius (R ≈ 220′′ ) (cf. Mulder & van Driel 1993; Mulder sion [van der Kruit 1974; 1976]), they also do not 1995; Mollenhoff, Matthias, & Gerhard 1995), and preclude a scenario for radially propagating star for- outer stellar ring near its de Vaucouleurs radius (R25 mation. Other investigators have modeled the inner ≈ 330′′ ). Figure 1 (extracted from the Digital Sky and outer rings in terms of resonant dynamics medi- Survey using the SkyView utility [McGlynn, Scollick, ated by bar or “oval” potentials interior to the rings & White 1996])12 shows the outermost portions of (Gerin et al. 1991; Shaw et al. 1993; Mollenhoff et M94, highlighting the oval disk and outer pseudoring. al. 1995; Mulder & Combes 1996), with the observed The inner starburst ring is a prominent source of non-circular motions resulting from dispersion orbits Hα, H I, and CO emission (Smith et al. 1991; Mulder near the Lindblad resonances (Buta 1988). & van Driel 1993; Gerin, Casoli, & Combes 1991). In this paper, we present and discuss new obser-11 Collisionally-induced “ring galaxies” such as the Cartwheel vational clues to the dynamical mechanisms govern- Galaxy are thought to be morphologically and dynamically ing the star formation in M94. The ultraviolet im- distinct from the more common “ringed galaxies” considered ages obtained by the Ultraviolet Imaging Telescope, herein (cf. Athanassoula & Bosma 1985; Marcum et al. 1992). in particular, reveal hitherto unrecognized patterns12 NASA’s SkyView facility (http://skyview.gsfc.nasa.gov) was of recent star formation whose presence lends fur- developed and is maintained under NASA ADP Grant NAS5- 32068 at NASA’s Goddard Space Flight Center. ther support to the hypothesis of galaxy evolution via bar-mediated resonances. The various imaging 3
  4. 4. and spectroscopic observations and reductions are de- IUE observations of standard stars (cf. Stecher et al.scribed in Section 2. The resulting FUV, NUV, Hα, 1992, 1997; Waller et al. 1995 and references therein).R, and I-band emission morphologies are presented Correction for image distortion produced by the mag-and compared in Section 3. Radial intensity profiles netically focused image intensifiers was carried out ac-and other photometric comparisons are discussed in cording to the procedures described by Greason et al.Section 4. UV and optical spectroscopy of the inner (1994). The resulting corrections amounted to a fewdisk and nucleus is presented in Section 5. Kinematic arcsec in the field center (which includes all of M94’sproperties and inferred dynamical scenarios are con- FUV emission) increasing to 10–20 arcsec near thesidered in Section 6. Our summary of the wavelength- edge of the 40 arcmin field of view. Astrometry wasdependent morphological, spectro-photometric, and tied to 10 compact knots evident in both the FUVdynamical properties of M94 appears in Section 7, and B-band images (see next subsection). Positionswherein evolutionary implications are discussed. in the resulting distortion-corrected image are good In the following Sections, we assume a distance to to ∼3 arcsec, and the spatial resolution is of similarM94 of 4.6 (75/H◦ ) Mpc, based on the galaxy’s re- magnitude.cession velocity of 345 km/s with respect to the Lo- The Hubble Space Telescope’s Faint Object Cam-cal Group ( Sandage & Tammann 1981). The corre- era (FOC)13 imaged the center of M94 in the near-sponding spatial scale is 22.3 pc/arcsec. Unresolved ultraviolet (λ◦ = 2300 ˚, ∆λ = 500 ˚) on 18, July A Asources imaged by the HST/FOC provide additional 1993 — before the optical repair mission — as partconstraints on the distance, as discussed in Section 4. of a UV imaging survey of 110 large nearby galax-We adopt a nominal inclination of 40◦ and major-axis ies (Maoz et al. 1995, 1996). After standard STScIposition angle of 120◦ , while recognizing that both pipeline processing, the 596 sec NUV exposure has aof these quantities may vary significantly with radius 22′′ × 22′′ field of view at 0.022′′ /pixel (see Figure 2aand with measuring technique (e.g. morphological vs. (Plate xxx)). The spherical aberration of HST’s pri-kinematic determinations) (Bosma, van der Hulst, & mary mirror resulted in a point-spread function (PSF)Sullivan 1977; Buta 1988; Mulder & van Driel 1993; featuring a sharp core with FWHM ≈ 0.05′′ and aboutMulder 1995; Mollenhoff et al. 1995; Wong & Blitz 15% of the total light surrounded by an extensive halo2000). of several arcsec radius containing most of the energy. Following Maoz et al. (1996), our calibration of the2. Observations and Reductions detected FOC counts into flux densities assume a con- version of 1.66 × 10−17 erg s−1 cm−2 ˚−1 /count s−1 , A A log of the ultraviolet and visible imaging is pre- while noting that the PHOTFLAM conversion in thesented in Table 1. A listing of complementary UV image header is 2.017×10−17 erg s−1 cm−2 ˚−1 /count Aand visible spectra is shown in Table 2. s−1 . Flux uncertainties are estimated at ∼5% over large areas increasing to ∼20% for compact sources2.1. Ultraviolet Imaging (Maoz et al. 1995, 1996, and references therein). The Ultraviolet Imaging Telescope (UIT) imagedM94 in the far-ultraviolet (λ◦ = 1521 ˚, ∆λ = A 2.2. Visible Imaging354 ˚) on 1995 March 12 as part of the 16-day A A wide-field (9.66′ × 9.66′) B-band image was ob-Spacelab/Astro-2 mission aboard the Space Shut- tained with the Palomar 5-m telescope and Tek3 CCDtle Endeavour. This wide-field telescope images 40- camera (1024 × 1024 pix) on 1994 February 11 underarcmin fields of view at ∼3 arcsec resolution. In the hazy skies. This 600-sec exposure is saturated in thecase of M94, the UIT image represents the only ex- central 2 arcmin, but contains high S/N detections oftant UV image of the entire galaxy (see Figure 2a the oval disk and parts of the outer ring. Astrometry(Plate xxx)). The 1040-sec exposure was obtained of this image is tied to the positions of several fore-with a dual-stage image intensifier with CsI photo- ground stars, as measured on a corresponding imagecathodes and was recorded on carbon-backed IIaOKodak film. After processing of the film and scan- 13 Data from the NASA/ESA Hubble Space Telescope were ob-ning of the emulsion, the resulting digitized “density tained at the Space Telescope Science Institute, which is oper- ated by the Association of Universities for Research in Astron-image” was fog-subtracted, flat-fielded, linearized to omy, Inc., under NASA contract No. NAS5-26555.“exposure units,” and calibrated to flux units using 4
  5. 5. in The Digitized Sky Survey (STScI 1994)14. toelectric photometry of Munoz-Tunon et al. (1989) Groundbased Hα, R, and I-band images of M94 and the (R−I) color profiles of Beckman et al. (1991). were obtained with the now-retired KPNO 0.9-m tele- scope and RCA-1 CCD camera (508 × 316 pix) on 2.3. UV Spectroscopy 17, February 1986.15 These images have 7.28′ × FUV (1200 ˚ to 2000 ˚) and NUV (2000 ˚ to 3200 A A A 4.53′ fields of view at 0.86′′ /pix. Sky conditions ˚) spectra of M94’s inner disk were obtained from A varied from photometric to hazy, yielding PSFs of the International Ultraviolet Explorer (IUE) archive. about 2′′ (FWHM). Astrometry is tied to 5 fore- The IUE data were accessed via the IUE Data Anal- ground stars that are common to the KPNO (Hα, ysis Center at NASA/GSFC16 and are representa- R, I-band) and Palomar (B-band) images. For cali- tive of the NEW Spectral Image Processing System bration purposes, spectrophotometric standard stars (NEWSIPS). (BD26◦ 2606 & HD84937) were imaged before and af- Table 2 lists the image numbers, dates, exposure ter the target imaging. Subtraction of the red contin- times, nominal positions and roll angles of the low- uum from the Hα-band image was carried out with resolution FUV (SWP) and NUV (LWP & LWR) IUE the R-band image according to the formulations in spectra. Here, SWP, LWP, and LWR respectively re- Waller (1990b), whereby corrections were made for fer to IUE’s short-wavelength prime, long-wavelength the ∼38% [N II]λλ6548, 6584 contribution to the to- prime, and long-wavelength redundant cameras. Fig- tal Hα + [N II] line emission and resulting ∼15% ure 3 shows the IUE apertures (20′′ × 10′′ ) on grey- contamination of the Hα image after transmission by scale images of the FUV emission, where the mapping the 36 ˚ bandwidth Hα filter. A pure red-continuum A is based on the nominal positions and roll angles of the image was also produced, by removing the contam- FUV (SWP) and NUV (LWP & LWR) observations. inating Hα + [N II] line emission from the R-band Although the accuracy of the nominal positions is of image (Waller 1990b) (see Figure 2a (Plate xxx)). order ±10′′ , Figure 3 indicates that the apertures Photometry of the Hα emission from the 7.3′ × 4.5′ were most likely sampling the inner disk rather than field yields a total flux of 9.9 × 10−12 erg cm−2 s−1 , the starburst ring. Table 2 notes which apertures 14% higher than that determined by Kennicutt & are filled with disk (d) and/or nuclear (n) emission, Kent (1983) within a 7′ -diameter aperture (after cor- based on visual inspection of the overlays. The limit- recting for a 38% [N II] contamination within their ˚ ing spectral resolution of these data is about 6 A. 100 ˚ bandwidth). The starburst ring (R = 30′′ A → 60′′ ) is measured to have f(Hα) = 9.6 × 10−12 2.4. Visible Spectroscopy erg cm−2 s−1 , which is nearly 1.8 times higher than that obtained by Smith et al. (1991) from the 75 Groundbased visible spectra (3800 ˚ to 7500 ˚) A A ˚ bandwidth image of Pogge (1989). Some of these A were obtained from the Lick 1-m Nickel telescope and discrepancies can be attributed to the varying band- Image Dissecting Spectrograph (IDS). The summed passes and corresponding uncertainties in the [N II] spectrum of the central region (8.1 arcsec circular emission being transmitted, vagaries in the continuum aperture) is a mean-flux average of 64 min in blue subtraction, problematic Hα absorption produced by and red grating settings, with 16 min in an inter- A-type stars in the inner disk (see Section 4.2), and mediate setting to ensure the overlap area was well final calibration (Smith et al. 1991). Because of hazy calibrated. The spectrum has been rebinned to 2.5-˚ A conditions during the I-band exposure, calibration of pixels from the original, which still oversamples the the I-band image was done by bootstrapping to the resolution of ≤10 ˚ FWHM. We note that adjacent A I-band radial intensity profiles resulting from the pho- pixels in the IDS spectrum are not statistically in- dependent, yielding detections similar to those of a14 The Digitized Sky Survey was produced at the Space Telescope non-centroiding photon counter which spreads single Science Institute (STScI) under U. S. Government grant NAG photons across several output pixels. W-2166. The digitized images are based on photographic data from the Palomar Observatory Sky Survey (produced by the We complement this composite spectrum with anal- California Institute of Technology & Palomar Observatory, and ysis of the image-dissector scanner data presented by funded by the National Geographic Society)15 Kitt Peak National Observatory is operated by the Association 16 The IUE database is currently available at of Universities for Research in Astronomy, Inc., under contract http://archive.stsci.edu/iue/ with the National Science Foundation. 5
  6. 6. Keel (1983) and discuss a new high-resolution spec- 3.2. Inner Disk trum, obtained with the 2.5-m Isaac Newton Tele- The UIT/FUV emission from the inner disk in- scope (INT) on La Palma, using the Intermediate- cludes concentric FUV arcs to the SW of the nucleus Dispersion Spectrograph with image photon-counting which can be traced for ∼π radians (∼700 pc). These system (IPCS) detector.17 The 1” slit was oriented arcs are of low contrast and, because of the noise char- approximately along the major axis (PA 135◦) for this acteristics of the UIT imaging, are not amenable to 1000-second exposure. There is useful signal over 45 typical contrast enhancement techniques (e.g. median spatial increments of 0.6′′ each. We consider equiva- filtering). However, they do show up as enhancements lent widths of the lines along with spectral slopes and in the radial distribution of intensities (see Figure discontinuities of the continuum emission. 2b). Typical spacing between the arcs is 9′′ (200 pc), with additional features at 15′′ and 34′′ . The arcs 3. Ultraviolet and Visible Morphologies themselves appear to show some substructure at the Figure 2a (Plate xxxx) shows the dramatically limits of resolution. different morphologies that are detected at ultravio- Contrary to a merging scenario (Maoz et al. 1995), let and visible wavelengths. The inner disk and bulge which would seem to be precluded by the lack of sig- component that is so prominent at R-band completely nificant tidal effects beyond the inner few hundred disappears in the FUV image. The oval disk at inter- parsecs, these features more likely indicate an orbital mediate radius also has no FUV counterpart. Instead, dynamic at work in the inner disk. At an interpolated the FUV image is characterized by (a) an extended orbital velocity of 140 km/s and corresponding shear and elongated nucleus, (b) a diffuse inner disk, (c) a rate of -1360 km−1 kpc−2 (see Section 6), such fea- bright inner ring at the perimeter of the inner disk tures could have been differentially swept out over a (R = 48′′ = 1.1 kpc), and (d) two 500-pc size knots timescale of ∼20–100 Myr. This estimated timespan exterior to the ring (R = 130′′ = 2.9 kpc). is consistent with a population of late B-type stars whose main-sequence lifetimes are of similar duration. 3.1. Nuclear Region The UIT/FUV image of the inner disk also shows The insert in Figure 2a (Plate xxxx) contains the a brighter arc to the north, just inside the starburst HST/FOC pre-COSTAR image of the nuclear region ring at a radius of 40′′ (see Figure 2b), along with (Maoz et al. 1995, 1996). This near-UV image shows widespread diffuse emission at a level amounting to a marginally-resolved nucleus (FWHM ≈ 0.1′′ ) em- ∼15% the mean surface brightness of the ring. The bedded in a bright core of ∼1′′ (22 pc) diameter along nature of these resolved and unresolved FUV compo- with a faint “mini-bar” that has a total length of 20′′ nents is uncertain, although some clues can be ob- (450 pc) at a position angle of approximately 30◦ . tained from the longer wavelength imagery. Low-level ripples in the emission (which are included At Hα the inner disk appears in absorption with in the estimate of the bar’s total length) are evident respect to the continuum emission from the underly- 2′′ , 5′′ and 9′′ to the SW of the nucleus. Maoz et al. ing population of stars. The strongest absorbers at (1995) attribute these features to bow shocks or tidal Hα are the photospheric atmospheres of B and A- arms resulting from a recent merger event. However, type stars, whose temperatures are sufficiently cool we see that the ripples overlap with the larger con- for Hydrogen to remain neutral and sufficiently warm centric arcs that are evident in the UIT/FUV image for significant population of the H Balmer (n=2) at projected radii of ∼9′′ and 15′′ (see next subsec- electronic energy level (cf. Mihalas 1978). In the tion). Similar ripples are also present in an archival UIT/FUV image, the diffuse light from the inner HST/WFPC2 V-band image of this region – with con- disk probably arises from these same stars. Indeed, nections to larger spiral arcs. To the north of the the UIT/FUV image represents the first view of this nucleus are two point sources whose flux densities are young stellar population, unconfused by the longer- consistent with those of single B-type supergiant stars wavelength emission from the older inner disk and (see Section 4). bulge components. Figure 4 compares a spatially-filtered R-band im-17 The Isaac Newton Telescope is operated by the Royal Green- age with an (R−I) color image of the inner disk. The wich Observatory on behalf of the SERC at the Spanish Ob- servatoriao del Roque de los Muchachos. spatially-filtered R-band image (Fig. 4a) was cre- 6
  7. 7. ated by median smoothing the R-band image over a of 40◦ , based on the H I study of Mulder & van Driel13′′ × 13′′ window and then subtracting the smoothed (1993).image from the original. The resulting fine-scale A comparison of the FUV and Hα morphologiesstructure includes the nuclear “mini-bar” previously reveals strong similarities. Figure 5a shows the Hαnoted by Mollenhoff et al. (1995) and Mulder (1995), emission, and Figure 5b shows the ratio of Hα andalong with a “dark” spiral arc of diminished emis- FUV intensities in the inner disk and ring. Exceptsivity to the west and other flocculent spiral struc- for the spiral-like ridges of enhanced Hα emission totures associated with the starburst ring. The archival the WNW and ESE, very little coherent variation inHST/WFPC2 V-band image of this region resolves this ratio is evident in the ring. In particular, thethe flocculent structure into spiral dust lanes of high intensity ratios do not show any radial displacementpitch angle which cross through the nearly circular in the FUV emission relative to that at Hα. From thisring. lack of radial structuring in the Hα/FUV ratio, we The (R − I) color image (Fig. 4b) shows redden- can conclude that little evidence is found for outwarding along the western spiral arc seen in (Fig. 4a) and or inward propagating starburst activity, as explainedin another arc to the NE, along with blue knots in the below.starburst ring. None of the fine-scale structures and A morphological comparison of Hα and FUV emit-reddened features in the optical inner disk have coun- ting regions provides a useful means of tracking se-terparts in the UIT/FUV image. Therefore, neither quential patterns of star formation in disk galaxies.emitting nor absorbing structures, nor reddening dust The Hα emission is dominated by the most massivelanes in the old disk can be defining the observed con- and short-lived stars (M ≥ 20 M⊙ ), while the FUVcentric FUV arcs to the SW and NE. We infer that the emission arises mostly from less massive, longer- livedFUV arcs are recently-generated structures of emit- stars (20 ≤ M/M⊙ ≤ 2.5). For an evolving star clusterting B and A-type stars and/or scattering clouds of with an IMF typical of local star-forming regions, thelow dust optical depth. We further speculate that the Hα emission typically reaches a maximum within aemitting and/or scattering FUV sources, being rela- million years of the star-forming episode, and quicklytively young, are distributed in a much thinner disk decays with an e-folding timescale of about 3 Myr.than that associated with the optical fine-scale struc- By contrast, the FUV emission reaches a maximumtures and reddened features, thus explaining the mor- at about 3 Myr (due to the onset of B-type super-phological differences between these two wavelength giants), decaying to 1/e in yet another 3 Myr. Theregimes. Further insights on the stellar content of the Hα/FUV ratio is seen to decrease by 2 orders of mag-inner disk can be gained from the IUE spectroscopy nitude after 10 Myr (e.g. Hill et al. 1995). Subse-presented in Section 6. quently, the Hα emission will vanish, while the FUV declines slowly for the next few 100 Myrs.3.3. Starburst Ring In the outward propagating starburst scenario, the The inner starburst ring in M94 is the single dom- Hα emission would concentrate where the propagat-inant emission feature in the FUV, Hα, and radio ing wavefront is located — on the outer perimetercontinuum bands. Rectification of the galaxy to its of the FUV-emitting ring. Inward propagating sce-nominal face-on orientation (PA = 120◦ i = 40◦ [Mul- narios would have the Hα emission interior to theder & van Driel 1993]) shows that the ring is almost FUV ring. Spatial displacements of Hα and FUVperfectly circular, with a mean radius of 47′′ (1.1 emission have been observed across the spiral armskpc) and FWHM of 21′′ (0.49 kpc). The latter mea- of M51 (O’Connell 1997; Petit et al. 1996), M74surement is based on doubling the measured HWHM (Marcum et al. 1997), and NGC 4258 (Courtes etof the emission beyond the mean radius, thereby ex- al. 1993). These displacements have been interpretedcluding the diffuse contribution interior to the ring. as the result of spiral density waves concentrating gasAdopting an inclination of 35◦ , as in the dispersion along the insides of the spiral arms and subsequentorbit model of Buta (1988), would yield a finite but migration of the evolving clusters past the spiral wavesmall ellipticity of 0.063, with a 16◦ offset between fronts. Given the resolution of our images (∼100 pc)the kinematic line of nodes and projected major axis. and characteristic timescale between Hα and FUVWe are not able to further constrain the ring’s orien- maxima (3 Myr), any residual propagation of star-tation, and so shall continue to assume an inclination burst activity would have to proceed at a speed less 7
  8. 8. than 35 km/s to avoid detection. Implications of this In summary, the bi-symmetric FUV knots have B-propagation speed limit are discussed in Section 7. band counterparts which appear to be part of the The arm-like enhancements in the Hα/FUV ratio complex spiral arm structure at these radii. Theircorrespond to similar features in a recent CO mapping symmetric prominence on opposite sides of the nu-with the BIMA interferometer (Wong et al. 2000). cleus, however, requires a dynamical explanation thatThese arm-like extensions away from the ring have is spatially more specific than that of the arms. Sim-further analogues at B-band, where a complex spiral ilarly, the starburst ring can be regarded as an espe-pattern is evident (see next subsection). cially bright, tightly wound component of the overall spiral pattern, whose prominence also indicates spe-3.4. Bi-Symmetric Knots and Associated Spi- cial dynamical circumstances. ral Structure Bi-symmetric knots or plumes have been noted in other ringed-barred galaxies, including NGC 1433 One of the most remarkable aspects of M94’s FUV (Buta 1986), NGC 7020 (Buta 1990), NGC 7098emission morphology is the pair of knots on diamet- (Buta 1995), and IC 4214 (Buta & Combes 1996).rically opposite sides of the nucleus. Both knots As discussed in these papers, the symmetric featureshave projected radii of approximately 130′′ (2.9 kpc). are likely tracing dynamical resonances that are con-A line drawn through the knots intersects the nu- nected with the general ring-bar phenomenon.cleus at a position angle of 105◦ , intermediate be-tween the PAs of the inner starburst ring (127◦ ) and 4. Photometric Resultsthe oval disk (95◦ ) (Mollenhoff et al. 1995). Thetwo knots are roughly 20′′ (500 pc) in size, with 4.1. Nuclear Photometrythe eastern knot showing the more complex struc-ture. Only the eastern knot shows significantly in our From the UIT and HST/FOC images, circularcontinuum-subtracted Hα image (Figure 5). Other aperture photometry of the nucleus out to a radius ofmuch smaller knots of FUV emission appear at low 5′′ yields m(F U V ) = 14.7 mag and m(N U V ) = 13.5S/N to the SW, SSW, ENE, and NNE. The SW, SSW, mag, respectively. The resulting (F U V −N U V ) colorand ENE knots have faint counterparts in the light of of 1.2 mag is significantly redder than those derivedHα, while all of these features have counterparts at from IUE spectra of the inner disk (see next Section).B-band (see below). Between 5′′ – 10′′ radii, the non-nuclear emission is Figure 6a and Figure 6b respectively show the significantly bluer than those derived using IUE, withB-band image before and after spatial filtering. The (F U V −N U V ) ≈ −0.3. The latter colors indicate thefiltering in this case involves median smoothing over presence of late B and hotter stars, depending on thea 30′′ × 30′′ window and then dividing the original amount of reddening (Fanelli et al. 1992). Estimatesimage by this smoothed image. Such median normal- of the reddening have been derived from the visibleized spatial filtering reveals fine-scale structure over colors (Smith et al. 1991; see also next subsection),a wide range of surface brightnesses (e.g. Waller et visible spectra and subsequent modelling of the stellaral. 1998). Here, it highlights the starburst ring and populations ( Taniguchi et al. 1996; Pritchett 1977).bisymmetric knots as significant enhancements above The resulting estimates of visual extinction are 0.5–the disk at B-band. The filtered image also reveals 1.0 mag in the nuclear region, corresponding to UVmany flocculent spiral arms outside of the ring, whose color excesses of E(F U V − N U V ) = 0.1–0.2 mag.relation with the knots is somewhat ambiguous. The Therefore, the corrected (F U V − N U V )◦ color of thewestern knot appears part of a dominant arm with a nucleus could be close to 1.0 mag, or the equivalent of“shingled” morphology connecting to the ring in the a stellar population with a late A-type stellar cutoffnortheast. The eastern knot appears to be associated (Fanelli et al. 1992).with several arms, including one which would be the The non-nuclear point sources in the HST/FOCsymmetric counterpart to the arm that links the west- near-UV image of the nuclear region provide help-ern knot to the ring. The marginally detected FUV ful checks on the distance to M94. For the brightestknots to the SW, SSW, ENE, and NNE all show en- point source, Maoz et al. (1996) lists a monochro-hanced B-band emission from associations of massive matic flux of fλ (2300 ˚) = 2.6 × 10−16 erg s−1 cm−2 Astars among the myriad spiral arms. ˚−1 (mλ (2300˚) = 17.9 mag). This source is unre- A A solved at a resolution of 0.1′′ and corresponding linear 8
  9. 9. scale of <2.2 pc at the assumed distance. Although an 9.45 mag, or M(FUV) = −18.91 mag at the adoptedextremely compact star cluster cannot be ruled out, distance of 4.6 Mpc.this exceptional UV source is most likely dominated The corresponding FUV luminosity of 1.5 × 1039by a single hot supergiant star. In the absence of ex- erg s−1 ˚−1 is the photometric equivalent of 2 × 104 Atinction, a B0-2 Ia-O supergiant star has M (N U V ) ≈ Orion nebulae (Bohlin et al. 1982) or about 90 30-−10.7 mag (Fanelli et al. 1992), which would imply a Doradus regions (as measured on an UIT/FUV im-distance modulus of (m − M ) = 28.6 mag and corre- age of 30 Dor out to a radius of 5 arcmin [67 pc]),sponding distance of 5.2 Mpc. Assuming nuclear color before correcting for the extinction in these sources.excesses of E(B − V ) = 0.15–0.27 mag ( Taniguchi et Adopting a Salpeter IMF with lower and upper massal. 1996) and Galactic-type extinction law, the corre- limits of 0.1 and 100 M⊙ , respectively, yields an un-sponding near-UV extinctions would be A(N U V ) ≈ corrected star formation rate of 0.15 M⊙ /yr. This1–2 mag, and the revised distances would be 3.3–2.1 SFR estimate assumes continuous star formation andMpc, respectively. The adopted distance of 4.6 Mpc includes the strong contribution of B supergiant starscan thus be regarded as a reasonable estimate, given to the overall luminosity. An overall visual extinctionthe uncertainties in stellar spectral type and extinc- of 1 mag would increase the global luminosity and in-tion. The second source, 1-arcsec to the E, is 6 times ferred SFR by about a factor of 11 (Hill et al. 1997).(1.9 mags) fainter – consistent with it being an A0- At this rate, it would have taken ∼10 Gyrs to trans-2 Ia-O supergiant at this distance, if subject to the form the dynamical mass of 1.6 × 1010 M⊙ that issame amount of extinction. Were the galaxy signif- present within 60′′ of the nucleus(Garman & Youngicantly closer, more abundant OBA-type giant and 1986) into the dominant stellar component that wemain-sequence stars would be resolved – which is not see today.the case. As a check on these starbirth estimates, we note that the measured Hα flux of 9.6 × 10−12 erg cm−24.2. Surface Photometry of the Disk s−1 converts to a luminosity of L(Hα) = 2.45 × 1040 Figure 7 shows radial profiles of the FUV surface erg/s, or only 5600 equivalent Orion nebulae. Thebrightness and cumulative flux. These profiles were origin of this discrepancy is deferred to the next sub-derived from annular-averaged photometry using el- section. Assuming Case B recombination and mul-liptical annuli consistent with the adopted position tiplying the Hα luminosity by 7.4 × 1011 yields aangle (120◦ ) and inclination (40◦ ). Since Galactic ex- photoionization rate of Ni = 1.8 × 1052 photons/sec,tinction is negligible towards M94 (de Vaucouleurs and corresponding star formation rate of 0.22 M⊙ /yr.et al. 1991), no correction was made. The result- The similarity of the FUV and Hα-based SFRs, be-ing surface brightness profile (Fig. 7a) is dominated fore correcting for extinction, suggests either that in-by light from the nucleus, inner disk, starburst ring, significant obscuration is present in the photometri-and an exponentially-declining disk that is punctu- cally dominant starburst ring, or that significant ob-ated by an enhancement from the bi-symmetric knots scuration exists with other mechanisms making upat R = 130′′ . The exponentially declining component for the greater attenuation of the FUV emission rel-between 50 and 80-arcsec radii has an e-folding scale- ative to the longer-wavelength Hα emission. Suchlength of only 10′′ (223 pc), similar to that found at mechanisms include (1) an excess contribution of non-R and I-bands (cf. Munoz-Tunon et al. 1989). The ionizing B-type stars to the total FUV emission, (2)FUV emission beyond the bi-symmetric knots also a reduction of the total Hα emission due to Hα ab-shows a steep decline with an estimated scalelength of sorption by the atmospheres of B and A-type stars,20′′ (446 pc), significantly shorter than the 70′′ scale- and (3) the absorption of EUV photons before theylength measured at R and I-bands in the outer disk ionize the gas and induce Hα emission. Likely EUV(Munoz-Tunon et al. 1989). absorbers include the nebular dust associated with The cumulative flux profile (Fig. 7b) indicates the H II regions as well as the various metal speciesthat the half-light radius matches that of the star- in the O-type stellar atmospheres themselves (cf. Hillburst ring, and that more than 80% of the total FUV et al. 1997; Waller, Parker, & Malumuth 1996). Weemission is contained within a radius of 60′′ (1.3 kpc). conclude in the next subsection that modest obscura-The total FUV flux from M94 is fλ (1520 ˚) = 6.02 × A tion plus non-ionizing contributions to the total FUV10−13 erg cm−2 s−1 ˚−1 , corresponding to m(FUV) = A emission best explain the observed levels of Hα and 9
  10. 10. FUV emission. Stellar absorption at Hα is proba- 1996]), and close to the mean of M51’s wide-rangingbly less than a 10% effect overall, given Hα emission GHRs (29 ± 23 ˚ [Hill et al. 1997]). Aequivalent widths of order 100 ˚ in the ring, and stel- A A model of steady-state star formation with alar absorption equivalent widths peaking at less than Salpeter-type IMF yields a somewhat lower ratio of 1610 ˚. A ˚, while a modeled 2 Myr-old starburst would match A The global (total) (F U V − V ) color of M94 is 1.21 our result (Hill et al. 1995 [see their Fig. 10]). Suchmag, which is characteristic of early-type disk galax- a short burst timescale is probably untenable, thusies with circumnuclear starburst activity (e.g. NGC indicating some need to correct our FUV flux for ex-1068, NGC 3351 [Waller et al. 1997]). The (R−I) col- tinction. For 1 mag of visual extinction, the resultingors shown in Figure 4b do not vary as much as those “corrected” Hα/FUV flux ratio would be lower byfound by Beckman et al. (1991) in their photometric a factor of ∼5, leading to a modeled burst age of 6study. Bootstrapping our measured intensity ratio in Myr — close to the age limit of the ionizing O stars.the nuclear region to a color of (R − I) = 0.45 mag as Based on these considerations alone, we surmise thatreported by Beckman et al. (1991), we obtain colors the FUV and Hα emission from the starburst ring arethat range from (R − I) = 0.3 mag in the rings’ star- subject to no more than 1 mag of visual extinctionburst knots to (R − I) = 0.5 mag in the arcs interior (on average). We also can conclude that the simi-to the ring. Beckman et al. (1991) obtain much bluer larity in uncorrected SFRs based on the global FUVcolors of order 0.1 mag near the ring. We attribute and Hα fluxes is probably coincidental. After correc-this discrepancy to our having used the Hα-band im- tion for 0.5–1.0 mag of visual extinction, the Hα fluxage to remove contaminating line emission from the would be increased by a factor of 1.4–2.1, while theR-band image ( Waller 1990b), thereby reducing the FUV emission would increase by a factor of 3.3–11.red flux from the knots in the ring by 20%. Compari- In other words, the FUV emission is tracing an ad-son of the contaminated and uncontaminated R-band ditional non-ionizing Population I component in theimages confirm our attribution. inner disk that is missed at Hα and, after correction If the reddened arcs are caused by dust, the corre- for extinction, is contributing to the overall calculatedsponding color excess relative to that of the starburst rate of star formation.knots would amount to E(B − V ) ≈ E(R − I) = 0.2 The bi-symmetric FUV knots to the west and eastmag, or the equivalent visual extinction of roughly 0.6 of the ring have fluxes of 3.0 × 10−15 and 3.8 × 10−15mag in excess of the knots. Allowing for bluer stellar erg cm−2 s−1 ˚−1 , respectively. This translates to the Apopulations in the knots could reduce the estimate equivalent of 100 and 127 Orion nebulae, respectively.of excess extinction in the arcs to negligible levels, Only the eastern knot shows significant Hα emission,while inclusion of some internal reddening in the knots with a flux of 2.57 × 10−13 erg cm−2 s−1 and hencewould increase the total estimated extinction by ≤1 an Hα/FUV flux ratio of 67 ˚— about twice that of Amag. Such low estimates for the extinction are simi- the Orion nebula — thus indicating greater FUV ob-lar to the spectroscopic results obtained by Pritchett scuration than is present in Orion or a hotter ionizing(1977) and by Taniguchi et al. (1996) in their stud- cluster. Examination of the spatially-filtered B-bandies of the nuclear region, where color excesses of 0.27 image (Figure 6b) reveals dark spiral-arm featuresmag and 0.15 mag, corresponding to AV = 0.86 mag at the position of the eastern knot which may cor-and 0.48 mag, were respectively obtained. respond to obscuring dust clouds. The western knot shows similar structures to one side. The lack of sig-4.3. Photometry of the Starburst Ring and nificant Hα emission from this knot is difficult to ex- Knots plain with dust, and is most likely due to the presence Photometry of the FUV and Hα emission from the of an older (τ > 10 Myr), non-ionizing stellar popu-starburst ring (between projected radii of 30′′ and lation.60′′ ) yields a mean Hα/FUV flux ratio (in equivalentwidth units) of 22 ˚. By way of comparison, we note A 5. Spectroscopic Resultsthat this flux ratio is somewhat lower than that found 5.1. FUV Spectroscopyin the Orion nebula (38 ˚[Bohlin et al. 1982]), within Athe range of flux ratios evident in M33’s giant H II Figure 8a shows an average of 5 IUE/SWP spec-regions (19 ˚ to 69 ˚ [Parker, Waller, & Malumuth A A tra of the extranuclear emission from the inner disk. 10
  11. 11. Based on the aperture mapping shown in Figure 3a an issue). The off-nuclear spectrum shown here issome emission from the nucleus may be present, but not only bluer but shows a diminished 2800 ˚ break Aat low levels. The wavelengths of commonly observed amplitude. Measured as a ratio of flux Fλ above andFUV absorption and emission lines are indicated on below 2800 ˚, the ratio is 2.5 at nucleus and 1.8 away Athe spectrum for comparison. Although the aver- from it. Using the spectral-break indices developedaged spectrum is of generally low S/N, several fea- by Fanelli et al. (1992), we obtain for the off-nucleustures can be identified. Of these, the most prominent averaged spectrum (Mg II 2800 – Mg I 2852) = 0.31are the absorption blends of S II (1250-1259) and Si II mag, (2609/2660) = 0.45 mag, and (2828/2921) =(1260,1265), the P-Cygni profile of S IV (1394,1443), 0.31 mag — all of which are consistent with a (B − V )the absorption blends of Fe III (1601-1611), Al III color of 0.2–0.3 mag, or the equivalent of a late A-(1600-1612), C II (1720-1722) and Al II (1719-1725), type main sequence star or late A to early F-typeand part of the broad absorption complex of Fe III giant/supergiant star.(1891-1988). The low-ionization lines, in particular, The off-nuclear continuum is likewise flatter in theare most characteristic of B and A-type stars. The SWP range, with little trace of the emission-like fea-strength of the blueshifted S IV absorption feature rel- tures around 1900 ˚ seen in the nuclear spectrum Aative to that of C IV (1550) along with the strong Si II (see Kinney et al. 1993). These properties can beabsorptions at 1260,1265 ˚ and 1527,1533 ˚ indicate A A accounted for, if recent star formation (few times 108a composite spectral type later than B3 but earlier years ago) contributes relatively more light off thethan B8 (Fanelli et al. 1992; Kinney et al. 1993; nucleus than on it, consistent with an aging burstWalborn, Parker, & Nichols 1995). The absorptions that is spatially more extended than the centrally-at 1470 ˚, 1780 ˚, and 1790 ˚ remain unidentified. A A A concentrated stars of the older disk and bulge. In emission, there is some evidence for C IV (1548-1551), He II (1640), N III] (1730,1750), and C III] 5.3. Optical Spectroscopy(1909) — much of which may be the result of LINER Figure 8c shows the spectrum of optical emissionactivity being scattered by the ISM in the inner disk. from the central D = 8.1 arcsec, as obtained with theThe C III] (1909) line emission may be responsible for Lick 1-m telescope and IDS. Absorption lines includefilling in part of the broad stellar absorption complex the Ca II K&H lines at 3934 ˚ and 3968 ˚, Hδ (4101), A Aof Fe III (1891-1988). CH G-band (4300), Hγ (4340), He II (4686), Hβ (4860), Mg I (5170), Fe I (5270), Na D (5890,5896),5.2. NUV Spectroscopy Hα (6563), and atmospheric absorption bands of O2 Figure 8b shows an average of 2 IUE/LWP spec- (6867) and H2 O (7186). Emission lines are restrictedtra of the disk. The most prominent spectral fea- to [N II] (6584) and [S II] (6731).tures are the absorption lines of Fe II (2609, 2750), The Hβ equivalent width of about 5.4 ˚ is huge for AMg II (2800), and Mg I (2852). Comparison with an old population typical of spiral bulges, even beforestellar spectra from the IUE Spectral Atlas (Wu et any correction for line emission. There has been staral. 1991) shows that Mg II (2800) is unusually weak formation in this region not long ago. The 4000-˚ Arelative to Mg I (2852); the line ratios in this spec- break is also suppressed, indicating the effective agetral range being consistent with light dominated by is much younger than for typical early-spiral bulges.late G-type stars, based on this cursory comparison. Following the Dressler and Shectman (1987) defini-However, the spectrum is too blue and the ampli- tion (ratio of Fν between 3950-4050 and 3750-3850tude of the 2800 ˚ break too small for this to be the A ˚), the observed break has Fν = 1.6, whereas values Awhole story. Comparison with the nuclear spectra of 1.9–2.0 are more usual.(IUE LWR 12221,12238 – as listed in Table 2) shows From the IDS/IPCS on the 2.5-m Isaac Newtonevidence for a spectrally smooth blue component off Telescope, we see that the nucleus (inner 1” or so) isthe nucleus proper. Also, the Mg II 2800 ˚ break is A bluer at optical wavelengths than its immediate sur-roughly 50% smaller than that evident in the nuclear roundings. Misalignment in the IPCS detector is lessspectrum (see below). than 0.1 pixel, so this is not an obvious instrumen- All the absorption features are broader in the off- tal effect. This nuclear blueing effect is substantial,nuclear spectrum, because the light there almost uni- amounting to about 30% over the 4400–5200 ˚ range Aformly fills the aperture (so centering shifts are not 11
  12. 12. (more or less equivalent to E(B − V ) = −0.3 mags, emission beyond 60 arcsec radius – further corrobo-where the mean (B − V ) color of the bulge is 0.91 rating the gas upper limit used here. The H I profile,mags, which is already bluer than ordinary Sa/Sb though of lower amplitude, shows enhancements atbulges [Keel & Weedman 1978]). Outside this area, the radii of the starburst ring and the bi-symmetricthe color along the slit is quite constant out to 15′′ knots.from the nucleus, where the signal begins to die out. Figure 9c shows the corresponding radial profileThis effect has no observed counterpart in the IUE of the gravitational stability index (Q). Here, we havespectra, because of their courser spatial resolution. considered the simplest case of the gaseous stabil- The prominent Hβ absorption line in the nuclear ity, without any coupling with the stellar component,INT/IPCS spectrum has FWHM 18–20 ˚, consistent A such thatwith values seen in mid-to-late A stars. Thus, thedata are consistent with seeing the main-sequence Q = Σcrit /Σgas = κσ/πGΣgas ,turnoff near this spectral type, rather than the super-giant dominance of a much younger population giving where the epicyclic frequency κ is closely linked to thesuperficially similar spectral features (as seen in the rotation curve v(R) throughstarbursting nucleus of NGC 4569 [Keel 1996]). 2v v dv The optical spectra confirm our imaging result that κ2 = ( + ),Hα is in absorption with respect to the continuum R R dRfrom the underlying population of A and B-type stars. and where σ is the gas velocity dispersion — a quan-By contrast, the neighboring line of [N II] (6584) is tity which is not well constrained but is probably ofseen in emission in the Lick/IDS spectrum. As pre- order 5–10 km/s. Here, we leave σ as an unknown.viously noted, Hβ is also dominated by photospheric The resulting radial profile of Q/σ out to the limits ofabsorption. The IDS/IPCS spectrum shows some Hγ the CO observations shows a shallow minimum at theemission at least at the nucleus, that is not swallowed radius of the starburst ring. Although this minimumby the stellar absorption line. is too broad and shallow to explain the more discrete and prominent starburst ring, it may help to explain6. Kinematics and Dynamics the relatively young (107 − 108 yr) stellar population pervading the inner disk. Figure 9a shows the rotation curve based on the H Figure 10 shows the orbital resonance diagramI observations of Mulder and van Driel (1993), where that results from the rotation curve along with thekey morphological features are noted. This curve is radii of key morphological features. A single wavequalitatively similar to that obtained from a recent pattern speed of 35 km s−1 kpc−1 (where d = 4.6interferometric mapping of CO (Wong & Blitz 2000) Mpc) would place the nuclear bar inside the Inner-but is on-average 12% lower, due to the adoption of Inner Lindblad Resonance (IILR), the starburst ringa 40◦ inclination compared to their 35◦ . Figure 9b between the IILR & Outer Inner Lindblad Resonanceshows the corresponding radial profiles of H I and H2 (OILR), the bisymmetric knots at the 4:1 “ultra-gas, the latter being derived from the CO observations harmonic” resonance, the oval disk terminating at co-of Gerin et al. (1991). rotation (CR), and the outer pseudo-ring at the Outer The molecular gas component clearly dominates Lindblad Resonance (OLR). Alternatively, a higherthe inner disk’s ISM and may continue to prevail pattern speed of 56 km s−1 kpc−1 (as modeled byat higher radii, where the CO emission has yet to Mulder & Combes (1996)) would move the starburstbe measured. The plotted extrapolation beyond the ring just outside the OILR, the bisymmetric knots tolast reliable measurement of CO emission is intended the CR radius, and the edge of the oval disk to theto provide an upper limit on the total gas and thus OLR. This latter model, however, fails to account fora lower limit on the Q stability index (see below). the outer ring without invoking additional patterns.The interferometric measurements by Wong & Blitz Further support for the proposed sequence of res-(2000) yield even lower extrapolated surface densi- onances comes from specific ratios of the correspond-ties of gas – consistent with our extrapolated upper ing radii (cf. Buta 1986). Given a flat rotationlimit. Moreover, FIR measurements with IRAS and curve, the modeled ratio of OLR and UHR radii isthe KAO (Smith et al. 1991) reveal negligible dust r(OLR)/r(UHR) = 2.6. If these OLR and UHR reso- 12
  13. 13. nances are respectively traced by the outer pseudor- to 1.5 M⊙ yr−1 — sufficient to build up the stellaring and bi-symmetric knots, then we obtain a ratio mass of the inner disk and bulge in ∼1010 yr.of radii equaling 2.54 – closely matching the modeled (5.) No detectable radial offsets between the Hα andratio. The theoretical ratio of OLR and CR radii, FUV rings, thus indicating a 35 km/s speed limit tobeing r(OLR)/r(CR) = 1.7, is also well matched by any outward or inward propagating star formationthe observed relative dimensions of the outer ring and in the ring, if such a mode is present.oval disk, where a ratio of 1.5 is obtained. (6.) Two 500-pc size FUV-emitting knots exterior to Admittedly, morphological tracers and rotation the ring on diametrically opposite sides of the nu-curves are insufficient to discriminate between these cleus. The bisymmetric knots and starburst ringand other possible kinematic patterns (cf. Buta & appear to be especially prominent parts of a com-Combes 1996). Further progress on constraining the plex spiral arm structure, as revealed in a spatially-resonant dynamics in M94 will require analysis of the filtered B-band image.complete velocity field in the disk (cf. Westpfahl & (7.) The starburst ring, bi-symmetric knots, ovalAdler 1996; Canzian & Allen 1997). A complete H I disk, and outer pseudo-ring as signposts of reso-mapping with the VLA has been made recently, whose nant dynamics in the disk of M94. More specifi-spectral and spatial resolution would be sufficient to cally, the radii of these features match those of var-derive a complete velocity field and its associated res- ious orbital resonances, given a pattern speed of 35onant states (D. Westpfahl, private communication). km s−1 kpc−1 at our adopted distance and incli-Until such an analysis is carried out, we think that nation. These orbital resonances are most likelythe resonant state diagrammed in Figure 10 best driven by some combination of the nuclear mini-explains the most features with the fewest conditions. bar and oval distortion in the disk. (8.) A shallow minimum of gravitational stability at7. Conclusions and Implications the radius of the starburst ring that extends inward Through UV-Optical imaging and spectroscopy, we into the inner disk. Although too broad to explainhave found new evidence for bar-mediated resonances the discrete starburst ring, the shallow minimumas the primary drivers of evolution in M94 at the may help to explain the 107 − 108 -yr old stellarpresent epoch. Our observational results include evi- population interior to the ring.dence for(1.) A 450-pc long nuclear “mini-bar” at both optical Although we can set a limit on the speed of out- and near-UV wavelengths. ward or inward propagating star formation in the(2.) An inner disk with diffuse FUV emission in ring, we cannot preclude the existence of such a mode. concentric arcs that do not match the fine-scale At a propagation speed of 35 km/s, a wave initiated in structures or reddened structures at visible wave- the nucleus could traverse the inner disk to the radius lengths. Since Hα is observed in absorption here, of the current starburst ring in only 31 Myr. There- the UIT/FUV image represents the first view of fore, it is possible that the 107 −108 -yr old stellar pop- this non-ionizing but relatively young disk compo- ulation detected in the inner disk is the result of such nent. an outward propagating wave. The striking differ- ence in emission morphologies at FUV and red wave-(3.) UV-Optical colors and spectral indices in the lengths provides further support for the starburst ring nucleus and inner disk that indicate B and A-type being a transient phenomenon which does not persist stars in the presence of modest extinction (AV ≤1 at any one radius for very long. Either these reso- mag) along with some LINER activity from the nant phenomena come and go, as the oval distortions nucleus itself. undergo secular evolution, or their operating radii mi-(4.) A 2.2 kpc diameter starbursting ring at the grate in response to other dynamical influences on the perimeter of the inner disk that is bright at FUV, stars and gas (cf. Combes 1994; Combes et al. 1995; Hα, and radio-continuum wavelengths. The level Friedli & Benz 1995). Otherwise, one must invoke of starbirth activity in this inner ring rivals the lev- strong radial inflows of stars from the starburst ring els observed in starbursting irregular galaxies such to populate the inner disk and bulge, a feat requiring as NGC 1569 and NGC 4449. The inferred star for- unusual circumstances – e.g. mergers. mation rate within the ring and inner disk amounts 13
  14. 14. The results reported herein may have important the Spacelab Office at NASA Headquarters underimplications with regard to observations of the most Project number 440-51. We are deeply grateful to thedistant observable galaxies. At redshifts of 1–5, the crew of STS-67 and the many people who helped make2-kpc diameter starburst ring in M94 would subtend the Astro-2 mission a success. WHW acknowledgesangles of only (0.7′′ – 1.0′′ )H◦ /75 in an Einstein-de partial support from NASA’s Astrophysics Data Pro-Sitter Universe (q◦ = 1/2) and (0.3′′ – 0.2′′ )H◦ /75 gram (071-96adp).in an open (Milne) universe (q◦ = 0) (cf. Narlikar1983). The UV-bright nuclear rings evident in NGC REFERENCES1097, NGC 1317, NGC 1433, NGC 1512, NGC 2997,NGC 4321, and NGC 5248 (Maoz et al. 1995; Maoz et Athanassoula, E. & Bosma, A. 1985, ARA&A, 23,al. 1996; Kuchinsky et al. 2000; Marcum et al. 2000) 147would subtend even smaller angles at the same red- Beckman, J. E., Varela, A. M., Munoz-Tunon, C.,shifts. Moreover, nuclear rings tend to have higher Vilchez, J. M., & Cepa, J. 1991, A&A, 245, 436FUV surface brightnesses than their larger counter- Bohlin, R. C., Hill, J. K., Stecher, T. P., and Witt,parts – the inner ring in M94 being a remarkable A. N. 1982, ApJ, 255, 87exception. Therefore, some of the “core-halo” mor-phologies that are evident at high-redshift in the rest- Bosma, A., van der Hulst, J. M., & Sullivan, W. T.,frame FUV (cf. Giavalisco et al. 1997) may, in fact be III 1977, A&A, 57, 373marginally-resolved representations of galaxies with Buta, R. 1986, ApJS, 61, 631starburst rings in their centers. Buta, R. 1988, ApJS, 66, 233 Gravitationally-lensed galaxies are fortuitously mag- Buta, R. 1990, ApJ, 356, 87nified, enabling resolutions of their structure at highS/N. An important precedent in this regard is the Buta, R. 1995, ApJS, 96, 39gravitationally-lensed “Pretzel Galaxy” which lies be- Buta, R. Purcell, G. B., & Crocker, D. A. 1995, AJ,hind the galaxy cluster 0024+1654 at an estimated 110, 1588redshift of 1.2 – 1.8 (Colley et al. 1996; Tyson et al. Buta, R. & Combes, F. 1996, Fund. Cosmic Phys.,1997). Detailed reconstructions of the multiply-lensed 17, 95galaxy show a clear annular morphology on a scale ofseveral kpc. If M94 and other nearby ringed galaxies Byrd, G., Rautiainen, P., Salo, H., Buta, R., &can be used as current-epoch analogues, the “Pret- Crocker, D. A. 1994, AJ, 108, 476zel Galaxy” and perhaps other marginally-resolved Canzian, B. & Allen, R. J. 1997, ApJ, 479, 723“core-halo” galaxies at high redshift may represent Clemens, D. P., Sanders, D. B., & Scoville, N. Z. 1988,youthful inner disks and bulges growing under the or- ApJ, 327, 139ganizing influence of oval or bar asymmetries(Friedli& Benz 1995; Waller et al. 1997). Conversely, if ev- Colley, W., Tyson, J. A., & Turner, E. L. 1996, ApJ,idence for starburst rings at high redshift proves to 461, L83be sparse, then massive inner disks featuring ring-bar Combes, F. 1994, in The Formation and Evolutiondynamics have yet to form in most systems, or star- of Galaxies, eds. C. Mun˜z-Tun˜n & F. Sa´ chez o o´ nbursting bulges are masking their presence. (Cambridge, UK: Cambridge U. Press), p. 317 Combes, F., Boiss´, P., Mazure, A., & Blanchard, A. e We thank David Adler, Gene Byrd, Francoise 1995, Galaxies and Cosmology, A&ALibrary (NewCombes, Daniel Friedli, and David Westpfahl for gen- York: Springer-Verlag)erously providing consultation on the dynamics of Courtes, G., Petit, H., Hua, C. T., Martin, P., Blecha,ringed-barred spiral galaxies. WHW is grateful to A., Huguenin, D., and Golay, M. 1993, A&A, 268,John Huchra and the OIR division of the Harvard- 419Smithsonian Center for Astrophysics for their kindhospitality during his visiting appointment at the de Vaucouleurs, G., de Vaucouleurs, A., Corwin,CfA. WHW also thanks Eric Murphy and Chris- H. G. Jr., Buta, R. J., Paturel, G., & Fouqu´ etine Winslow, Tufts undergraduates who helped craft 1991, Third Reference Catalogue of Bright Galax-some of the graphics. UIT research is funded through ies (RC3) (New York: Springer-Verlag) 14
  15. 15. Dressler, A., & Shectman, S. A. 1987, AJ, 94, 899 Marcum, P. M, et al. 1997, in the Ultraviolet UniverseDuric, N. & Dittmar, M. R. 1988, ApJ, 332, L67 at Low and High Redshift: Probing the Progress of Galaxy Evolution, eds. W. H. Waller, M. N.Elmegreen, B. G. 1992, in The Galactic Interstellar Fanelli, J. E. Hollis, & A. C. Danks (New York: Medium, Saas-Fee Advanced Course 21, eds. D. AIP Press), p. 88 Pfenniger & P. Bartholdi (New York: Springer- Verlag), p. 248 Marcum, P. M., et al. 2000, ApJS, in pressElmegreen, B. G. 1994, ApJ, 425, L73 McGlynn, T., Scollick, K., & White, N. 1996, in New Horizons from Multi-Wavelength Sky Surveys, IAUFanelli, M. N., O’Connell, R. W., Burstein, D., & Wu, Symp. 179 (Dordrecht: Kluwer), p. 465 (see also C.-C. 1992, ApJS, 82, 197 http://skyview.gsfc.nasa.gov/skyview.html)Friedli, D. & Benz, W. 1995, A&A, 301, 649 Mihalas, D. 1978, Stellar Atmospheres 2nd ed. (SanGarman, L. & Young, J. 1986, A&A, 154, 8 Francisco: Freeman)Gerin, M., Casoli, F., & Combes, F. 1991, A&A, 251, Mollenhoff, C., Matthias, M., & Gerhard, O. E. 1995, 32 A&A, 301, 359Giavalisco, M., et al. 1997, in the Ultraviolet Universe Mulder, P. S. & van Driel, W. 1993, A&A, 272, 63 at Low and High Redshift: Probing the Progress Mulder, P. S. 1995, A&A, 303, 57 of Galaxy Evolution, eds. W. H. Waller, M. N. Fanelli, J. E. Hollis, & A. C. Danks (New York: Mulder, P. S. & Combes, F. 1996, A&A, 313, 723 AIP Press), p. 221 Munoz-Tunon, C., Prieto, M., Beckman, J., & Cepa,Greason, M. R., Offenberg, J. D., Cornett, R. H., Hill, J. 1989, Astrophys. Space Sci., 156, 301 R. S., & Stecher, T. P. 1994, PASP, 106, 1151 Narlikar, J. V. 1983, Introduction to Cosmology (Boston:Gusten, R. & Mezger, P. G. 1982, Vistas in Astron- Jones & Bartlett Pub. Inc.), p. 129 omy, 26, 159 O’Connell, R. W. 1997, in The Ultraviolet UniverseHill, J. K. et al. 1995, ApJ, 438, 181 at Low and High Redshift: Probing the ProgressHill, J. K. et al. 1997, ApJ, 477, 673 of Galaxy Evolution, eds. W. H. Waller, M. N. Fanelli, J. E. Hollis, & A. C. Danks (New York:Keel, W. C. & Weedman, D. W. 1978, AJ, 83, 1 AIP Press), p. 11Keel, W, C. 1983, ApJ, 269, 466 Parker, J. Wm., Waller, W. H., & Malumuth, E. M.Keel, W. C. 1996, PASP, 108, 917 1996, in The Interplay between Massive Star For-Kenney, J. D. & Jogee, S. 1997 in Star Formation mation, the ISM, and Galaxy Evolution, eds. D. Near and Far, Seventh Astrophysics Conference, Kunth, et al. (Paris: Editions Frontieres), p. 485 AIP Conference Proceedings 393, eds. S. S. Holt Petit, H., Hua, C. T., Bersier, D., and Courtes, G. & L. G. Mundy (New York: AIP Press), p. 247 1996, A&A, 309, 446Kennicutt, R. C. & Kent, S. M. 1983, AJ, 88, 1094 Pogge, R. 1989, ApJS, 71, 433Kinney, A. L., Bohlin, R. C., Calzetti, D., Panagia, Pritchett, C. 1977, ApJS, 35, 397 N., & Wyse, R. F. G. 1993, ApJS, 86, 5 Sandage, A. & Tammann, G. A. 1981, A RevisedKuchinski, L. E., et al. 2000, ApJS, in press, (astro- Shapley-Ames Catalog of Bright Galaxies (Wash- ph/000211) ington, D.C.: Carnegie Institution of Washington)Maoz, D., Fillippenko, A. V., Ho, L. C., Rix, H., Bah- Shaw, M. A., Combes, F., Axon, D. J., & Wright, G. call, J. N., Schneider, D. P., & Macchetto, F. D. S. 1993, A&A, 273, 31 1995, ApJ, 440, 91 Smith, B.J., Lester, D. F., Harvey, P. M., & Pogge,Maoz, D., Fillippenko, A. V., Ho, L. C., Macchetto, R. W. 1991, ApJ, 373, 66 F., D., Rix, H.-W., & Schneider, D. P. 1996, ApJS, Smith, B. J. & Harvey, P. M. 1994, ApJ, 425, 91 107, 21 Stecher, T. P. et al. 1992, ApJ, 395, L1Marcum, P. M., Appleton, P. N., & Higdon, J. L. 1992, ApJ, 399, 57 Stecher, T. P. et al. 1997, PASP, 108, 584 15
  16. 16. STScI 1994, The Digitized Sky Survey (San Francisco: Wu, C.-C., et al. 1991, The IUE Ultraviolet Spectral ASP) Atlas Addendum I (Greenbelt: NASA/GSFC)Taniguchi, Y., Ohyama, Y., Yamada, T., Mouri, H., & Yoshida, M. 1996, ApJ, 467, 215Tenorio-Tagle, G., Munoz-Tunon, C., Perez, E., & Melnick, J. 1997, ApJ, 490, L179Tyson, J. A., Kochanski, G., & Dell’Antonio, I. 1997, in The Ultraviolet Universe at Low and High Red- shift: Probing the Progress of Galaxy Evolution, eds. W. H. Waller, M. N. Fanelli, J. E. Hollis, & A. C. Danks (New York: AIP Press), p. 204van der Kruit, P. C. 1974, ApJ, 188, 3van der Kruit, P. C. 1976, A&A, 52, 85Walborn, N., Parker, J. Wm., & Nichols, J. S. 1995, IUE Atlas of B-Type Spectra from 1200 to 1900 ˚, A NASA Ref. Publ. 1363Walker, C. E., Lebofsky, M. J., & Rieke, G. H. 1988, ApJ, 325, 687Waller, W. H. 1990a, Ph.D. Dissertation, University of MassachusettsWaller, W. H. 1990b, PASP, 102, 1217Waller, W. H., Gurwell, M., & Tamura, M. 1992, AJ, 104, 63Waller, W. H., et al. 1995, AJ, 110, 1255Waller, W. H., Parker, J. Wm., & Malumuth, E. M. 1996, in Cosmic Abundances, eds. S. S. Holt & G. Sonneborn, ASP Conf. Series #99 (San Francisco: ASP), p. 354Waller, W. H., et al. 1997, in The Ultraviolet Uni- verse at Low and High Redshift: Probing the Progress of Galaxy Evolution, eds. W. H. Waller, M. N. Fanelli, J. E. Hollis, & A. C. Danks (New York: AIP Press), p. 39Waller, W. H., Varosi, F., Boulanger, F., & Digel, S. W. 1998, in New Horizons from Multi-Wavelength Sky Surveys, IAU Symp. 179, eds. B. J. McLean, D. A. Golombek, J. E. Hayes, & H. E. Payne B. Lasker & M. Postman (Dordrecht: Kluwer), p. 194, (astro-ph/9612233)Westpfahl, D. J. & Adler, D. S. 1996, in The Min- nesota Lectures on Extragalactic Neutral Hydro- gen, ed. E. D. Skillman, ASP Conference Series #106 (San Francisco: ASP), p. 96Wong, T., & Blitz, L. 2000, ApJ, 540, 771, (astro- This 2-column preprint was prepared with the AAS L TEX A ph/9912503) macros v4.0. 16
  17. 17. Fig. 1.— A wide-field J-band image (extracted from knots in the starburst ring.the Digital Sky Survey) shows the outermost por-tions of the (R)SA(r)ab galaxy M94, highlighting the Fig. 5.— (a.) — Inner disk in the light of Hα. Afterovoid disk and outer stellar ring. The field of view scaling and subtracting the underlying red-continuumis 22.67′ × 22.67′. North is up and East to the left. emission, the residual Hα line emission shows concen-Corresponding fields of the 9.66′ ×9.66′ B-band image trations in the starburst ring and in (only) one of theand the 7.28′ ×7.28′ R-band, Hα and FUV images are bi-symmetric FUV knots. The nucleus and innermostindicated. disk show a net deficit, due to Hα absorption by the atmospheres of the B and A-type stars that domi-Fig. 2.— (a.) — UV and R-band imaging of M94. nate the light in these regions. (b.) — Ratio of HαThe field of view is 7.28′ × 4.53′. North is up and and FUV emission in the inner disk: No radial dis-East to the left. The UIT’s FUV image shows the placement in the Hα/FUV intensity ratio is evidentstarburst ring in high contrast against a mostly dark across the starburst ring, contrary to outward or in-disk. Exterior to the ring are two hitherto unrecog- ward propagating starbirth scenarios.nized 500-pc size bi-symmetric knots on diametricallyopposite sides of the nucleus. The HST’s NUV image Fig. 6.— (a.) — Deep B-band image, taken with(see inset) shows a 450-pc long nuclear mini-bar that the Palomar/Hale 5-m telescope. The field of viewhad been previously inferred from photometric anal- is 9.66′ × 9.66′ with the central 2 arcmin saturated.yses of optical-band images. By comparison, the R- (b.) — Complex of spiral structure. The same B-band image shows the underlying bulge and oval disk band image has been spatially filtered to highlightmade up of cooler and typically older stars. (b.) — the intermediate and small-scale structure. The nu-Radial distribution of FUV intensities (surface bright- clear region was saturated and hence shows no struc-nesses) reveals strong enhancements at the nucleus ture. In the disk, however, a complex spiral pattern isand starburst ring, along with regularly-spaced low- evident. The starburst ring and bi-symmetric knotslevel enhancements that are associated with arcs of appear to be especially prominent parts of this over-FUV emission interior to the ring. all spiral structure. The knots are resolved into as- sociations of hot stars, thus confirming their relativeFig. 3.— Location of IUE apertures with respect to youth.the inner disk and nucleus of M94. Numbers refer tothe entries in Table 2. Positional accuracy is esti- Fig. 7.— (a.) — Radial distribution of annular-mated at ±10′′ . (a.) — Overlay of IUE’s far-UV averaged FUV surface brightness in M94, expressed(SWP) apertures on the UIT/FUV image of M94, in magnitudes. (b.) — Radial distribution of theshowing both nuclear and disk-dominated observa- cumulative FUV flux in M94, showing most of thetions. (b.) — Overlay of the IUE’s near-UV (LWP FUV light arising from the central R = 1.3 kpc.and LWR) apertures on the UIT/FUV image of M94. Fig. 8.— (a.) — FUV spectrum of the inner disk,Fig. 4.— (a.) — The nuclear mini-bar and inner based on the signal-weighted average of 5 IUE/SWPdisk: Spatial filtering of the R-band image (shown disk-dominated spectra. (b.) — NUV spectrum ofin Fig. 2) shows a mini-bar with the same approx- the inner disk, based on the signal-weighted averageimate extent and position angle as that seen in the of 2 IUE/LWP disk-dominated spectra. (c.) — Op-HST’s NUV image. Optical and UV spectroscopy tical spectrum of the central D = 8.1 arcsec, as ob-of this region indicates a stellar population with an tained with the Lick/Shane 1-m telescope and Imageearly main-sequence turnoff (A4-A7) superposed on Dissecting Spectrometer (IDS).an older G-type population belonging to the central Fig. 9.— (a.) — Rotation curve (radial distribu-disk and bulge. The inner disk shows dark spiral tion of orbital velocities), based on the H I obser-arcs of relatively lower surface brightness that connect vations of Mulder and van Driel (1993), where keywith the starburst ring. (b). — Color morphology: morphological features are noted. (b.) — RadialThe (R − I) color image is coded so that dark fea- profile of gas surface densities. The H I gas densitiestures denote relatively red colors and bright features come from Mulder and van Driel (1993), and the H2denote bluer colors. The image shows reddened arcs are derived from the CO observations of Gerin et al.to the west and northeast along with especially blue (1991). Beyond a radius of 75′′ , the H2 surface densi- 17
  18. 18. ties have been extrapolated and most likely representupper limits (see text). This was done to provide a(probable) lower limit on the gravitational stabilityindex (Q). (c.) — Gravitational stability index (Q),as normalized by the gas velocity dispersion V(disp)(= σ in text). Beyond a radius of 75′′ , the extrapo-lated value of Q represents a (probable) lower limit.As shown, the starburst ring and inner disk appearto be situated in a shallow minimum of Q.Fig. 10.— Radial profile of orbital frequencies andcorresponding resonances. From the H I rotationcurve shown in Fig. 9a., the resulting orbital res-onance diagram shows key morphological features co-incident with important resonances. Here, the solid,short-dashed, and long-dashed lines respectively tracethe angular frequencies Ω, Ω±κ/4, and Ω±κ/2. A sin-gle pattern speed of 35 km s−1 kpc−1 (where d = 4.6Mpc and i = 40◦ ) would place the nuclear bar insidethe Inner Inner Lindblad Resonance (IILR) [whereΩp = Ω − κ/2], the starburst ring between the IILR& Outer Inner Lindblad Resonance (OILR), the bi-symmetric knots at the “ultra-harmonic” resonance(UHR) [where Ωp = Ω − κ/4], the oval disk termi-nating at the “co-rotation” resonance (CR) [whereΩp = Ω], and the outer pseudo-ring at the OuterLindblad Resonance (OLR) [where Ωp = Ω + κ/2]. 18

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