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Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

Optical Jets in Radio Galaxies

W.B. Sparks, S. de Koff, S. Baum, A. Capetti, J. Biretta, D. Golombek, F. Macchetto
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

P. McCarthy
Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101-1000, USA

G. Miley
Sterrewacht, Postbus 9513, 2300 RA Leiden, The Netherlands

 

Abstract:

We have obtained WFPC2 snapshots of a large number of 3CR radio galaxies. A primary aim was to assess the frequency with which optical synchrotron jets occur, and to determine their characteristics and those of the host galaxies. We have discovered several new jets, and have additional candidates. A preliminary statistical analysis indicates that radio sources with associated optical jets are intrinsically smaller than those without. While beaming appears to contribute in some cases, this would imply it is not the dominant effect. The optical jets sources are genuinely small sources.

Keywords: Jets

Introduction

One of the most extraordinary phenomena associated with radio emission from galaxies is the presence of jets. They appear from radio observations to be ubiquitous, to the point where they are almost taken for granted! But, we still do not even know how these jets are triggered, what they are made of, and how they are collimated over vast distances. It is clear that jets unambiguously connect the active galactic nucleus to the radio lobes, and that, therefore, in some way they represent an important means of energy transport.

A small number of the radio jets are also known to show optical non-thermal emission, generally thought to be optical synchrotron emission because of its similarity to the radio morphology and high degree of linear polarization. Optical jets are interesting in the study of this phenomenon because the short lifetime of optically emitting synchrotron electrons localizes the sites where things are happening: shocks or other processes of particle acceleration. They offer an opportunity to probe jet interiors and even to investigate the motion of the jet, while providing additional or supplementary physical diagnostics to the radio.

Before HST was launched, the number of optical jets known to be associated with extragalactic radio sources was small: M87, 3C 273, 3C 66B and PKS0521-36. An additional jet was discovered with the Faint Object Camera prior to the refurbishment mission by Crane et al. (1993) in 3C 264. Thomson et al. (1993) obtained optical HST imaging polarimetry of the jet of 3C 273.

Following the refurbishment mission in 1993, there have been detailed observations of the jet in M87: polarization and proper motion in particular, see Biretta et al. (1996). Bahcall et al. (1995) observed the jet of 3C 273 with the WFPC-2 in detail. There has also been a survey of 3CR extragalactic radio sources, one of the purposes of which was to determine the frequency with which optical jets occur, their characteristics and those of the hosts. A description of the results from part of that survey forms the remainder of this paper.

Observations

Approximately 270 extragalactic radio sources were observed during the Cycle 4 WFPC-2 snapshot survey of 3CR sources. All observations used the F702W () filter with the target located in the PC chip. Typical exposures range from three to 10 minutes, and fine lock guidance was used throughout.

A variety of scientific questions were targeted by these observations, including the nature of the cores of radio galaxies, the presence of dust, the alignment effect, quasar host characteristics, and more. The high spatial resolution and sensitivity of HST enables a vastly improved detectability for optical jet or jet-like emission in radio galaxies as compared to ground-based observations.

Results

Low Redshift Sources

For analysis purposes we divided the sample by redshift. At low redshift, , the survey provided its first newly discovered optical jet in 3C 78, or NGC1218, Sparks et al. (1995). The optical image of the jet is remarkably similar to the radio with a bright narrow inner region, and a sudden flaring and fading.

Also at low redshift, the galaxy hosting previously known jet of 3C 264 revealed a surprising facet: a curious ring-like structure that is probably due to a circular face-on dust disc, see Fig. 1. As in 3C 78, the jet suddenly flares and fades, however, the interesting observation is that this happens essentially exactly at the outer boundary of the dust disc. Although other work has shown a tendency for jets and discs to lie perpendicular to one another, Kotanyi & Ekers (1979), de Koff et al. (1996b) from this survey, in the case of 3C 264, the strange morphology leads to the possibility that the jet is emerging in the plane of the dust disc, and is perhaps optically luminous because of the additional deceleration, Baum et al. (1996).

 
Figure: Image of 3C 264 showing optical synchrotron disc embedded within probably face-on dust disc.

Intermediate Redshift

For a first statistical study into the prevalence of jets, we have examined the 77 galaxies in the redshift range . The data are presented in de Koff et al. (1996a). Over this range, the spatial resolution of WFPC-2 observations is comparable to the resolution obtained on the nearest galaxies with good ground-based imaging.

Our analysis is somewhat subjective at the present time, since quantitative radio to optical ``overlays'' have not yet been made; detailed limits have not been calculated on the optical emission nor on the optical to radio spectral index. Hence, there may be additional jet candidates in the final analysis. A second caveat is that the broad filter used transmits optical line emission for essentially all redshifts, and there is likely to be a contribution in some cases to the morphology from such gaseous emission, rather than the pure continuum of optical synchrotron emission.

In the survey, there are three excellent candidates for new synchrotron jets, a galaxy showing a pair of optical hotspots, and a further six candidate jet hosts. These are discussed in more detail below. Very importantly, there are candidate two-sided optical jets which, if confirmed as synchrotron sources, will have profound implications for jet physics and the relevance of relativistic beaming. Fig. 2 illustrates some of the candidates we have identified, with the bar showing the direction of the radio source.

 
Figure: Example jet candidates from 3CR snapshot survey. The short bar indicates the direction of the radio axis.

From these numbers, we infer that between 4 and 10 percent of radio galaxies show relatively prominent optical jets.

 
Figure: 3C 346 showing a narrow, curved optical synchrotron jet emerging from the brighter component of a double galaxy.

3C 346 shows a convincing example of an optical synchrotron jet. The radio source and jet was studied by Dey & van Breugel (1994) who also found an optical hotspot at the location of the radio peak. The new WFPC-2 image shows a double galaxy, with a fine, curved feature starting at the nucleus of the westerly galaxy, brightening at a sharp kink (the ``hotspot''), and then fading further out from the galaxy, shown in Fig. 2 and in detail in Fig. 3. This is to all intents and purposes identical to the VLA image of Dey & van Breugel (1994) and leads us to conclude the optical emission is also non-thermal synchrotron. The bright emission from the `kink' suggest the possibility of a helical geometry in which that portion of the jet points closest to the observer, thereby boosting the emission at the turning point. That is, the large changes in amplitude for the emission may be offering us data on the intrinsic `beam pattern' of optical radiation. Alternatively, enhanced synchrotron emissivity at bending shocks in the jet may also contribute bright emission at the kinks.

3C 200 shows a straight linear feature starting at the nucleus of the galaxy and fading smoothly to larger distances. Again, this is exactly the behavior of the radio image, Bogers et al. (1994), where the radio jet emerges at the same position angle as the linear optical feature.

3C 268.3 shows a very highly elongated optical morphology, dominated by the appearance of the candidate jet structure. On both sides of the center, the image shows narrow plateaus very similar to the structure of the northern component in the radio image of Akujor et al. (1991). The additional complexity of the structure renders identification of this source as a jet somewhat less secure, however, it is crucially important because this may be the first two-sided optical jet.

3C 213.1 shows a typical elliptical galaxy type host morphology, although with some evidence for structure in the form of sharp edges or shells. Remarkably, however, there are two unresolved optical point sources either side of the object. These coincide, to the accuracy with which we can measure, with two classical radio hotspots, Spencer et al. (1989). The nature of the optical emission is unknown, however, such features are classically interpreted as the working surfaces where the jet is disrupted by the ambient medium. Other optical synchrotron hotspots are known, and in the case of M87, even on the opposite side to the famous jet, Sparks et al. (1992), Stiavelli et al. (1992). These new observations may be providing another example of a two-sided optical pair of hotspots.

Discussion: Beaming or Not?

Relativistic beaming is often invoked to explain the characteristics of optical jet emission: the jets are one-sided, they typically are associated with prominent nuclei, they are small and curved. Beaming would enhance the detectability of a jet in the optical by both boosting the intensity and also by shifting the synchrotron break frequency (generally in the infra-red) to shorter wavelengths. Circumstantial evidence in support of this idea comes also with the Laing-Garrington effect, whereby Faraday rotation is least on the lobe where the (radio) jet is visible.

3C 346: supports the idea. The galaxy appears in the ``compact steep spectrum'' source list of Fanti et al. (1990). However, Fanti et al. (1990) argue that in fact the source is an interloper, and that its unusual characteristics are most naturally explained if it is an `end-on' classical double source. This is an attractive explanation for the optical appearance, with the optical emission brightest where the jet points towards us, and the highly curved structure arises naturally due to foreshortening effects on an intrinsically more gradually curved jet.

On the other hand, Fraix-Burnet (1992) advocates environmental influences to explain the one-sidedness of jets. Statistics of source size, and the curious appearance of 3C 264 count against the dominance of beaming. Twin optical hotspots and two-sided optical jets would also argue against a beaming hypothesis since there is no obvious reason why symmetric hotspots should be beamed (although Wilson & Scheuer (1983) do find relatively small asymmetries in beamed hotspots). Sparks et al. (1995) looked at the angular sizes of the associated radio sources and found that the ones with optical jets are smaller than those without. While superficially consistent with the beaming notation, in detail the distributions are clearly not simply due to a randomly oriented set of sources of roughly the same size. The high number of small radio sources requires that they be genuinely small, intrinsically, and these are the population that show optical jets. Optical hotspots also showed the same correlation and while beaming is a possibility, it is less likely in that case, especially since there are definite examples of two-sided synchrotron hotspots.

Conclusions

By utilizing the highly efficient snapshot mode of observing, we have surveyed a very large number of radio galaxies with HST WFPC-2 imaging. The high spatial resolution and improved contrast over the host galaxies greatly improves the detectability of optical synchrotron jets. We have discovered new optical jets in the survey and for the first time may begin to make meaningful statistical comparisons of the properties of sources containing optical jets to the remainder.

Acknowledgments:

Support for this work was provided by NASA through grant number GO-5476.01-93A from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.

References:

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W. B. Sparks, S. de Koff, S. Baum, A. Capetti, J. Biretta, D. Golombek, F. Macchetto, P. McCarthy, and G. MileySparks et al.Optical Jets in Radio Galaxies



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