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THE ASTROPHYSICAL JOURNAL, 465 : L5–L8, 1996 July 1᭧ 1996. The American Astronomical Society. All rights reserved. Printed...
L6                                                                             BAUM ET AL.                                ...
No. 1, 1996                                     OBSCURATION RINGS IN HERCULES A                                           ...
L8                                                             BAUM ET Koff, S., Baum, S. A., Biretta, J., Golombek,...
PLATE L2                            FIG. 1.—Gray-scale image of the planetary camera F702W image of Her ABAUM et al. (see ...
PLATE L3  FIG. 2.—Idealized sketch of the interlocking “rings” superposed on the gray-scale image of Her A. Arrows indicat...
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  1. 1. THE ASTROPHYSICAL JOURNAL, 465 : L5–L8, 1996 July 1᭧ 1996. The American Astronomical Society. All rights reserved. Printed in U.S.A.HUBBLE SPACE TELESCOPE OBSERVATIONS OF OBSCURATION RINGS IN HERCULES A: IMPLICATIONS FOR ENERGY TRANSPORT IN POWERFUL RADIO GALAXIES STEFI A. BAUM, CHRISTOPHER P. O’DEA, SIGRID DE KOFF,1 WILLIAM SPARKS, JEFFREY J. E. HAYES, MARIO LIVIO, AND DANIEL GOLOMBEK Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Received 1996 March 4; accepted 1996 April 12 ABSTRACT We have used the Hubble Space Telescope to obtain snapshot images of Hercules A, the host galaxy to the powerful radio source 3C 348, through a broadband red filter. We report the discovery of interlocking, kiloparsec-scale rings of obscuration aligned near the radio axis and slightly offset from the galaxy’s nucleus. We discuss possible models for these rings and their implications for models of energy transport in extragalactic radio jets. Subject headings: dust, extinction — galaxies: active — galaxies: individual (Hercules A) — galaxies: interactions — galaxies: ISM — galaxies: jets 1. INTRODUCTION the host galaxy of 3C 348 (Sadun & Hayes 1993; Hayes et al. 1996) contrasts sharply with the low surface brightness of the The astrophysics of the launching, collimation, and propa- galaxy itself as seen by HST. Two faint, dark, interlocking ringsgation of jets is one of the outstanding problems in the study with centers offset 11"5 from the nucleus of Her A areof powerful radio-loud active galactic nuclei. In this Letter, we apparent at low S/N in both of these images. The measuredreport the discovery with the Hubble Space Telescope (HST) of geometrical properties of the rings are summarized in Table 1,a phenomenon that has the potential to shed new light on the and an idealized sketch based on these properties is presentedtransport of energy in powerful radio jets. We present HST in Figure 2 (Plate L3), where we also indicate the location ofWide Field Planetary Camera 2 (WFPC2) observations, taken the nucleus of 3C 348, the orientation of the radio source’sthrough the F702W broadband red filter, of Hercules A, the axis, and the companion galaxy of the powerful radio source 3C 348. These images The two rings are slightly elliptical and exhibit differentshow two laterally unresolved, interlocking rings of obscura- morphologies. The eastern, smaller, ring is oriented with itstion slightly offset from the nucleus, roughly along the radio center and its long axis directly along the radio jet’s axis andjet’s axis. We discuss possible origins of these rings and their has a characteristic width of 1"5 (3 kpc). It appears to beimplications for the nature of the central engine and radio jets centered on a small but resolved optically emitting feature thatin Her A. We adopt H0 ϭ 75 and q0 ϭ 0.5, which yields a scale is elongated along the radio axis. The nature of this feature isof 12 kpc arcsec Ϫ1 at the redshift of Her A ( z ϭ 0.154). unknown—it could be optical synchrotron emission from a 2. OBSERVATIONS AND REDUCTION knot in the jet, a region of star formation, or an emission-line region. By contrast, the western, larger, ring is oriented with its We obtained two 300 s exposures of Her A, with the target center 130Њ from the radio source’s axis and its major axiscentered in the planetary camera of WFPC2, using the broad- virtually perpendicular to the radio axis. This ring has aband red F702W filter. These observations were obtained characteristic width of 2"25 (5.5 kpc).during the course of the HST 3CR Snapshot Survey (de Koff The dark rings appear at low S/N in our snapshot images,et al. 1994). In de Koff et al. (1996), the snapshot data are and it is clear that longer integration, multicolor images arepresented for the sources in the 3CR with redshifts warranted to confirm their existence and to further study their0.5 Ͼ z Ͼ 0.1, including 3C 348, and we refer the reader to nature. Nevertheless we believe the features are likely to bethat paper for a more detailed description of the observations real and not an artifact of the observations or our eyes. Weand data reduction. have, to date, examined similar WFPC2 images of over 200 The two 300 s frames were combined to reject cosmic rays. 3CR galaxies, and in this source alone have we identified suchTwo dark, interlocking rings are visible in the single images as rings. Her A has long been known to be one of only twowell as the combined, cosmic-ray–rejected image, but in all powerful radio sources that show closed loops or bubbles ofcases they are seen at very low signal-to-noise ratio (S/N) synchrotron-emitting plasma in its large-scale radio structure(near the limit of our ability to detect them). We found that (Dreher & Feigelson 1984; van Breugel & Fomalont 1984).smoothing the images by a 0"08 Gaussian produced the best Thus, Her A has now been shown to be blowing both darkvisual representation of the rings. optical rings and radio bubbles. 3. RESULTS 4. DISCUSSION In Figure 1 (Plate L2), we show a gray-scale representationof our HST images. The bright, high surface brightness, The rings appear as dark regions in which the underlyingelliptical companion located 14Љ northwest of the nucleus of stellar light from the host galaxy is not seen. Thus they appear to be rings or shells of absorbing material. Below, we first 1 Leiden University. discuss several possible mechanisms for the absorption and L5
  2. 2. L6 BAUM ET AL. Vol. 465 TABLE 1 PROPERTIES OF OBSCURATION RINGS a Major Axis Minor Axis P.A. P.A. to Nucleus Ring (kpc) (kpc) (deg) (deg) East . . . . . . 2.8 2.3 90 90 West. . . . . . 4.8 4.2 0 120 a Estimated from the HST images.then discuss possible origins for the rings and their implica-tions for models of jets in extragalactic radio sources. 4.1. Origin of the Obscuration We consider two potential mechanisms for the absorption ofthe underlying stellar light, (1) dust and (2) Thomson scatter-ing by electrons. The contrast ratio between the obscured ring and thesurrounding stellar light is roughly 1Ϻ4. If the absorption isexponential, then ␶ ϭ Ϫln 1 Ϫ ͩ ⌬S S ͪ , (1) FIG. 3.—Plot of the ring/bubble diameter (perpendicular to the jet’s axis) aswhere S is the background flux and ⌬S is the depth of the a function of distance from the nucleus.absorption. This yields an optical depth of ␶ 1 1.4. The changein magnitude of the background light is ⌬m ϭ Ϫ2.5 ϫ log(1 Ϫ ⌬S/S) Ϫ1 2 1.5 mag. At the wavelength of the F702W 4.2. Origin of the Rings and Their Associationfilter, A(F702W) 3 2E(B Ϫ V ). This implies E(B Ϫ V ) 2 with the Radio Source0.75. If the dust-to-gas ratio is approximately the Galactic We consider several possible origins for the optical obscu-value, then, from Burstein & Heiles (1978), ration rings. First, they may, of course, bear no relation to the N͑H͒ 2 5.0 ϫ 1021 ͓E͑B Ϫ V ͒ ϩ 0.06͔ cm Ϫ2 , (2) radio jets or the nuclear activity but represent, for example, Ϫ2 remnants from a merger or resonant structures in the hostwhich yields N(H) 2 4.1 ϫ 10 cm . 21 galaxy. It is clear that the optical obscuring rings are not in an While, in principle, we should be able to determine whether equilibrium configuration, however; they must be transient orthe obscuring material is distributed in a bubble or a true ring evolving features. Since 3C 348 is a radio-loud, active sourceby looking for obscuration that is internal to the ring, the S/N with clear radio jets and lobes (a phenomenon present in onlyin the current data is too low to allow an investigation of this 11% of all galaxies at Her A’s absolute magnitudes), and sincequestion. Regardless of the true distribution of the obscuring the radio structure of 3C 348 is distinguished even within thatmaterial, we can approximate the path length through it at the class of select sources by the presence of closed loops orobserved ring to be half the diameter of the ring. Assuming, bubbles or radio synchrotron– emitting plasma, it is certainlytherefore, a path length of 1Љ (2 kpc), we derive an average gas worthwhile to seek origins for the rings that are related to thatdensity in the ring of nH 2 0.7 cm Ϫ3 for gas with a filling factor activity.of unity. If the gas is clumpy, the average density will be higher. In Figure 3, we plot the diameter perpendicular to the radio If the absorption is instead due to Thomson scattering by axis of the observed optical and radio rings as a function ofelectrons with density ne and path length r, the optical depth radius from the nucleus. Any model that seeks to explain theis given by optical and radio rings with a common mechanism must be ␶ ϭ ne ␴ T r , (3) able to explain the relation shown; the optical and radio rings follow a roughly linear relation between diameter and dis-where ␴ T is the Thomson scattering cross section. For a path tance. We consider three possibilities below.length of 2 kpc and an optical depth of 1.4 we thus have One possibility is that the rings are dusty molecular cloudsne 2 340 cm Ϫ3 . Depending on the temperature of the gas in that have been entrained and transported along the jets orthat instance, we might expect significant line emission from within a turbulent sheath around the jet. The clouds mightsuch a dense ionized gas. Therefore spectroscopy of the rings have been pulled into ringlike structures by turbulent eddies inshould be obtained. the propagating jet flow. Those same turbulent eddies might A priori, then, with the current observation, we cannot manifest themselves as the radio bubbles seen on the largedistinguish between a dust or an electron-scattering origin for scale. Thus this model would suggest a common origin for thethe observed obscuring rings since, in both cases, the derived small-scale (1kpc) optical rings and the large-scale (10s ofdensities are certainly within the realm of feasibility. Interest- kpc) radio bubbles.ingly, the HST images do not show any other evidence of dust A second possibility is that the rings might be produced byin the galaxy in the form of dust disks or filaments. We also expanding bubbles of hot gas, which either produce the opacityfind no strong evidence for distorted optical isophotes in the by compressing dust along their outer edges or via electronhost or companion galaxy. scattering off the hot gas itself. To explain the two bubbles on
  3. 3. No. 1, 1996 OBSCURATION RINGS IN HERCULES A L7alternate sides of the nucleus, we would posit that hot gas timescale involved have changed with time, since, while therebubbles were ejected roughly along the radio axis and at are dramatic wiggles apparent in the large-scale lobe structureroughly (though perhaps not exactly) the same times. If these of Her A, the inner radio jet (within 115 kpc of the nucleus)are hot, expanding bubbles, since the bubbles are roughly as is remarkably straight; (2) there has been a wide-angle nuclearlarge (in diameter) as their distance from the nucleus, their outflow that has swept dense cold gas into a bipolar structureexpansion and ejection velocities must be roughly equal. This in the inner few kiloparsecs of 3C 348. To explain thesuggests that the dynamics of the rings are “bubble-like,” asymmetry of the two optical rings with the wobbling, etchingrather than “jetlike.” The possibility that optical line– emitting jet model, one must presume that either the wide-anglegas might be associated with the optical rings should be outflow was asymmetric on opposite sides of the nucleus orexplored, as such gas would allow a direct measurement of the that the jets themselves have slightly different orientations onbubbles’ propagation and expansion velocities. the two sides of the source. This model would also need to If the small-scale optical rings are expanding hot gas bub- explain why the rings are seen in obscuration; presumably, thebles, do they share a common origin with the large-scale radio role of the jets must be to compress that gas and/or dust in thebubbles? Early models of radio jets hypothesized that, rather conical wind, thereby increasing its density.than being continuously ejected in hydromagnetic flows, they Of the three models for the origin of the optical and radiomight be composed of a series of plasmons— clouds of hot, rings presented here, the plasmon model seems the mostradio-emitting plasmons pinched off in periodic ejections from straightforward, and it most naturally explains the roughlythe nucleus (e.g., De Young & Axford 1967; Christiansen linear relationship observed between the rings’ diameters and1969; Jaffe & Perola 1973; Pacholczyk & Scott 1976; Chris- distance from the nucleus. Given the present data, however,tiansen, Pacholczyk, & Scott 1981). Plasmon models fell into other models cannot be excluded.disfavor when Very Large Array observations showed radiojets to be linear and continuous. Plasmon models are energet-ically unfavorable compared to continuous or jet-flow models, 5. SUMMARYas a result of the large adiabatic losses suffered during the We have presented HST WFPC2 broadband red images ofexpansion of the bubbles as they move outward into the lobes. the powerful radio galaxy Hercules A and reported theFrom our observed optical rings to the radio bubbles, expan- detection of two faint dark rings of obscuration with ansion factors of only 14 in radius are seen, implying adiabatic interlocking, “bipolar” appearance. These rings are 12Љenergy losses of a factor of 14. Plasmon models require (4 kpc) in diameter and are offset by 11"5 (3 kpc), roughlycontinuous reacceleration of synchrotron particles by shocks along the radio axis. The morphologies and orientations of thealong the expanding plasma bubble or in instabilities along the two rings are distinct; one is oriented along the radio axis andsurfaces of those bubbles as they interact with the ambient appears to be centered on a small optical feature that is itselfmedium. Deep radio observations of the inner few kiloparsecs oriented along the radio axis. The other, located on theof Her A should be undertaken to determine if the obscuring opposite side of the nucleus, has its long axis roughly perpen-optical rings are visible in the radio as bubbles or plasmons. If dicular to the radio axis and its center offset by 130Њ from thata relationship between the dust bubble and visible radio axis. The radio source associated with Her A, 3C 348, wasstructure is discovered, this would support a model for the already known from VLA imaging to have a unique radioradio source in terms of ejection of discrete plasmons rather structure with radio bubbles/close loops in its eastern radio jetthan a continuous jet (Dreher & Feigelson 1984). We note and lobe. We suggest that the optical rings may be caused bythat sporadic relativistic ejection of radio-emitting plasmoids dust obscuration or by electron scattering. We have consideredin a double-jet geometry has been observed in the Galactic scenarios in which they are related to the radio jet. Thesesuperluminal transient sources GRS 1915ϩ105 (Mirabel & obscuring rings may trace the interaction of precessing radioRodrı ´guez 1994) and GRO I1655Ϫ40 (Hjellming & Rupen jets with the ambient medium or may be due to shells swept up1995). by expanding radio plasmons. The plasmon model explains the A third possibility is that the optical obscuring rings in Her observed roughly linear relationship between bubble diameterA are produced in a fashion similar to the way in which and distance from the nucleus most naturally.optically emitting rings and bubbles are produced in stellar Further, deeper, multicolor optical observations and deeperoutflows in Galactic objects, e.g., planetary nebulae (Livio high-resolution VLA radio images will be required to deter-1996) and luminous blue variables (Nota et al. 1995). In these mine the nature of these fascinating optical structures andobjects it is believed that a wind from a central object is shaped their relationship to the activity in Her A. It will also bevia a density contrast in the ambient medium into a bipolar important to determine whether Her A (and possibly 3C 310)outflow. The interaction of a precessing or wobbling jet with are “special” in their energy transport properties, or whetherthis preexisting surface can cause a ring to be traced out (see, they tell us that episodic ejections rather than continuouse.g., Livio 1996). Such a model has been discussed in the outflow are common in powerful radio galaxies.context of the offset rings in SN 1987A (Burrows et al. 1995).Precession or wobbling of the radio jet’s axis in Her A issupported by the observed wiggles in the large-scale radio jets This work was supported by NASA through grant GO-and the overall point symmetry of the envelope of the radio 5476.01 from the Space Telescope Science Institute, which isstructure (see Dreher & Feigelson 1984). If this scenario is operated by the Association of Universities for Research incorrect, it would imply that (1) the precession cone and Astronomy, Inc., under NASA contract NAS 5-26555. REFERENCESBurrows, C. J., et al. 1995, ApJ, 452, 680 Christiansen, W. 1969, MNRAS, 145, 327Burstein, D., & Heiles, C. 1978, ApJ, 225, 40 Christiansen, W. A., Pacholczyk, A. G., & Scott, J. S. 1981, ApJ, 251, 518
  4. 4. L8 BAUM ET Koff, S., Baum, S. A., Biretta, J., Golombek, D., Macchetto, F. D., Jaffe, W. J., & Perola, G. C. 1973, A&A, 26, 423 McCarthy, P. J., Miley, G. K., & Sparks, W. 1994, BAAS, 185, 107 Livio, M. 1996, in preparationde Koff, S., et al. 1996, in preparation Mirabel, I. F., & Rodrı´guez, L. F. 1994, Nature, 371, 46De Young, D. S., & Axford, W. I. 1967, Nature, 216, 129 Nota, A., Livio, M., Clampin, M., & Schulte-Ladbeck, R. 1995, ApJ, 448, 788Dreher, J. W., & Feigelson, E. D. 1984, Nature, 308, 43 Pacholczyk, A. G., & Scott, J. S. 1976, ApJ, 203, 313Hayes, J. J. E., et al. 1996, ApJ, submitted Sadun, A. C., & Hayes, J. J. E. 1993, PASP, 105, 379Hjellming, R. M., & Rupen, M. P. 1995, Nature, 375, 464 van Breugel, W., & Fomalont, E. B. 1984, ApJ, 282, L5
  5. 5. PLATE L2 FIG. 1.—Gray-scale image of the planetary camera F702W image of Her ABAUM et al. (see 465, L5)
  6. 6. PLATE L3 FIG. 2.—Idealized sketch of the interlocking “rings” superposed on the gray-scale image of Her A. Arrows indicate the locations of the nucleus of 3C 348, thecompanion galaxy, and the radio source’s axis.BAUM et al. (see 465, L5)