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 ...
THE ASTROPHYSICAL JOURNAL, 465 : L5–L8, 1996 July 1 q 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 the host galaxy of 3C 348 (Sadun & Hayes 1993; Hayes et al. 1996) contrasts sharply with the low surface brightness of the galaxy itself as seen by HST. Two faint, dark, interlocking rings with centers offset 11"5 from the nucleus of Her A are apparent at low S/N in both of these images. The measured geometrical properties of the rings are summarized in Table 1, and an idealized sketch based on these properties is presented in Figure 2 (Plate L3), where we also indicate the location of the nucleus of 3C 348, the orientation of the radio source’s axis, and the companion galaxy. The two rings are slightly elliptical and exhibit different morphologies. The eastern, smaller, ring is oriented with its center and its long axis directly along the radio jet’s axis and has a characteristic width of 1"5 (3 kpc). It appears to be centered on a small but resolved optically emitting feature that is elongated along the radio axis. The nature of this feature is unknown—it could be optical synchrotron emission from a knot in the jet, a region of star formation, or an emission-line region. By contrast, the western, larger, ring is oriented with its center 1308 from the radio source’s axis and its major axis virtually perpendicular to the radio axis. This ring has a characteristic width of 2"25 (5.5 kpc). The dark rings appear at low S/N in our snapshot images, and it is clear that longer integration, multicolor images are warranted to confirm their existence and to further study their nature. Nevertheless we believe the features are likely to be real and not an artifact of the observations or our eyes. We have, to date, examined similar WFPC2 images of over 200 3CR galaxies, and in this source alone have we identified such rings. Her A has long been known to be one of only two powerful radio sources that show closed loops or bubbles of synchrotron-emitting plasma in its large-scale radio structure (Dreher & Feigelson 1984; van Breugel & Fomalont 1984). Thus, Her A has now been shown to be blowing both dark optical rings and radio bubbles.
1. INTRODUCTION
The astrophysics of the launching, collimation, and propagation of jets is one of the outstanding problems in the study of powerful radio-loud active galactic nuclei. In this Letter, we report the discovery with the Hubble Space Telescope (HST) of a phenomenon that has the potential to shed new light on the transport of energy in powerful radio jets. We present HST Wide Field Planetary Camera 2 (WFPC2) observations, taken through the F702W broadband red filter, of Hercules A, the host galaxy of the powerful radio source 3C 348. These images show two laterally unresolved, interlocking rings of obscuration slightly offset from the nucleus, roughly along the radio jet’s axis. We discuss possible origins of these rings and their implications for the nature of the central engine and radio jets in Her A. We adopt H0 5 75 and q0 5 0.5, which yields a scale of 12 kpc arcsec 21 at the redshift of Her A ( z 5 0.154). 2. OBSERVATIONS AND REDUCTION
We obtained two 300 s exposures of Her A, with the target centered in the planetary camera of WFPC2, using the broadband red F702W filter. These observations were obtained during the course of the HST 3CR Snapshot Survey (de Koff et al. 1994). In de Koff et al. (1996), the snapshot data are presented for the sources in the 3CR with redshifts 0.5 . z . 0.1, including 3C 348, and we refer the reader to that paper for a more detailed description of the observations and data reduction. The two 300 s frames were combined to reject cosmic rays. Two dark, interlocking rings are visible in the single images as well as the combined, cosmic-ray–rejected image, but in all cases they are seen at very low signal-to-noise ratio (S/N) (near the limit of our ability to detect them). We found that smoothing the images by a 0"08 Gaussian produced the best visual representation of the rings. 3. RESULTS
4. DISCUSSION
In Figure 1 (Plate L2), we show a gray-scale representation of our HST images. The bright, high surface brightness, elliptical companion located 140 northwest of the nucleus of 1
The rings appear as dark regions in which the underlying stellar light from the host galaxy is not seen. Thus they appear to be rings or shells of absorbing material. Below, we first discuss several possible mechanisms for the absorption and
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TABLE 1 PROPERTIES
OF
OBSCURATION RINGS a
Ring
Major Axis (kpc)
Minor Axis (kpc)
P.A. (deg)
P.A. to Nucleus (deg)
East . . . . . . West. . . . . .
2.8 4.8
2.3 4.2
90 0
90 120
a
Estimated from the HST images.
then discuss possible origins for the rings and their implications for models of jets in extragalactic radio sources. 4.1. Origin of the Obscuration We consider two potential mechanisms for the absorption of the underlying stellar light, (1) dust and (2) Thomson scattering by electrons. The contrast ratio between the obscured ring and the surrounding stellar light is roughly 1;4. If the absorption is exponential, then
S
t 5 2ln 1 2
DS S
D
,
(1)
where S is the background flux and DS is the depth of the absorption. This yields an optical depth of t 1 1.4. The change in magnitude of the background light is Dm 5 22.5 3 log (1 2 DS/S) 21 2 1.5 mag. At the wavelength of the F702W filter, A(F702W) 3 2E(B 2 V ). This implies E(B 2 V ) 2 0.75. If the dust-to-gas ratio is approximately the Galactic value, then, from Burstein & Heiles (1978), N~H! 2 5.0 3 1021 @E~B 2 V ! 1 0.06# cm 22 ,
(2)
22
which yields N(H) 2 4.1 3 10 cm . While, in principle, we should be able to determine whether the obscuring material is distributed in a bubble or a true ring by looking for obscuration that is internal to the ring, the S/N in the current data is too low to allow an investigation of this question. Regardless of the true distribution of the obscuring material, we can approximate the path length through it at the observed ring to be half the diameter of the ring. Assuming, therefore, a path length of 10 (2 kpc), we derive an average gas density in the ring of nH 2 0.7 cm 23 for gas with a filling factor of unity. If the gas is clumpy, the average density will be higher. If the absorption is instead due to Thomson scattering by electrons with density ne and path length r, the optical depth is given by 21
t 5 ne s T r ,
(3)
where s T is the Thomson scattering cross section. For a path length of 2 kpc and an optical depth of 1.4 we thus have ne 2 340 cm 23 . Depending on the temperature of the gas in that instance, we might expect significant line emission from such a dense ionized gas. Therefore spectroscopy of the rings should be obtained. A priori, then, with the current observation, we cannot distinguish between a dust or an electron-scattering origin for the observed obscuring rings since, in both cases, the derived densities are certainly within the realm of feasibility. Interestingly, the HST images do not show any other evidence of dust in the galaxy in the form of dust disks or filaments. We also find no strong evidence for distorted optical isophotes in the host or companion galaxy.
FIG. 3.—Plot of the ring/bubble diameter (perpendicular to the jet’s axis) as a function of distance from the nucleus.
4.2. Origin of the Rings and Their Association with the Radio Source We consider several possible origins for the optical obscuration rings. First, they may, of course, bear no relation to the radio jets or the nuclear activity but represent, for example, remnants from a merger or resonant structures in the host galaxy. It is clear that the optical obscuring rings are not in an equilibrium configuration, however; they must be transient or evolving features. Since 3C 348 is a radio-loud, active source with clear radio jets and lobes (a phenomenon present in only 11% of all galaxies at Her A’s absolute magnitudes), and since the radio structure of 3C 348 is distinguished even within that class of select sources by the presence of closed loops or bubbles or radio synchrotron– emitting plasma, it is certainly worthwhile to seek origins for the rings that are related to that activity. In Figure 3, we plot the diameter perpendicular to the radio axis of the observed optical and radio rings as a function of radius from the nucleus. Any model that seeks to explain the optical and radio rings with a common mechanism must be able to explain the relation shown; the optical and radio rings follow a roughly linear relation between diameter and distance. We consider three possibilities below. One possibility is that the rings are dusty molecular clouds that have been entrained and transported along the jets or within a turbulent sheath around the jet. The clouds might have been pulled into ringlike structures by turbulent eddies in the propagating jet flow. Those same turbulent eddies might manifest themselves as the radio bubbles seen on the large scale. Thus this model would suggest a common origin for the small-scale (1kpc) optical rings and the large-scale (10s of kpc) radio bubbles. A second possibility is that the rings might be produced by expanding bubbles of hot gas, which either produce the opacity by compressing dust along their outer edges or via electron scattering off the hot gas itself. To explain the two bubbles on
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alternate sides of the nucleus, we would posit that hot gas bubbles were ejected roughly along the radio axis and at roughly (though perhaps not exactly) the same times. If these are hot, expanding bubbles, since the bubbles are roughly as large (in diameter) as their distance from the nucleus, their expansion and ejection velocities must be roughly equal. This suggests that the dynamics of the rings are “bubble-like,” rather than “jetlike.” The possibility that optical line– emitting gas might be associated with the optical rings should be explored, as such gas would allow a direct measurement of the bubbles’ propagation and expansion velocities. If the small-scale optical rings are expanding hot gas bubbles, do they share a common origin with the large-scale radio bubbles? Early models of radio jets hypothesized that, rather than being continuously ejected in hydromagnetic flows, they might be composed of a series of plasmons— clouds of hot, radio-emitting plasmons pinched off in periodic ejections from the nucleus (e.g., De Young & Axford 1967; Christiansen 1969; Jaffe & Perola 1973; Pacholczyk & Scott 1976; Christiansen, Pacholczyk, & Scott 1981). Plasmon models fell into disfavor when Very Large Array observations showed radio jets to be linear and continuous. Plasmon models are energetically unfavorable compared to continuous or jet-flow models, as a result of the large adiabatic losses suffered during the expansion of the bubbles as they move outward into the lobes. From our observed optical rings to the radio bubbles, expansion factors of only 14 in radius are seen, implying adiabatic energy losses of a factor of 14. Plasmon models require continuous reacceleration of synchrotron particles by shocks along the expanding plasma bubble or in instabilities along the surfaces of those bubbles as they interact with the ambient medium. Deep radio observations of the inner few kiloparsecs of Her A should be undertaken to determine if the obscuring optical rings are visible in the radio as bubbles or plasmons. If a relationship between the dust bubble and visible radio structure is discovered, this would support a model for the radio source in terms of ejection of discrete plasmons rather than a continuous jet (Dreher & Feigelson 1984). We note that sporadic relativistic ejection of radio-emitting plasmoids in a double-jet geometry has been observed in the Galactic superluminal transient sources GRS 19151105 (Mirabel & Rodrı´guez 1994) and GRO I1655240 (Hjellming & Rupen 1995). A third possibility is that the optical obscuring rings in Her A are produced in a fashion similar to the way in which optically emitting rings and bubbles are produced in stellar outflows in Galactic objects, e.g., planetary nebulae (Livio 1996) and luminous blue variables (Nota et al. 1995). In these objects it is believed that a wind from a central object is shaped via a density contrast in the ambient medium into a bipolar outflow. The interaction of a precessing or wobbling jet with this preexisting surface can cause a ring to be traced out (see, e.g., Livio 1996). Such a model has been discussed in the context of the offset rings in SN 1987A (Burrows et al. 1995). Precession or wobbling of the radio jet’s axis in Her A is supported by the observed wiggles in the large-scale radio jets and the overall point symmetry of the envelope of the radio structure (see Dreher & Feigelson 1984). If this scenario is correct, it would imply that (1) the precession cone and
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timescale involved have changed with time, since, while there are dramatic wiggles apparent in the large-scale lobe structure of Her A, the inner radio jet (within 115 kpc of the nucleus) is remarkably straight; (2) there has been a wide-angle nuclear outflow that has swept dense cold gas into a bipolar structure in the inner few kiloparsecs of 3C 348. To explain the asymmetry of the two optical rings with the wobbling, etching jet model, one must presume that either the wide-angle outflow was asymmetric on opposite sides of the nucleus or that the jets themselves have slightly different orientations on the two sides of the source. This model would also need to explain why the rings are seen in obscuration; presumably, the role of the jets must be to compress that gas and/or dust in the conical wind, thereby increasing its density. Of the three models for the origin of the optical and radio rings presented here, the plasmon model seems the most straightforward, and it most naturally explains the roughly linear relationship observed between the rings’ diameters and distance from the nucleus. Given the present data, however, other models cannot be excluded. 5. SUMMARY
We have presented HST WFPC2 broadband red images of the powerful radio galaxy Hercules A and reported the detection of two faint dark rings of obscuration with an interlocking, “bipolar” appearance. These rings are 120 (4 kpc) in diameter and are offset by 11"5 (3 kpc), roughly along the radio axis. The morphologies and orientations of the two rings are distinct; one is oriented along the radio axis and appears to be centered on a small optical feature that is itself oriented along the radio axis. The other, located on the opposite side of the nucleus, has its long axis roughly perpendicular to the radio axis and its center offset by 1308 from that axis. The radio source associated with Her A, 3C 348, was already known from VLA imaging to have a unique radio structure with radio bubbles/close loops in its eastern radio jet and lobe. We suggest that the optical rings may be caused by dust obscuration or by electron scattering. We have considered scenarios in which they are related to the radio jet. These obscuring rings may trace the interaction of precessing radio jets with the ambient medium or may be due to shells swept up by expanding radio plasmons. The plasmon model explains the observed roughly linear relationship between bubble diameter and distance from the nucleus most naturally. Further, deeper, multicolor optical observations and deeper high-resolution VLA radio images will be required to determine the nature of these fascinating optical structures and their relationship to the activity in Her A. It will also be important to determine whether Her A (and possibly 3C 310) are “special” in their energy transport properties, or whether they tell us that episodic ejections rather than continuous outflow are common in powerful radio galaxies. This work was supported by NASA through grant GO5476.01 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
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PLATE L2
FIG. 1.—Gray-scale image of the planetary camera F702W image of Her A BAUM et al. (see 465, L5)
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, the companion galaxy, and the radio source’s axis. BAUM et al. (see 465, L5)
