THE ASTROPHYSICAL JOURNAL, 523 : 575È584, 1999 October 1( 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A. VERY EXTENDED X-RAY AND Ha EMISSION IN M82 : IMPLICATIONS FOR THE SUPERWIND PHENOMENON MATTHEW D. LEHNERT1 Max-Plank-Institut fur extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany TIMOTHY M. HECKMAN Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 AND KIMBERLY A. WEAVER NASA Goddard Space Flight Center, Greenbelt, MD 20771 ; and Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 Received 1999 January 29 ; accepted 1999 May 11 ABSTRACT We discuss the properties and implications of the recently discovered 3@5 ] 0@9 (3.7 ] 0.9 kpc) region . . of spatially coincident X-ray and Ha emission about 11@ (11.6 kpc) to the north of the prototypical starburst/superwind galaxy M82. The total Ha Ñux from this ridge of emission is 1.5 ] 10~13 ergs s~1 cm ~2, or about 0.3% of the total M82 Ha Ñux. The implied Ha luminosity of this region is 2.4 ] 1038 ergs s~1. Di†use soft X-ray emission is seen over the same region by the ROSAT PSPC and HRI. The PSPC X-ray spectrum is Ðtted by thermal plasma absorbed by only the Galactic foreground column density (N 3.7 ] 1020 cm~2) and having a temperature of kT 0.80 ^ 0.17 keV. The total unab- sorbed ÑuxH from the ridge is 1.4 ] 10~13 ergs cm~2 s~1 (D2.2 ] 1038 ergs s~1), comprising about 0.7% of the total X-ray emission from M82. We evaluate the relationship of the X-ray/Ha ridge to the M82 superwind. The Ha emission could be excited by ionizing radiation from the starburst that propagates into the galactic halo along the cavity carved by the superwind. However, the main properties of the X-ray emission can all be explained as being due to shock heating driven as the superwind encounters a massive ionized cloud in the halo of M82 (possibly related to the tidal debris seen in H I in the inter- acting M81/M82/NGC 3077 system). This encounter drives a slow shock into the cloud, which contrib- utes to the excitation of the observed Ha emission. At the same time, a fast bow shock develops in the superwind just upstream of the cloud, and this produces the observed X-ray emission. This interpretation would imply that the superwind has an outÑow speed of roughly 800 km s~1, consistent with indirect estimates based on its general X-ray properties and the kinematics of the inner kiloparsec-scale region of Ha Ðlaments. The alternative, in which a much faster and more tenuous wind drives a fast radiative shock into a cloud that then produces both the X-ray and Ha emission, is ruled out by the long radiative cooling times and the relatively quiescent Ha kinematics in this region. We suggest that wind-cloud interactions may be an important mechanism for generating X-ray and optical line emission in the halos of starbursts. Such interactions can establish that the wind has propagated out to substantially greater radii than could otherwise be surmised. This has potentially interesting implications for the fate of the outÑowing metal-enriched material, and bears on the role of superwinds in the metal enrichment and heating of galactic halos and the intergalactic medium. In particular, the gas in the M82 ridge is roughly 2 orders of magnitude hotter than the minimum ““ escape temperature ÏÏ at this radius, so this gas will not be retained by M82. Subject headings : galaxies : active È galaxies : individual (M82) È galaxies : ISM È galaxies : nuclei È galaxies : starburst 1. INTRODUCTION Reuter et al. 1994 ; Moran & Lehnert 1997 ; Ptak et al. 1997 ; Strickland, Ponman, & Stevens 1997). It is perhaps impossi- The history of the study of M82 is a long and rich one. ble for any paper, let alone one this brief, to do justice toM82 is the prototypical starburst galaxy, and since it is one this rich history. We refer the reader to excellent overviewsof the nearest (D 3.63 Mpc ; Freedman et al. 1994) and on M82 in, for example, Bland & Tully 1988 and Telescobrightest members of that class, it has been studied at (1988).almost every wavelength ranging from c-rays through low- M82 is also the prototype of the ““ superwind ÏÏ phenome-frequency radio waves (e.g., Bhattacharya et al. 1994 ; non (Axon & Taylor 1978 ; McCarthy, Heckman, & vanBregman, Schulman, & Tomisaka 1995 ; Courvoisier et al. Breugel 1987 ; Bland & Tully 1988 ; Heckman, Armus, &1990 ; Lynds & Sandage 1963 ; Shopbell & Bland-Hawthorn Miley 1990 ; Bregman et al. 1995 ; Strickland et al. 1997 ;1998 ; Larkin et al. 1994 ; Petuchowski et al. 1994 ; Seaquist, Shopbell & Bland-Hawthorn 1998). These Ñows areKerton, & Bell 1994 ; Hughes, Gear, & Robson 1994 ; powered by the collective kinetic energy and momentum injected by stellar winds and supernovae in starburst gal- 1 Visiting observer at Kitt Peak National Observatory of the National axies. While recent studies have established the ubiquity ofOptical Astronomy Observatories, operated by AURA under contract superwinds in local starburst galaxies (e.g., Lehnert &with the National Science Foundation. Heckman 1996 ; Dahlem, Weaver, & Heckman 1998, here- 575
576 LEHNERT, HECKMAN, & WEAVER Vol. 523after DWH), there is still only a rough understanding of subsequently Ðtted with various models using the packageboth their dynamics and the processes responsible for the XSPEC.observed optical and X-ray emission (e.g., Strickland 1998 We have also inspected the archival ROSAT HRI dataand references therein). Without such understanding, it is for M82. These data have lower sensitivity than the PSPCdifficult to quantitatively assess the role that superwinds data, and provide no spectral information. However, theyhave played in the formation and evolution of galaxies do provide signiÐcantly higher spatial resolution (D6A(Kau†mann & Charlot 1998 ; Somerville, Primack, & Faber FWHM on-axis and D15A at the position of the X-ray1998) and in the chemical enrichment, heating, and possible ridge). The data we have used comprise a 53 ks exposuremagnetization of the intergalactic medium (Gibson, Loe- taken in 1995 April and May. We have used these data onlywenstein, & Mushotzky 1997 ; Ponman, Cannon, & to study the morphology of the X-ray ridge.Navarro 1999 ; Kronberg, Lesch, & Hopp 1999). In this paper we discuss the optical and X-ray properties 2.2. Optical Imagingof the spatially correlated region of X-ray and Ha line emis- Our optical images of M82 were taken on the night of UTsion recently reported by Devine & Bally (1999, hereafter 1995 February 27, using the KPNO 0.9 m telescope. WeDB) to be located roughly 11 kpc above the disk of M82, far observed with a Tek 20482 CCD and through the optics ofbeyond the region in which the superwind has been pre- 0.9 m, yielding a scale of 0A68 pixel~1 and a total Ðeld of .viously investigated. We are motivated by the important view of over 23@. The night was not photometric, with cirrusimplications for the superwind theory of understanding around all night and intermittent patches of thicker clouds.such a region. If this region could be shown to be physically M82 was observed through two Ðlters, a narrowband Ðlterrelated to the superwind that M82 is driving, then it implies with a FWHM of 29 A and a central wavelength of 6562 A, that winds driven even by energetically modest starbursts and a wider Ðlter with a FWHM of 85 A and a centralsuch as M82 are able to reach large distances into the halo, wavelength of 6658 A. The Ðrst Ðlter e†ectively isolated Hamaking them more likely to be able to escape the gravita- and the underlying continuum and did not include emissiontional potential of the galaxy and thereby supply mass, from either [N II] lines at 6548 and 6584 A. The second Ðlter metal, and energy to the intergalactic medium (IGM). Such provided a line-free measure of the continuum and was useda Ðnding would have implications for our understanding of for continuum subtraction of the Ðrst narrowband Ðlter tothe evolution of the IGM, the halos of galaxies, and quasar produce a Ha lineÈonly image of M82. The total integrationabsorption-line systems. Our study of this emission region time in each Ðlter was 1200 s.also provides some unique clues to the physics that under- The images were reduced in the usual way (bias-sub-lies the observed X-ray and optical emission from super- tracted and Ñat-Ðelded, and then the narrowband contin-winds, and strongly suggests that wind-cloud collisions in uum image was aligned and scaled to subtract thestarburst galaxy halos are an important emission process. continuum from the narrowband Ha image) using the reduction package IRAF.2 2. OBSERVATIONS AND DATA REDUCTION Since the data were taken under nonphotometric condi- tions, Ñuxing the data directly using observations of 2.1. X-Ray Data spectrophotometric standards is impossible. However, The ROSAT PSPC data were obtained during the period obtaining Ñuxes and hence luminosities is very important toof 1991 March through October and had a total integration our analysis. Therefore, to estimate the Ha Ñux and contin-time of about 25 ks. M82 was placed approximately in the uum Ñux density of the o†-band Ðlter of the continuum andcenter of the Ðeld for all the observations, and the source emission-line nebulae of M82, we bootstrapped from thesize is small enough so that there is no obscuration of the Ñux given in a 5A8 circular aperture centered on knot C .X-ray emission from M82 due to the mirror ribs. from OÏConnell & Mangano (1978). Knot C was chosen We extracted a spectrum from a region of di†use X-ray because it was easily identiÐable in our images and appearsemission located about 11@ north of M82, and coincident in a relatively uncomplicated region of line emission. Thiswith the region of Ha emission reported by DB. We will procedure yields a Ha Ñux from M82 of about 4.6 ] 10~11subsequently refer to this region as the ““ ridge ÏÏ of emission. ergs cm~2 s~1, which compares very well with the Ha ÑuxThis area of X-ray and optical emission is oriented roughly from M82 estimated by McCarthy et al. (1987) ofparallel to the major axis of M82, and was denoted as 4.5 ] 10~11 ergs cm~2 s~1 over a similar region 90@@ ] 90@@.sources 8 and 9 in DWH. We extracted source counts in an All subsequent results for the Ha Ñux and luminosity ofelliptical region with major and minor axes of 3@3 and 1@8, . . various features within the Ha image of M82 will rely onrespectively, oriented in a position angle of 43¡. This region this bootstrapping procedure.is just sufficient to encompass all of the obvious X-ray emis- To estimate the continuum Ñux density, we used the Ñuxsion, but excludes the soft point source at the east-northeast density of OÏConnell & Mangano (1978) taken at 6560 A. end of the ridge of emission that is discussed in ° 3.2. We Since the center of our narrowband continuum emission ishave also extracted source counts for the portion of the 6658 A, we are thus assuming that the continuum has aridge that is contaminated by this point source using a constant Ñux density between 6560 and 6658 A. To see ifcircular aperture with a diameter of 1@75. The background . this extrapolation procedure is accurate, we compared thefor these spectra was taken from a large region in which Ñux in R of M82 in a 35A aperture of 1.97 ] 10~13 ergsthere were no apparent background sources to the south cm~2 s~1 A~1 (from NED, based on data originally and west of M82. The relative distance of the backgroundregion from the center of the PSPC Ðeld was approximately 2 IRAF is distributed by the National Optical Astronomy Observa-the same as that for the ridge of emission. The spectrum was tories, which are operated by the Association of Universities for Researchthen grouped such that each channel contained at least 25 in Astronomy, Inc., under cooperative agreement with the Nationalcounts. The extracted background-subtracted data were Science Foundation.
No. 2, 1999 VERY EXTENDED EMISSION IN M82 577obtained by Johnson 1966). If we also take the total counts that the Ha line emission has a very complex morphology in the 6658 A Ðlter and determine the Ñux density by that is highly suggestive of outÑowing gas, with loops, ten-extrapolating the conversion factor from measuring knot C drils, and Ðlaments of line emission pointing out of theof OÏConnell & Mangano (1978), we Ðnd a Ñux density of plane of the galaxy.2.11 ] 10~13 ergs cm~2 s~1 A~1 in a 35A aperture. Our An especially interesting feature reported by DB is theextrapolated Ñux is within 10% (0.1 mag), and thus this faint ridge of emission about 11@ north of M82 at a positionextrapolation is reasonably accurate. angle of about [20¡ from the nucleus. This ridge is then about 11.6 kpc (in projection) north of M82 at the adopted 3. RESULTS distance of 3.63 Mpc (Freedman et al. 1994). The dimen- sions of the ridge down to surface brightnesses of about 3.1. Optical Imaging 3.5 ] 10~17 ergs s~1 cm ~2 arcsec~2 are about 3@5 long and . The Ha and narrowband continuum images of M82 are 0@9 wide, with the long axis being approximately parallel to .quite spectacular (Figs. 1 and 2). We Ðnd that the line emis- the major axis of M82. At the assumed distance of M82, thission is very extended, with discernible Ha line emission size translates into approximately 3.7 kpc ] 0.9 kpc, andextending to about 11 kpc above the plane of M82. This the total Ha Ñux from this ridge of emission is 1.5 ] 10~13very faint emission can be seen even more clearly in the ergs s~1 cm ~2. This Ñux is reasonably close to the estimatedeeper Ha image obtained by DB. Like previous authors in DB of 1.2 ] 10~13 ergs s~1 cm~2 and yields an Ha lumi-(e.g., Bland & Tully 1988 ; McCarthy et al. 1987), we Ðnd nosity of 2.4 ] 1038 ergs s~1. The total Ha Ñux for the 69 52 50 48 46 DECLINATION 44 42 40 38 36 09 57 00 56 30 00 55 30 00 54 30 RIGHT ASCENSION FIG. 1.ÈGray scale showing the Ha image and contours showing the ROSAT PSPC image of M82. The lowest surface brightness emission visible in theHa corresponds to a Ñux of 3.5 ] 10~17 ergs s~1 cm ~2 arcsec~2. The spatial coincidence of the Ha and X-ray emission is quite good. The images werealigned using the alignment determined for the continuum image. Coordinates are shown for J2000.
578 LEHNERT, HECKMAN, & WEAVER Vol. 523 FIG. 2.ÈGray scale showing a narrowband continuum image (central wavelength approximate 6658 A) and contours showing the smoothed (p 10@@ Gaussian) ROSAT PSPC image of M82. The images were aligned using what is likely to be a background active galactic nucleus and galaxy that were bothvisible in the optical continuum and X-ray images. Coordinates are shown for J2000.whole of M82 is approximately 4.6 ] 10~11 ergs s~1 cm ~2. This point source has an R magnitude of 12.2. We willThus, the ridge of emission constitutes approximately 0.3% discuss its likely nature in the next section.of the total Ha emission from M82 (not corrected for inter-nal extinction). While the ridge appears rather structureless 3.2. X-Ray Imaging and Spectroscopyin our Ha image, the deeper image published by DB shows The optical images described above were aligned with thea collection of bright knots and loops or Ðlaments con- X-ray images, starting with the coordinates given for thenected to and immersed in the more di†use and pervasive X-ray data from ROSAT and using the positions of starsemission. within the frame from the Guide Star Catalog and coordi- The narrowband line-free continuum image reveals a nates given in the header from the digital sky survey. Wecomplex morphology that is as interesting as the Ha image. then slightly adjusted the X-ray pointing by identifying andIn addition to the obvious complex dust absorption that is aligning X-ray sources on the digital sky survey image ofseen in the image (with some dark features oriented perpen- the region. The close spatial correspondence between thedicular to the plane of the galaxy), we also see Ðlaments of ridge of optical and X-ray emission about 11@ to the north ofcontinuum emission extending out of the plane of the M82 is obvious (Fig. 1).galaxy (although the Ðlaments are not entirely obvious in We do not display the ROSAT HRI image of the ridge,Fig. 2). These Ðlaments have twists and turns as seen pro- since these data show only that the ridge is largely di†usejected on the sky, but generally point perpendicular to the and structureless with the 15A (260 pc) o†-axis spatialplane of the galaxy. They extend outward almost 3@ to the resolution of the HRI. The only substructure is a pointnorthwest and almost 2@5 to the southeast, corresponding to . source at the easternmost end of the ridge. This sourcea projected distance out of the plane of about 3 kpc. Even appears to coincide with a stellar object visible in Figure 2though the continuum emission is in many ways as compli- (the relatively bright starlike object located about 40A east-cated as that seen in the Ha image, we do not observe any northeast of knot I in Fig. 2 of DB). This is most likely acontinuum emission from the ridge of spatially coincident foreground star, as we will brieÑy discuss below.HaÈX-ray emission. Using the counts-toÈÑux density con- Examination of the 0.25, 0.75, and 1.5 keV PSPC maps ofversion for the narrowband line-free continuum image and M82 presented by DWH) shows that the X-ray morphologythe same aperture as used to measure the Ha Ñux (but with of the ridge is energy dependent. The east-northeast portionthe point sources removed from the area), we Ðnd an upper of the ridge (source 9 in DWH) is relatively much morelimit for the possible continuum emission from the ridge of conspicuous in the 0.25 keV map compared to the rest offainter than 15.8 R mag. This then implies an absolute R the ridge (source 8 in DWH). This reÑects the contami-magnitude fainter than [12.0. nation of the easternmost ridge emission by the While we Ðnd no evidence for any di†use, spatially (presumably) unrelated point source visible in the HRIextended optical continuum associated with the Ha ridge, image.we do Ðnd a relatively bright point source in the line-free As described in ° 2.1 above, we have extracted a PSPCimage that is coincident with the X-ray point source seen at spectrum of the main body of the ridge from a region thatthe east-northeast end of the ridge in the ROSAT HRI data. excluded the point source. The relatively small number of
No. 2, 1999 VERY EXTENDED EMISSION IN M82 579detected photons precludes Ðtting complex multi- late-type dwarf star, and this would also be consistent withcomponent models, and we Ðt a model of a single thermal its soft ROSAT PSPC spectrum (e.g., Rosner, Golub, &(““ MEKAL ÏÏ) component to the spectrum. We are moti- Vaiana 1985).vated to use a thermal model by the detailed X-ray spec-troscopy of the brighter portions of the M82 outÑow 4. DISCUSSION(DWH ; Moran & Lehnert 1997 ; Strickland et al. 1997). The 4.1. Is the Ridge of Emission Associated with M82 ?Ðts strongly preferred a low-absorbing column, so wesimply froze the column at the Galactic value of N 3.7 Being able to isolate the ridge of emission to the north of H] 1020 cm~2. The data quality were not adequate to Ðt for M82 allows us to investigate its nature separately from thethe metal abundance, so we froze this at the solar value. rest of M82. Could this ridge of emission be due to anotherChanging the assumed metal abundance did not signiÐ- galaxy near M82, or is it instead gas excited by the M82cantly a†ect the best-Ðt temperature, but does directly a†ect starburst ?the normalization of the Ðt. The best-Ðt temperature was We have found that the ridge of X-ray and Ha emissionkT 0.76 ^ 0.13 keV (90% conÐdence interval with a has fairly high X-ray and Ha luminositiesÈboth lumi-reduced s2 B 1.3 ; Fig. 3). The relatively high temperature is nosities are in the range observed for star-forming dwarfsimilar to that measured in the ROSAT PSPC spectroscopy galaxies (e.g., Hunter, Hawley, & Gallagher 1993 ; Heckmanof the innermost region of the M82 outÑow (D0.7 keV ; et al. 1995). However, we see no evidence for any opticalDWH ; Strickland et al. 1997). The total unabsorbed Ñux in continuum emission from this ridge. The limit for the levelthe 0.1È2.4 keV band from this part of the ridge of emission of continuum emission implies that any putative galaxyis 8.5 ] 10~14 ergs cm~2 s~1. must be of very low luminosity, comparable to that of the For the east-northeast part of the ridge, the emission of local dwarf spheroidal galaxies (Mateo 1998). Moreover,which is contaminated by the point source described above, the limit on the continuum emission implies an Ha equiva-we have Ðtted a two-component MEKAL model (Fig. 3). lent width of greater than 180 A. Examining the sample ofBoth components were frozen at solar abundances ; one galaxies with M fainter than [15 in the Ha survey of Bcomponent (the ridge emission) had an absorbing column nearby dwarf irregular galaxies by Hunter et al. (1993), wefrozen at N 3.7 ] 1020 cm~2, and the other (the stellar Ðnd that these have typical global Ha equivalent widths of a Hpoint source) had an absorbing column frozen at 0. This Ðt few to a few tens of A. The only such galaxy with an Hayields kT 0.85 ^ 0.17 keV for the ridge and kT D 0.1 keV equivalent width comparable to our lower limit for the M82for the point source. The corresponding unabsorbed Ñuxes ridge is DDO 53. This galaxy has a ratio of H I mass to Haare 5.8 ] 10~14 ergs cm~2 s~1 and 3.5 ] 10~15 ergs cm~2 luminosity (in solar units) of 420. The corresponding H Is~1 for the ridge and point source, respectively. Combining mass for the M82 ridge would have to be 2.5 ] 107 M . _the Ðts for the two regions of the ridge, the total X-ray Ñux The H I map in Yun et al. (1993) implies a maximum H Iis then 1.4 ] 10~13 ergs cm~2 s~1. This comprises about mass in the M82 ridge that is only about 106 M . More- _0.7% of the total unabsorbed X-ray emission from M82 in over, while the M82 ridge has an X-ray luminosity similarthe PSPC band (e.g., Moran & Lehnert 1997 ; DWH). The to that of the dwarf starburst galaxy NGC 1569 (Heckmantotal X-ray luminosity of the ridge of emission is 2.2 ] 1038 et al. 1995), the M82 ridge would have a ratio of X-ray toergs s~1 (in the 0.1È2.4 keV band). optical continuum luminosity that would be over 40 times As stated above, the east-northeast point source is identi- higher than NGC 1569. Thus, it is clear that the ridge inÐed with a stellar object having an R magnitude of 12.2 M82 has properties that are very di†erent from those of(jF 1.6 ] 10~10 ergs cm~2 s~1). The X-ray/optical Ñux star-forming dwarf galaxies. This argument, together with j the strong morphological connection between the M82ratio suggests that the most likely nature of this source is a ridge and the M82 outÑow (see DB), leads us to conclude that this ridge is not a dwarf star-forming galaxy. Other possibilities for this region, such as a cloud in the halo of the Milky Way, are also highly unlikely. The H I detected in the data of Yun et al. (1993) and the Ha emission from the spectrum of DB have velocities such that they are both likely to be associated with M82 and not the Galaxy (Burton & Hartmann 1997 ; Ha†ner et al. 1998 ; Reynolds et al. 1995). Moreover, while Galactic high-velocity H I clouds often show soft X-ray excesses (Kerp et al. 1996, 1999), the associated X-ray emission is very soft (B0.1È0.2 keV ; see discussion in Kerp et al. 1996). 4.2. Photoionization of the Ridge by M82 The ridge is evidently related to, and excited by, M82. In the case of the Ha emission, two possibilities are obvious. Either the superwind in M82 has worked its way out to B12 kpc above the plane of the galaxy and is shocking halo material that appears in plentiful supply around M82 (Yun FIG. 3.ÈROSAT PSPC spectrum of the di†use X-ray emission from et al. 1993, 1994), or Lyman continuum radiation from thethe ridge to the right of the point source (circles) and superimposed on thepoint source (crosses). Data below 0.6 keV are ignored in the latter case to starburst is ionizing gas at very large distances from theeliminate the point source. Also shown are the best-Ðt MEKAL plasma disk. Only the Ðrst possibility could apply to the X-raymodels (see text) folded through the instrumental response. emission from the ridge.
580 LEHNERT, HECKMAN, & WEAVER Vol. 523 Let us suppose that ambient gas is being photoionized by luminosity in the ROSAT bandpass of 0.1È2.4 keV impliesthe stellar population of M82. The rate of ionizing photons that the X-rayÈemitting gas has an electron density offrom M82 is approximately 1054 s~1 (McLeod et al. 1993 n 5.4 ] 10~3f ~1@2 cm~3, a pressure P B 2n kT e,X X X e,Xand references therein). Using the projected size and dis- 1.3 ] 10~11f ~1@2 dynes cm~2, a cooling time t Btance from M82 estimated for the ridge of emission given in 3kT /n " X2.7 ] 108f 1@2 yr, mass M 2.0 ] 106f 1@2cool e,X X X Xthe previous section, and assuming that the ridge is really a M , and a total thermal energy of 8 ] 1054f 1@2 ergs. Simi- _ X““ box ÏÏ with a depth equal to its projected length (i.e., 3.7 larly, assuming that conditions of case B recombination andkpc ] 3.7 kpc), we Ðnd that the ridge will intercept about a temperature of 104 K holds in the region producing the8 ] 1051 ionizing photons s~1. This of course is an upper Ha emission, and that this region also occupies a volume oflimit, since some (most ?) of the ionizing photons will be 3.7 ] 1065f cm~3, then the implied density and mass ofabsorbed or dust-scattered before they reach distances as Ha this gas are n 4.3 ] 10~2f ~1@2 cm~3 and M 8.0 e,Ha Ha Hagreat as 11.6 kpc out of the plane of M82. We can estimate ] 106f 1@2 M , respectively. If we make the physically rea- Ha _the rate of photons necessary to give the Ha luminosity of sonable assumption that this cooler gas is in rough pressurethe ridge (2.4 ] 1038 ergs s~1) by assuming that the gas is in balance with the hot X-ray gas, then the minimum pressurephotoionization equilibrium, i.e., the number of recombi- of 1.3 ] 10~11 dyne cm~2 would require that the Ha-nations equals the number of absorptions. Using the emitting gas is highly clumped ( f D 8 ] 10~5). This in Hanumbers for the case B e†ective recombination rate of Ha turn implies that n 5 cm~3 and M 1.1 ] 105 M .from Osterbrock (1989), we Ðnd that it is necessary to have e,Ha Ha _ We note that a small volume-Ðlling factor for this gas isa rate of D 2 ] 1050 ionizing photons s~1. This is D40 consistent with the complex knotty, Ðlamentary structuretimes smaller than the upper limit for the number of seen in the deep Ha image in DB.photons that M82 is able to provide, so only about 2% to The total bolometric luminosity and ionizing luminosity3% of the ionizing radiation initially emitted in the direc- of M82 (McLeod et al. 1993 and references therein) can betion of the ridge needs to make it to this distance. Such an used together with the starburst models of Leitherer &““ escape fraction ÏÏ is consistent with Hopkins Ultraviolet Heckman (1995) to predict the rate at which the starburstTelescope (HUT) spectra of starbursts below the Lyman injects mechanical energy and momentum. While these pre-edge (Leitherer et al. 1995 ; Hurwitz, Jelinsky, & Dixon dictions will depend to some degree on the evolutionary1997), and it seems plausible that the optimal path along state of the starburst (cf. Figs. 66È69 in Leitherer &which ionizing radiation could escape from a starburst is Heckman 1995), we estimate that the mechanical luminosityalong the cavity carved out by the superwind. and momentum Ñux provided by M82 are D2.5 ] 1042 ergs DB have made similar arguments in favor of a starburst s~1 and D2 ] 1034 dyne, respectively. For this windphotoionization origin for the Ha emission. However, mechanical luminosity, the rate at which the X-ray ridgephotoionization by stars does not produce the hot X-ray intercepts the energy is 2.5 ] 1041)~1 ergs s~1, where ) isemission that is coincident with the Ha emission. Instead, w w the total solid angle into which the wind Ñows () ¹ 4n).shock heating by the superwind may be an efficient mecha- w of the Comparing this heating rate to the X-ray luminositynism for producing coincident X-ray and line-emitting ridge (2.2 ] 1038 ergs s~1) shows that the wind could easilyplasma. We now explore the energetic plausibility of this power the emission. It could also supply the ridgeÏs esti-model, and revisit the issue of the role of photoionization in mated thermal energy in a timescale of 106 f 1@2) yr (much° 4.5. X w less than the wind/starburst lifetime). Matching the observed high pressure in the X-ray ridge is 4.3. Shock Energetics and the Basic Physical Properties of more challenging. A wind momentum Ñux of 2.5 ] 1034 the Ridge dyne leads to a wind ram pressure at a radius of 11.6 kpc of The ridge of emission has an X-ray luminosity of 2 ] 10~11 )~1 dyne cm~2. Consistency with the X-ray2.2 ] 1038 ergs s~1 and a temperature of 9 ] 106 K. w pressure estimated above would require that the X-ray gasAssuming that the shock is strong and adiabatic, we have has a large volume-Ðlling factor ( f D unity) and that theT (3/16)[(kv2)/k], where k is the mass per particle, X M82 wind is rather well collimated () D 1.5 sr). This latter s wv postshock shock velocity, and k is BoltzmannÏs constant. is the constraint is consistent with the morphology of the outÑow sThus, to produce gas with T 9 ] 106 K requires a shock (e.g., Goetz et al. 1990 ; McKeith et al. 1995 ; Shopbell &speed of 820 km s~1. Such a high shock speed is quite Bland-Hawthorn 1998). Alternatively, we can use thereasonable considering that M82 has been estimated to empirical measurements of the gas pressure in the innerhave a deprojected outÑow velocity in optical line emitting region of the M82 nebula (Heckman et al. 1990) andÐlaments observed along the minor axis of roughly 660 km extrapolate these outward, assuming that the wind rams~1 (Shopbell & Bland-Hawthorn 1998), and the outÑowing pressure falls as r~2. The pressure at a radius of 1 kpc wasX-ray gas could in principle have a velocity as high as 3000 measured to be D5 ] 10~10 dyne cm~2, and so the predict-km s~1 (Chevalier & Clegg 1985). It would take material ed wind ram pressure at a radius of 11.6 kpc would betraveling at 820 km s~1, B1.4 ] 107 yr to travel the 11.6 D4 ] 10~12 dyne cm~2, about a factor of 3 smaller thankpc from M82 to the ridge of emission. Since this is approx- the estimated value (for f 1). In view of the uncertaintiesimately the estimated lifetime of the starburst in M82 (Rieke in the estimated physical X parameters for the X-ray ridge, weet al. 1993 ; Bernloehr 1992 ; Schreiber 1998 ; Satyapal et al. regard this level of disagreement as acceptable.1997), it is very plausible that the starburst could drive a If the optical line emission is also shock-excited by theshock over its lifetime this far out of the galactic plane. outÑowing wind, then the luminosity observed at Ha should If we assume that we are observing a ““ box ÏÏ of emission be consistent with the wind energetics. We can estimate thewith dimensions of 3.7 ] 3.7 ] 0.9 kpc, the total volume is amount of emission from a shock as follows. The total3.7 ] 1065 cm~3. Assuming that this region is occupied by energy dissipated in a shock is L 1 oAv3 1.0 ] 1034hot gas Ðlling a fraction f of this volume, the measured shock 2 s n A v3 ergs s~1, where A is the shock surface area in X 0 kpc s kpc
No. 2, 1999 VERY EXTENDED EMISSION IN M82 581kpc2, n is the preshock density (cm~3), and v is in units of ] 10~3 cm~3. Thus, for example, if the shock speed in the 0 skm s~1. From Binette, Dopita, & Tuohy (1985), using the cloud is v 100 km s~1, then n D 0.07 cm~3. Weequation above, we have L (Ha) B 3L (Hb) s,cloud cloud B note further that the velocity of the bow shock is just the shock shock3L /100 1.7 ] 1041 n A (v /820)3 ergs s~1, if 90 di†erence between the outÑow velocity of the superwind shock 0 kpc sv 1000 km s~1. Adopting the geometry above, the total itself and that of the shocked cloud. Assuming that latter ssurface area that the ridge would present to the wind would velocity is much smaller than the former (as suggested bybe 3.7 kpc ] 3.7 kpc. The observed Ha luminosity of the the kinematic information provided by DB), the wind veloc-ridge (2.4 ] 1038 ergs s~1) then requires that n (v /820)3 ity implied by the X-ray temperature is D820 km s~1. To s1.0 ] 10~4, while the pressure estimated in the0X-ray ridge drive a wind with a terminal velocity of 820 km s~1, hot gas(1.3 ] 10~11 dynes cm~2) requires n (v /820)2 8 ] 10~4. would need to be injected into the starburst at a mean 0 sThus, provided that the shock speed is greater than 100 km mass-weighted temperature of 1 ] 107 K (Chevalier &s~2, it can both produce the observed Ha emission-line Clegg 1985). This temperature is consistent with X-ray spec-luminosity and be consistent with the pressure estimated for troscopy of the hot gas in the core of M82 (DWH ; Moran &the hot shocked gas observed in the X-rays. Lehnert 1997 ; Strickland et al. 1997). We have seen above that the radiative cooling time in the 4.4. T he Relationship Between the Ha and X-Ray Emission X-ray ridge is D300 times longer than the characteristic What is the physical relation between the Ha and X-ray dynamical time for the ridge. This implies that the windemission in this type of model, in which the superwind bow shock is adiabatic. This can also be seen by comparingdrives a shock into an ambient cloud in the halo of M82 ? the total rate of energy dissipation in the shock (L 1o shockOne possibility is that the shock driven into the cloud by A v3 7 ] 1040 ergs s~1 for the parametersthe wind is fast enough (i.e., D800 km s~1) to produce the 2 wind ridge bow estimated above) to the observed X-ray luminosity of theX-ray emission, and that the shocked gas then cools radi- ridge (2.2 ] 1038 ergs s~1). Thus, the thickness of the X-rayatively down to temperatures of the order of 104 K and ridge in the direction of the outÑow is set by the dimensionsproduces the observed Ha emission via recombination. The of the region in which adiabatic expansion and coolingdifficulty with this simple model is that the estimated radi- causes the X-ray surface brightness to drop precipitously.ative cooling time in the X-ray gas (D3 ] 108 yr) is very This region will be of the order the size of the obstacle in thelong compared to the characteristic dynamical time for the Ñow that has caused the bow shock to develop (the cloud).X-ray ridge, t D (0.9 kpc/820 km s~1) D 1 ] 106 yr. This is consistent with the observed X-ray morphology and dynSimply put, this model would require the X-ray ridge to be the good spatial correlation seen between the Ha and X-rayD300 kpc thick, and the Ha emission would be at the back emission. We have also estimated previously that the super-edge of this very thick cooling zone. An additional per- wind can supply the ridge with its estimated thermal energysuasive argument against this model is provided by the content in a timescale of the order of 106 yr. This is roughlyrather quiescent kinematics of the Ha-emitting material equal to the dynamical time in the ridge (the timescale forreported by DB. Their optical spectra show that the Ha adiabatic cooling), as required.emission in the ridge spans a velocity range of only about We conclude that it is energetically feasible for the cloud102 km s~1, which is inconsistent with material cooling to be shock heated by a superwind driven by the starburstbehind a shock with a velocity nearly an order of magnitude in M82. A wind/cloud shock interpretation leads to nolarger. While the relatively small di†erence DB report uncomfortable estimates of the luminosity, covering frac-between the mean velocity of the ridge and M82 proper tion, or timescales. In addition, as noted previously, there is(blueshift of 50È200 km s~1) could perhaps be explained as ample evidence that M82 lies in an ““ H I halo ÏÏ (Yun et al.a projection e†ect in an outÑow seen nearly in the plane of 1993, 1994). It is likely that the outÑowing gas has inter-the sky, this e†ect could not explain the narrowness of the cepted and shock heated some of this halo material, whichHa emission line in the ridge (D102 km s~1) without is observed to be clumpy over large scales (see Yun et al.appealing to an unreasonably idealized shock geometry 1993). We also note that perhaps one might have expected(which is bellied by the morphological complexity of the Ha to Ðnd stronger evidence a priori for such interactions onemission in the image published by DB). the north side of the plane of M82 compared to the south A much more plausible alternative is that we are actually side ; Yun et al. (1994) Ðnd that the velocity gradient in theobserving two physically related shocks in the M82 ridge. H I is much higher on the northern side (12 km s~1 kpc~1)We propose that as the superwind encounters a large cloud of the plane of M82 compared to that on the southern sidein the halo of M82, it drives a relatively slow (of order 102 (3È6 km s~1 kpc~1). This higher gradient may be related tokm s~1) radiative shock into the cloud. At the same time, our observation of cospatial Ha and thermal X-ray emis-this encounter also results in a fast (820 km s~1) stand-o† sion at large distances from the plane of M82.bow shock in the superwind Ñuid upstream from the cloud.The slow cloud shock (aided and abetted by ionizing radi- 4.5. W here is the Cloud ?ation from M82) produces the Ha emission. This is consis- We have shown that a simple model of the interactiontent with the quiescent Ha kinematics. The fast bow shock between a fast (D800 km s~1) wind and a massive cloud canin the wind produces the X-ray emission and is predicted to quantitatively explain the physical and dynamical proper-only be o†set from front edge of the cloud by a fraction of ties of the ridge. However, it a fair question to ask why thethe size of the cloud (Klein, McKee, & Colella 1994 ; putative cloud is not observed in the sensitive H I mapBedogni & Woodward 1990). Pressure balance between the published by Yun et al. (1993). More speciÐcally, we havetwo shocks requires that o v2 o v2 , where estimated above that the high pressures in the X-ray gas wind bow cloud shock,cloudo and o are the preshock densities in the wind and and the low velocity of the shock driven into the cloud by wind cloudcloud, respectively. The observed X-ray pressure and tem- the wind together mean that the preshock density of theperature imply that v D 820 km s~1 and n D1 cloud must be of the order of 10~1 cm~3. For a cloud with a bow wind,ion
582 LEHNERT, HECKMAN, & WEAVER Vol. 523line-of-sight dimension of 3.7 kpc, the implied cloud column obviously has important implications for our understand-density is then D1021 cm~2. The H I map in Yun et al. ing of superwinds. It emphasizes that one must be careful in(1993) constrains the H I column density of the ridge to be using the observed spatial scales of the extended emission in2.7 ] 1019 cm~2. This implies that the cloud must now starbursts to either access whether or not the wind materialbe fully ionized. is able to escape the potential of the host galaxy or to This could be accomplished by either the wind-driven estimate the dynamical age of the outÑow.shock or the ionizing radiation from M82. The time for the It is interesting in this vein to consider the long-term fateshock to traverse the cloud will be D107 yr for v of the hot gas in the X-ray ridge of M82. To do so, we will shock,cloud100 km s~1, comparable to the estimated age of the star- compare the observed temperature of the gas to the ““ escapeburst and its superwind. Similarly, the velocity of a photo- temperature ÏÏ at that location in the M82 halo. Followingionization front moving though the cloud is just v B Wang (1995), the escape temperature for hot gas in a galaxy i n~1 (e.g., Osterbrock 1989), where is the incident potential with an escape velocity v is given by T ^ 1.1 LyC cloud LyC es esÑux of ionizing radiation. The observed Ha luminosity and ] 105 (v /100 km s~1)2 K. The combined CO ] H I rota-the assumed cross sectional area that the cloud presents to es tion curve of M82 shows a roughly Keplerian decrease withthe starburst (3.7 ] 3.7 kpc) imply that the average value of radius for radii greater than 0.4 kpc, and v has dropped to is 1.5 ] 106f ~1 s~1 cm~2, where f is the fraction of rot 60 km s~1 at a radius of 3.5 kpc (Sofue 1998). If this LyC abs absthe incident ionizing photons absorbed by the cloud (i.e., Keplerian fallo† continues to larger radii, the impliedallowing the cloud to be optically thin). For a preshock escape velocity at a radius of 11.6 kpc is only 45 km s~1,density of 0.07 cm~3 in the cloud, the implied velocity is and the corresponding escape temperature is T ^ 2 ] 104v B 210f ~1 km s~1, and the time for the front to cross the K. This is 450 times cooler than the observed es X-ray emis- i abscloud is about 4 ] 106f ~1 yr. Thus, it is plausible that an sion. Even if we assume instead that M82 has an isothermal absinitially neutral cloud such as those seen in the H I maps of dark matter halo with a depth corresponding to a circularYun et al. (1993, 1994) has indeed been fully ionized in the velocity of 60 km s~1 (the maximum permitted by thetime since the starburst and its superwind turned on. directly measured rotation curve) and further assume that Our rough estimates suggest that the timescale for the this halo extends to a radius of 100 kpc, the escape velocitycloud to be fully photoionized is likely to be somewhat at r 11.6 kpc is about 150 km s~1 and T 2.5 ] 105 K.shorter than the timescale for it to be fully shock ionized. If es This is still a factor of 36 below the observed temperature ofcorrect, this would mean that the Ha emission produced by the gas in the ridge. This implies that the observed hot gasthe gas cooling and recombining behind the shock would (and by inference, the superwind itself) will be able to escapehave a high density and pressure, while the rest of the the gravitational potential of M82.ionized cloud (the as-yet unshocked material) would be at a The X-ray/Ha ridge has particularly interesting implica-much lower pressure and density. We speculate that the tions for the physics of superwind emission. While the exis-bright knots and loops of Ha visible in the image in DB may tence of superwinds is now well established, the process(es)correspond to shock-excited emission from gas with a high by which they produce X-ray emission is not clear. Thispressure, while the fainter and more di†use emission may topic has been nicely reviewed by Strickland (1998). If thecome from the low-pressure starburst-photoionized gas that outÑowing wind consists purely of the thermalized ejectahas not yet been shocked. from supernovae and stellar winds, then the wind Ñuid is hot (D108 K prior to adiabatic cooling) and is so tenuous 4.6. Implications for the ““ Superwind ÏÏ Phenomenon that it will be an insigniÐcant source of X-ray emission It is often difficult to associate discrete X-ray sources seen (Chevalier & Clegg 1985 ; Suchkov et al. 1994). Variousin the halos of starburst galaxies with activity in the star- alternatives have been proposed to boost the X-ray lumi-burst galaxy itself. Often, it is assumed that most discrete nosity produced by a superwind : (1) Substantial quantitiesobjects observed in the halos of starbursts are more distant of material could be mixed into the wind in or near theobjects (usually QSOs) seen in projection. Finding a spatial starburst as the stellar ejecta interact with the starburstassociation between Ha emission at the redshift of the star- ISM (the wind could be centrally ““ mass loaded ; ÏÏ Suchkovburst galaxy and X-ray emission is an irrefutable way of et al. 1996 ; Hartquist, Dyson, & Williams 1997). (2) Largebeing able to associate the X-ray emission directly with quantities of gas could be added in situ as the wind over-activity within the starburst galaxy. takes and then evaporates or shreds gas clouds in the galac- Having argued that this cospatial region of soft X-ray tic halo (e.g., Suchkov et al. 1994). These clouds could eitherand Ha emission must be due to the outÑowing superwind, be preexisting clouds in the halo or material from the diskit is now evident that at least some starburst galaxies are of the starburst galaxy that has been carried into the haloable to drive material to much larger distances from their by the wind. (3) The tenuous wind Ñuid could drive a shocknuclei than would be inferred using the relatively high into a denser volume-Ðlling galactic halo, with the observedsurface brightness X-ray or line emission that is generally X-ray emission arising from the shocked-halo materialobserved along their minor axes (see, e.g., Lehnert & rather than the wind Ñuid itself.Heckman 1996 ; Heckman et al. 1990 ; DWH). In M82, for In the case of the ridge of emission in the halo of M82, weexample, the inner ““ continuous ÏÏ region of bright Ha and have argued that the observable X-ray emission arises asX-ray emission extends only about 5@ from the nucleus the wind Ñuid, which, due to adiabatic expansion andalong the minor axis. The detached ““ ridge ÏÏ of emission that cooling, would otherwise have an X-ray surface brightnesswe have discussed is at a projected angular separation from below the level detectable in the existing ROSAT image,the nucleus of over twice this distance (about 11@). Thus, encounters a few-kiloparsec sized cloud in the halo. Wewithout making the association between the Ha and X-ray have argued that the observed X-ray emission is producedemission, a variety of interpretations for the source of the in the bow shock created in the superwind just upstream ofX-ray emission from this region would be possible. This the cloud. This process might be particularly relevant to the
No. 2, 1999 VERY EXTENDED EMISSION IN M82 583case of M82, which is immersed in an extensive system of context. The speciÐc model we favor is one in which thetidally liberated gas clouds, shared with its interaction part- encounter between the superwind and the cloud drives aners M81 and NGC 3077. Such material would also provide slow radiative shock into the cloud (producing some of thea potential source of optical line emission either by inter- Ha emission) and also leads to an adiabatic X-rayÈemittingcepting a small portion of the ionizing radiation produced bow shock in the superwind Ñuid upstream from the cloud.by the central starburst or as it is shock heated by the The cloud is now fully ionized, and so is not detected in thesuperwind. Given the strong connection between galaxy H I maps of Yun et al. (1993, 1994). The wind-cloud inter-interactions/mergers and the starburst phenomenon (e.g., action process may be especially relevant to M82, the haloSanders & Mirabel 1996 and references therein), this of which contains tidally liberated gas clouds. However, itmechanism may be generally applicable to superwinds. may well occur generally in powerful starbursts (which are usually triggered by galaxy interactions or mergers). 5. SUMMARY AND CONCLUSIONS The existence of a region of spatially coincident X-ray We report an analysis of a newly discovered (Devine & and Ha emission with strong evidence for wind/cloud inter-Bally 1999) region of spatially coincident X-ray and Ha action is important for several reasons. First, our analysisemission to the north of the prototypical starburst/ sheds new light on the physical processes by which galacticsuperwind galaxy M82. At the distance of M82 (3.63 Mpc) winds produce X-ray emission and implies that wind/cloudthe dimensions of this correlated ridge of emission are collisions are an important emission-producing process.3.7 ] 0.9 kpc, and it lies at a projected distance of about The emission ridge in M82 highlights the potential pitfalls11.6 kpc above the plane directly along the minor axis of in using the observed X-ray or Ha emission to trace the sizeM82 (and hence along the axis of the superwind). We and infer the ages or fates of galactic winds ; the winds maymeasure a total Ha Ñux from the ridge of 1.5 ] 10~13 ergs extend to radii far larger than the region over which theys~1 cm ~2. This Ñux yields a total Ha luminosity of have densities and temperatures high enough to produce2.4 ] 1038 ergs s~1. The total Ha Ñux for the whole of M82 observable emission. Wind-cloud encounters then o†er usis approximately 4.6 ] 10~11 ergs s~1 cm ~2. Thus, the Ha the possibility of ““ lighting up ÏÏ the wind at radii that areridge constitutes approximately 0.3% of the total observed otherwise inaccessible. The X-ray ridge in M82 implies thatHa Ñux from the galaxy. X-ray emission is seen over the the wind is able to reach large heights above the plane of thesame region as the Ha emission by the ROSAT PSPC and galaxy, thereby increasing the likelihood that it will be ableHRI. With the 15A (260 pc) o†-axis spatial resolution of the to eventually escape the galaxy potential well. Indeed, theHRI, the ridge is di†use and featureless except for a observed gas temperature exceeds the local ““ escape(probably unrelated) soft point source at its east-northeast temperature ÏÏ from the M82 gravitational potential well byend. The best Ðt to the X-ray emission from the ridge is a about 2 orders of magnitude. This result strengthens thesingle thermal component with a temperature of argument that starburst-driven galactic winds are impor-kT 0.80 ^ 0.17 keV, absorbed by a column density N tant sources of the metal enrichment and heating of the H3.7 ] 1020 cm~2. The absorption is consistent with the fore- intergalactic medium.ground Galactic column. The total unabsorbed Ñux fromthe ridge of emission is 1.4 ] 10~13 ergs cm~2 s~1, and its The authors wish to thank Kitt Peak National Observa-luminosity of 2.2 ] 1038 ergs s~1 comprises about 0.7% of tory for their generous allocation of observing time andthe total X-ray emission from M82 in the 0.lÈ2.4 keV their sta† for making sure that the observations wereROSAT band. carried out efficiently and e†ectively. We thank Ed Moran, We Ðnd that the observed Ha emission from the ridge David Strickland, and Crystal Martin for illuminating dis-could be produced via photoionization by the dilute radi- cussions about the physics of superwinds, and Davidation from the starburst propagating into the M82 halo Devine for communicating his discovery paper in advancealong the path cleared by the superwind. On the other of its publication. We would like to express our sinceresthand, the X-ray emission from the ridge can be most natu- thank you to the referee for carefully reading the manu-rally explained as the result of shock heating associated script and for providing us with detailed comments thatwith an encounter between the starburst-driven galactic have substantially improved this presentation. This work““ superwind ÏÏ and a large photoionized cloud in the halo of was supported in part by NASA LTSA grants NAGW-3138M82. Shock heating may also contribute signiÐcantly to the to T. H. and NAGW-4025 to K. 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