Received 1996 August 23; accepted 1996 December 30

Transcription

1 THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 110:191È211, 1997 June ( The American Astronomical Society. All rights reserved. Printed in U.S.A. HUBBL E SPACE T EL ESCOPE IMAGING OF COMPACT STEEP SPECTRUM RADIO SOURCES W. H. DE VRIES,1,2 C. P. OÏDEA,1 S. A. BAUM,1 W. B. SPARKS,1 J. BIRETTA,1 S. DE KOFF,1,3 D. GOLOMBEK,1 M. D. LEHNERT,3 F. MACCHETTO,1 P. MCCARTHY,4 AND G. K. MILEY3 Received 1996 August 23; accepted 1996 December 30 ABSTRACT We present Hubble Space T elescope WFPC2 images taken through a broad red Ðlter () of 30 Third Cambridge Catalog compact steep spectrum (CSS) radio sources. We have overlaid radio maps taken from the literature on the optical images to determine the radio-optical alignment and to study detailed correspondence. All CSS sources for which the relative orientation between the optical and radio can be measured display good alignment between the optical and radio emission down to the lowest redshift in the sample, z D 0.1. The alignment e ect does not occur at this relatively low redshift for the large-scale 3CR radio sources, which tend to show a signiðcant alignment only at z [ 0.6, as shown by McCarthy et al., Chambers et al., and de Ko et al. We Ðnd candidates for optical synchrotron hot spots in 3C and 3C 380 and an optical jet in 3C 346. Subject headings: galaxies: compact È galaxies: structure È quasars: general È radio continuum: galaxies 1. INTRODUCTION Thus CSS sources are important because (1) they probe the ISM of the host galaxy and (2) they provide clues to the Compact steep spectrum (CSS) radio sources are origin and evolution of powerful radio sources. Because of compact (1È20 kpc), powerful (log P [ 26.5 W Hz~1) 178 the small angular sizes of the radio sources, typically a few radio sources with steep radio spectra (a\[0.5, where arcseconds, the 0A.1 resolution of HST is necessary for a S P la) (Kapahi 1981; Peacock & Wall 1982; Fanti et al. detailed study of the relationship between the optical and 1990; Fanti & Fanti 1994). They are a signiðcant fraction radio morphologies. Here we present Hubble Space T elescope WFPC2 images of 30 CSS sources in the 3CR. The (D30%) of the bright radio-source population selected at centimeter wavelengths (Kapahi 1981; Peacock & Wall results are discussed in De Vries et al. (1997, hereafter Paper 1982). However, their origin and their place in the current II). radio galaxy taxonomy are not well understood. The existence of such sources was originally established by the 2. THE HST DATA Jodrell Bank radio-link interferometer in the 1960s (Palmer Our sample is the 3CR sources (Spinrad et al. 1985) in the et al. 1967). Subsequent high-resolution imaging (e.g., van Fanti et al. (1990) complete sample of CSS sources. The Breugel, Miley, & Heckman 1984; Spencer et al. 1989; selection criteria used by Fanti et al. were (1) projected Sanghera et al. 1995) has shown that they tend to resemble linear size of less than 20 kpc, (2) Ñux density at 178 MHz small-scale versions of the large-scale classical doubles in in excess of 10 Jy, (3) Galactic latitude o b o [ 10, (4) the Third Cambridge Catalog (3CR) sample. declination d[]10, and (5) monochromatic radio power There are currently two main hypotheses concerning CSS log P [ 26.5 WHz~1, where we have scaled the Fanti et sources. They could be younger versions of the large-scale 178 al. criteria using our adopted values of H \ 75 km s~1 classical doubles (e.g., Carvalho 1985; Hodges & Mutel Mpc~1 and q \ 0.5. Note that since the 0 original com- 1987; Mutel, Su, & Song 1990; De Young 1993; Fanti et al. pilation of the 0 CSS source list by Fanti et al. evidence has 1995; Begelman 1996; Readhead et al. 1996a, 1996b; OÏDea accumulated that some sources are larger than the 20 kpc & Baum 1996). In this scenario, they will evolve into the cuto (e.g., OÏDea 1997, and references therein). These large-scale powerful Fanaro & Riley (1974; FR) class II sources are footnoted in Tables 1 and 2. radio galaxies or perhaps weaker FR class I sources if they The HST observations were carried out as part of the decline signiðcantly in luminosity as they age (OÏDea & 3CR snapshot survey program (Sparks et al. 1995; De Ko Baum 1996). Alternately, they may be conðned to their et al. 1996; McCarthy et al. 1997), except for the obserpresent size scales through interaction with a dense ambient vations of 3C 237 and 3C 286, images of which were taken medium (van Breugel et al. 1984; Wilkinson et al. 1984; from the HST archive. Most sources were observed for Fanti et al. 1990; OÏDea, Baum, & Stanghellini 1991; 300 s using the WFPC2 (described by Biretta et al. 1996), Carvalho 1994). In this scenario these frustrated ÏÏ sources with the source centered in the WFPC2. The Ðlter never become large-scale doubles. was used, which (convolved with the system response) has a mean wavelength of 6868 Ó and an e ective width of Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Ó. The sample consists of 30 sources, of which 14 are identi- MD Ðed as galaxies and 16 as quasars. Since snapshot observing 2 Kapteyn Astronomical Institute, P.O. Box 800, NL-9700 AV, Groningen, The Netherlands. our sample is a random subsample of the CSS sources in the depended only on our ability to schedule the observation, 3 Leiden Observatory, P.O. Box 9513, NL-2300 RA, Leiden, The Netherlands. 3CR. Observational parameters and source parameters can 4 Observatories of The Carnegie Institution, 813 Santa Barbara Street, be found in Table 1. The mean redshift of our sample is 0.80, Pasadena, CA so the typical largest linear size of 10 kpc translates to 191

2 192 DE VRIES ET AL. Vol. 110 TABLE 1 CSS SUBSAMPLE FROM 3CR Exposure Time Number Resolution 3CR Designation IdentiÐcationa z Vb (s) of Frames m c lim k d lim Radio Map (arcsec) Reference 3C Q MERLIN 5 GHz C G MERLIN 5 GHz C G MERLIN 5 GHz C G VLA 8.4 GHz C G VLBI 0.6 GHz C Q VLA 8.4 GHz C 147e... Q VLA 8.4 GHz C Q MERLIN 1.7 GHz C Q VLA 8.4 GHz C Q VLA 15 GHz C G VLA 8.4 GHz C 216e... Q VLA 5 GHz C G VLA 8.4 GHz C G MERLIN 5 GHz C G VLA 8.4 GHz C G VLA 15 GHz C Q VLBI/EVN 1.6 GHz C Q VLA 8.4 GHz C Q VLBI 0.6 GHz C Q MERLIN 5 GHz C 299e... G VLBI 1.7 GHz C G VLA 8.4 GHz C G VLA 8.4 GHz C Q VLA 8.4 GHz C Q MERLIN 5 GHz C G VLBI 0.6 GHz C 346e... G VLA 15 GHz C 380e... Q MERLIN 1.7 GHz C Q VLA 4.9 GHz C Q VLA 15 GHz NOTE.ÈEVN \ European VLBI Network. a Q \ quasar, G \ galaxy. b Ground-based. c 3 p limiting magnitude for a point source. d 3 p limiting surface brightness in mag arcsec~2. e Source may be larger than the 20 kpc cuto for CSS sources. REFERENCES.È(1) Sanghera et al. 1995; (2) van Breugel et al. 1992; (3) Akujor & Garrington 1995; (4) Bogers et al. 1994; (5) Nan Rendong et al. 1991; (6) Akujor et al. 1991; (7) Spencer et al. 1991; (8) Reid et al. 1995; (9) Lonsdale et al approximately 45 pixels (the WFPC2 has a pixel scale of 0A.0455) across the source, providing ample resolution for our purposes. The redshift distribution of the sources can be found in Figure 1. We give the limiting magnitude for a stellar object and the surface-brightness limit (in mag arcsec~2) in Table 1 for each source. For point sources we assumed, as a limiting case, a stellar point-spread function (PSF) with a diameter of 2 pixels, with a signal at the edge of the circle of 3 p, where p is the standard deviation of the sky mean. Using an approximation to the PSF, we calculated the total Ñux within the aperture. We approximated the PSF by Ðtting Gaussians to radially averaged proðles of unsaturated stars on di erent locations on the WFPC2 chip. An average FWHM of 2.87 pixels was found, giving a total Ñux of 11.2 p for such a threshold ÏÏ star. In order to allow comparison with ground-based observations we give an estimate of our surface -brightness sensitivity. For the surface-brightness limit we use S \ N ] [3p/(N)1@2] as an estimate for the signal within 1 arcsec2, with N being the number of pixels per arcsec2 (approximately 483) and 3 p the detection limit for one pixel; the total Ñux works out to be about 66 p per arcsec2. The values for the individual frames can be found in Table 1, but a typical brightness limit is 24.6 mag arcsec~2 in. We expect the cosmological (1 ] z)4 surface- FIG. 1.ÈRedshift distribution of CSS galaxies and quasars. The redshift bin size is 0.3. The dashed line gives the summed distribution.

3 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 193 TABLE 2 OBSERVATIONAL RESULTS Optical Sizea Optical Sizeb Optical P.A. 3CR Designation m (arcsec) (kpc) (deg) * Remarks 3C C [10 3C [5 3C C C [23 3C 147c C C C C optical hotspots 3C 216c [4 3C C uncertain P.A. 3C [13 3C C C C C C 299c uncertain angle 3C [10 3C [12 3C C C C 346c optical jet 3C 380c optical knots 3C C a Determined as the largest distance lying within the 3 p contours. Error is estimated at 0A.1. Not calculated for PSF-dominated sources. b Calculated from the size in arcseconds assuming H \ 75 km s~1 Mpc~1, q \ 0.5. c Source may be larger than the 20 kpc cuto for CSS 0 sources. 0 brightness dimming to push the outer parts of the CSS galaxies into the noise region at z B 1. In Paper II we will quantify this discussion further Data Reduction The HST WFPC2 data were processed through the standard WFPC2 pipeline, namely the calwp2 routine in the STSDAS package in IRAF. This pipeline takes care of, among other things, bias and dark subtraction and Ñat Ðelding. We removed cosmic-ray events by using the crrej task in cases for which two exposures were taken. Since the cosmic rays are the only objects that change between the frames, crrej e ectively removes most of the cosmic rays. For those sources with only single exposures we used the cosmicrays task, which tries to identify the events by their steep count gradients. This technique works quite well for cosmic rays hitting the chip at roughly perpendicular angles, but unfortunately a large number of events have grazing incidence angles. Thus, some cosmic ray residuals may still be present in some of the frames. As a last step in the reduction, we rotated the frame so that north is up. The photometry on the sources was done with the IRAF APPHOT package. To convert the counts into magnitudes, we used the Vega zero point of mag for the Ðlter. The qphot task was then used to obtain the magnitudes. Since almost all sources are single objects, this task was sufficiently accurate for our purposes. The typical magnitude error is 0.01 mag, but by extending the integration aperture out into the sky, we found Ñuctuations in magnitude on the order of 0.05 magnitude in the fainter sources. The listed errors therefore are our conservative estimates. We also determined the Ñux within an aperture corresponding to a Ðxed radius of 1 kpc. The photometry results can be found in Table Radio Data The radio data are taken from the literature. The maps are selected to have roughly the same resolution and overall extent as the optical images. We preferentially used VLA A array images at 5, 8.4, and 15 GHz and MERLIN images at 1.7 and 5 GHz. The VLA angular resolution ranges at these frequencies from 0A.35 (5 GHz) to 0A.10 (15 GHz). The typical MERLIN angular resolution is somewhat better than 0A.10. However, maps at these resolutions are not available for all sources, so for some of them we overlaid VLBI maps (angular resolution a few tens of milliarcseconds). These VLBI maps often turned out to be too small in angular extent to be useful (e.g., the maps of 3C 119 and 3C 287). The most suitable radio maps were scanned in using a Ñatbed scanner preset at the resolution needed to obtain maps of similar pixel scales. The scanned radio image was then overlaid on the optical image. The absolute astrometry is not accurate enough to allow us to blindly overlay the radio structure on top of the optical, since the error in the absolute HST coordinates depends partly on the positional inaccuracy in the HST

4 194 DE VRIES ET AL. Vol. 110 FIG. 2.ÈHST and radio images of an unbiased subsample of CSS radio sources from the 3CR. For each set of images we give the gray scale of the image in the WFPC2 (top), the contour plot of the image in the WFPC2 (middle), and the contours of the radio image (described intable 1) overlaid on the gray-scale WFPC2 image (bottom). Guide Star Catalog, with a cited 1 p of 0A.33 for the northern hemisphere (see also the WFPC2 instrument handbook, Biretta et al. 1996). The 3 p relative positional error box is therefore D1A at best. In the cases where there is a positive identiðcation of the radio core, this core is placed over the optical core, which is taken to be the highest intensity point in the optical. When no radio core position is known, we placed the radio structure as symmetrically as possible with

5 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 195 respect to the optical. There are some sources for which this procedure is problematical. In the source 3C 266 it is not clear what the optical center is, so we placed the radio structure along the largest optical structure seen; this may not be a physically correct placing. A similar problem arises for the source 3C 299, for which the core position is not very well known (van Breugel et al. 1992; Liu & Pooley 1991). From the latterïs radio map, an angular distance for the core of D2A.5 (P.A. D 60 ) from the northern lobe is inferred, but with a large uncertainty. We positioned the radio core

6 196 DE VRIES ET AL. Vol. 110 over the brightest optical concentration, namely the southwest end of the bright structure in the right-hand corner (notice that the radio core is not visible in the radio map we overlaid). Once the relative registration is complete, the opticalradio alignment angle (*) was determined by the angle between the orientations of the global extents of the optical and radio emissions. Many of the quasars are stellar in appearance in our images, making a determination of the relative alignment unfeasible. For 15 of the 30 sources in our sample a determination is possible; the results can be found in Table 2. In most cases, we estimate the alignment

7 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 197 angle to be accurate to a few degrees. In the cases of 3C 266 and 3C 299 the uncertainty in the relative positioning results in a larger error. In 3C 266 the radio emission is at least parallel to most of the optical emission, and in 3C 299 the lobe positional uncertainty results in variations in * from approximately [30 to Optical Data The optical-radio overlaid images are presented in Figure 2. The contours in the optical images start at 3 p above sky and increase exponentially inward as (3 ] 2N)p above sky, N \ Notice that, in spite of the CSS galaxiesï large

8 198 DE VRIES ET AL. Vol. 110 redshift range (Fig. 1), the galaxies maintain their extended appearance. On the other hand, most quasars (except 3C 43, 3C 216, 3C 277.1, 3C 343, and 3C 455) are stellar in appearance and their images are dominated by the PSF. 3. DISCUSSION OF INDIVIDUAL SOURCES 3C 43 (quasar, z \ 1.459, m \ 20.59).ÈThe optical light is slightly extended to the south, along the direction of the radio jet, before it bends to the northeast. The identiðcation of the core with the northern knot in the jet was made by Sanghera et al. (1995). 3C 49 (galaxy, z \ 0.621, m \ 20.70).ÈThe host galaxy displays an arc of emission to the west. This arc connects to the western emission (contour plot) at lower

9 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 199 contour levels. The optical-radio alignment angle is measured as 10. 3C 67 (galaxy, z \ 0.310, m \ 18.77).ÈThis cen- trally concentrated source displays some prominent emission north and south of the nucleus, and the inferred optical axis has a position angle of 0. The Sanghera et al. (1995) core candidate is placed on top of the peak in the optical emission. The inferred alignment angle is 5. 3C 93.1 (galaxy, z \ 0.243, m \ 19.11).ÈThe radio map is a VLA map at 8.4 GHz from Akujor & Garrington

10 200 DE VRIES ET AL. Vol. 110 (1995). The source is not well resolved by these observations, so an assessment of the alignment angle is not possible. 3C 119 (galaxy, z \ 1.023, m \ 20.20).ÈThe optical spectrum of this object (Eracleous & Halpern 1994) con- Ðrms its reclassiðcation as a galaxy (Spinrad et al. 1985). However, the optical light in our HST image is dominated by the nucleus, which is more consistent with the morphology of the CSS quasars in our sample. Its total magnitude is also 2 mag brighter than that predicted by the Hubble Ðt to

11 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 201 the CSS galaxies. The absence of signiðcant extended optical emission prevents us from determining an alignment angle. 3C 138 (quasar, z \ 0.759, m \ 18.48).ÈThis object is one of the CSS quasars where some extended emission is present. The optical axis position angle is 90. We placed the radio core, as identiðed by van Breugel et al. (1992) and corroborated by Dallacasa et al. (1995), on top of the optical peak, resulting in an alignment angle of 23. 3C 147 (quasar, z \ 0.545, m \ 17.21).ÈOur image is dominated by the PSF. The two bright spots to the south of the source may be companion objects possibly associated

12 202 DE VRIES ET AL. Vol. 110 with the quasar. Superluminal motion has also been detected in this source (Alef, Preuss, & Kellermann 1990). 3C 186 (quasar, z \ 1.063, m \ 17.49).ÈOur image is dominated by the PSF. 3C 190 (quasar, z \ 1.195, m \ 18.92).ÈThis PSF- dominated quasar shows no signiðcant extended optical emission. 3C 191 (quasar, z \ 1.956, m \ 17.80).ÈAs we Ðnd in most CSS quasars, this source is heavily PSF dominated. 3C (galaxy, z \ 0.194, m \ 17.75).ÈThis galaxy has the largest optical extent in our sample. Clearly visible is the large extension to the southeast, as well as two optical counterparts to the radio hot spots. The good coincidence between the optical core and hot spots e ectively

13 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 203 rules out a chance alignment of cosmic rays (this image was only imaged once, so an unbiased removal of cosmic rays with crrej was not possible). Polarimetric observations to determine the nature of the optical emission in the northern hot spot are in progress. 3C 216 (quasar, z \ 0.670, m \ 18.80).È Superluminal motion has been detected in this source (Barthel, Pearson, & Readhead 1988). The radio source appears to extend beyond the 20 kpc cuto for CSS sources (e.g., Taylor, Ge, & OÏDea 1995). The optical image shows

14 204 DE VRIES ET AL. Vol. 110 that the source is extended toward the northeast, in the general direction of the spiral galaxy in the upper left corner. It is not known if the spiral is physically associated with 3C 216. At a redshift of 0.67, the visible extent of the spiral galaxy would be 10 kpc. Its magnitude is ^ The 5 GHz VLA radio map (Reid et al. 1995) shows a prominent extension in the same direction as the optical extension. The alignment angle is measured as 4. The optical contour map shows that the core isophotes are extended in the southeast direction. Careful comparison with a high-resolution VLBI radio map of the jet (Fejes, Porcas, & Akujor 1992) shows a good correspondence

15 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 205 between the radio jet and the suggested extension in the optical isophotes, hinting at the existence of a possible small-scale ([0A.1) optical jet. 3C 237 (galaxy, z \ 0.877, m \ 21.08).ÈThe F622W optical emission of this source is strongly aligned with the radio emission. Notice that the radio emission straddles the optical, i.e., the regions of highest intensity do not coincide. The alignment angle is D0. 3C 258 (galaxy, z \ 0.165, m \ 19.61).ÈThis oddly shaped galaxy displays an arc-like high surface-brightness structure and a larger and fainter tail fanning out to the northeast, roughly perpendicular to the bright central part.

16 206 DE VRIES ET AL. Vol. 110 The overlaid radio map (5 GHz MERLIN; Akujor et al. 1991) is at right angles with the central region, but is more or less aligned with the faint tail. This situation makes the alignment angle determination somewhat arbitrary, and we did not determine the alignment angle. 3C 266 (galaxy, z \ 1.275, m \ 21.25).È Determining the nucleus in this image of highly Ðlamentary structure is difficult. It is not clear if the bright spot to the east of the long Ðlament is associated with the source. We therefore aligned the radio source with the elongated emis-

17 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 207 sion to the west of the bright spot. The position of the radio axis di ers by 13 from that of the optical. 3C (galaxy, z \ 0.371, m \ 20.19).ÈThis galaxy displays a large tail to the southeast, approximately 5.5 h~1 kpc long. The detection of a probable core by Lu dke et al. (1997) close to the northern lobe causes the rather asymmetric positioning relative to the optical center. The inferred alignment angle is 8. 3C (quasar, z \ 0.321, m \ 17.66).ÈThis object is one of the few quasars to have readily detectable extended emission (like 3C 380). There is a bright arc of emission to the northwest of the nucleus. We deðned the

18 208 DE VRIES ET AL. Vol. 110 optical axis to run from the nucleus through the bright patch of emission to the northwest. The optical and radio axes are coincident, and we list an alignment angle of 0. 3C 286 (quasar, z \ 0.849, m \ 17.45).ÈThe HST image is dominated by the PSF. An intervening absorber at z B 0.7 (Steidel et al. 1994) is present to the southeast of the quasar, at a distance of D 2A (note the slight asymmetry in the PSF). Another faint extended feature can be seen D10A to the east of the quasar, though its relationship to the quasar is currently unknown. The radio jet extends in the opposite direction to this emission and we did not determine an alignment angle.

19 No. 2, 1997 HST IMAGING OF COMPACT RADIO SOURCES 209 3C 287 (quasar, z \ 1.055, m \ 17.47).ÈThis quasar is completely PSF dominated, and no extended optical emission is seen. The radio image (0.6 GHz VLBI; Nan Rendong et al. 1991) has a much smaller scale than the optical. 3C 298 (quasar, z \ 1.436, m \ 16.45).ÈAlthough this source is also heavily PSF dominated, an apparent excess of emission is present to the southeast; the outer contours are not centered at the nucleus. The e ect is too weak, though, to assign an optical axis to the system. 3C 299 (galaxy, z \ 0.367, m \ 19.64).ÈThe HST image shows complex structure including a system of arcs or Ðlaments to the northeast of the multimodal structure around the galaxy nucleus. The radio source is actually an asymmetric double with a separation of B13A (Liu & Pooley 1991; van Breugel et al. 1992) and is larger than the cuto for CSS sources. The radio component overlaid on the HST image is the brighter east lobe. The optical center is taken to be the brightest spot in the lower right-hand corner. From maps presented in Liu & Pooley, we measured a core-lobe distance of D2.6A in P.A. of D60, with large uncertainties. Since the scale of our optical/radio map is only 4A ] 4A, there is some freedom in positioning the lobe relative to the core by varying the position angle slightly. The radio core is not visible in the radio map we used (1.7 GHz VLBI; Spencer et al. 1991), but since the radio core is taken to coincide with the optical center we can deðne both the radio and optical axes. We measured an alignment angle of 8. This value is not as accurate as in other sources; lobe positional uncertainty gives rise to a 1 p error of D10. 3C (galaxy, z \ 0.267, m \ 18.73).ÈThis galaxy displays complex internal structure. The major axes of the isophotes twist over D35. Also, the presence of high surface-brightness structure to the southeast of the nucleus suggests a disturbed system, probably undergoing a merger. The radio structure aligns with the high surface-brightness features to within 10. With a lower optical resolution the optical axis would have been taken as the northèsouth axis, leading to an erroneous alignment angle of D40. 3C (galaxy, z \ 1.132, m \ 21.18).ÈThis high-redshift galaxy still shows a remarkable amount of Ðlamentary extended emission of high surface brightness, even though the (1 ] z)4 cosmological surface-brightness dimming a ects this source. The radio aligns within 12 of the optical. 3C (quasar, z \ 0.910, m \ 17.06).ÈRadio structure extends beyond the PSF-dominated optical emission. Because of the lack of extended optical emission, it is not possible to determine an alignment angle. 3C 343 (quasar, z \ 0.988, m \ 20.04).ÈThis quasar is elongated along P.A. D 70 and is not very centrally concentrated. Unfortunately the radio map (5 GHz MERLIN; Akujor et al. 1991) does not adequately resolve the radio structure, though it does suggest that the optical and radio emissions have similar shapes. 3C (galaxy, z \ 0.750, m \ 20.93).ÈThe optical and radio appearances and extents of this source are rather similar. We measured an alignment angle of 9. 3C 346 (galaxy, z \ 0.161, m \ 18.37).ÈThe classi- Ðcation of 3C 346 as a CSS source is probably incorrect (e.g., Dey & Van Breugel 1994; Cotton et al. 1995). Instead, the source may be a large-scale FR II radio source seen nearly end on. Both ellipticals in the image have the same redshift, implying that 3C 346 is an interacting system (Dey & Van Breugel 1994). Furthermore, Dey & Van Breugel identiðed the brightest hot spot with excess UV emission at the same position in their ground-based data. They concluded that this UV emission is due to synchrotron emission. We measured the sum of the magnitudes of the three visible knots in our image to be 20.8 ^ 0.1; the error is mainly due to the fact that the sources are superimposed on the galaxy halo. This magnitude (centered at 700 nm) corresponds to F B 14.2 ^ 1.3 kjy, where we used ] 10~18 ergs l s~1 Ó~1 as a conversion factor (the PHOTFLAM keyword in the image header, see the HST data handbook). This result is a factor of 2 lower than Dey & Van BreugelÏs estimate of 24.7 ^ 2.0 kjy at 645 nm. In our HST image we Ðnd a one-to-one correspondence between optical and radio knots in the jet, including knots B, C, and D (as deðned by van Breugel et al. 1992). This very strong correspondence makes any mechanism other than optical synchrotron emission very unlikely. Measuring the alignment angle is somewhat superñuous for this source; we adopt a value of 0. 3C 380 (quasar, z \ 0.692, m \ 16.86).ÈThis source also has a very detailed one-to-one correspondence between the optical and radio emission. Both optical hot spots to the northèwest are closely matched by peaks in the radio emission, so these optical hot spots probably originate from synchrotron radiation. Because of the close correlation, we assume an alignment angle of 0. Superluminal motion in this source was found by Wilkinson (1990), implying a viewing angle nearly perpendicular to the plane of the sky. The intrinsic source size is therefore signiðcantly larger than the 20 kpc CSS limit. 3C 454 (quasar, z \ 1.757, m \ 18.42).ÈThe HST image is PSF dominated and no signiðcant extended optical emission is detected. The radio map (Lonsdale, Barthel, & Miley 1993; 4.9 GHz VLA) is not the most detailed map available (Nan Rendong et al. 1991; Akujor et al. 1991; Spencer et al. 1991), but has a radio extent roughly comparable to our optical image. 3C 455 (quasar, z \ 0.543, m \ 19.91).ÈThe quasar is slightly extended along P.A. 50. The radio structure bends toward the ends into a C ÏÏ shape, decreasing the close match in the inner lobe region (alignment angle D0 ) to about 11 overall. 4. THE NATURE OF THE ALIGNED COMPONENTS Some discussion on the nature of the aligned optical and radio components will be given here; a more detailed account is deferred to Paper II. Various scenarios for the origin of the aligned components have been suggested, each with a di erent spectral signature. Since we do not have the necessary detailed spectral information needed to distinguish between the various scenarios, we can only make an educated guess (see Paper II). Some likely candidates are (1) jet-induced star formation (e.g., McCarthy et al. 1987; Rees 1989), in which the outward-propagating jet shock induces star formation in the interstellar medium, so that the resulting emission is continuum emission from a young stellar population, (2) shock-ionized gas (e.g., Bicknell, Dopita, & OÏDea 1997), in which the same jet shock ionizes the ambient interstellar medium, so that the radiation will be line emission only, (3) scattered nuclear light (e.g., di Serego Alighieri et al. 1989; Dey et al. 1996), in which continuum light of the active galactic nucleus is scattered into

20 210 DE VRIES ET AL. Vol. 110 the line of sight o dust particles, producing a polarized observed UV spectrum, and (4) optical synchrotron emission, in which the radio and optical emission are produced by the same population of electrons, resulting in a high spatial coincidence of radio and optical structures like hot spots and jets (e.g., 3C 346). See also McCarthy (1993) for a complete review of the alignment e ect. The Ðlter used is rather broad (FWHM D 2200 Ó), so various emission lines can be present in the images at various redshifts. At least one major emission line can contaminate the optical images for any given redshift in our sample range of z \ 0.16È1.96. In Paper II we will utilize ground-based spectroscopic data for a subset of our sources and calculate the line-emission contribution (integrated over the source) to the passband image. Given our insufficient spatial information, even a signiðcant amount of contamination (up to 25%) does not imply that the aligned component is due to line emission, i.e., the line emission may come from the nucleus rather than from the extended components. Only in the few cases where no lines are present in the groundbased spectra can we rule out line emission as the dominant mechanism of the aligned emission. Most of these sources turn out to be the ones with the detailed correspondences (3C 213.1, 3C 346 and 3C 380, for which optical synchrotron emission is the working hypothesis for the emission). In any case, because of the large range in redshift and the alignment detected in the sources, we can conclude that the alignment e ect is present over a large range of rest-frame wavelengths, from the UV to the red. 5. SUMMARY AND CONCLUSIONS We present HST WFPC2 images through a broad R-band Ðlter of a sample of 30 3CR CSS radio sources. We obtain the following results. (1) All CSS sources for which the relative orientation between the optical and radio can be measured display good alignment between the optical and radio emission down to the lowest redshift in the sample, z D 0.1. The alignment e ect does not occur at this relatively low redshift for the large-scale 3CR radio sources, which tend to show signiðcant alignment only at z [ 0.6 (McCarthy et al. 1987; McCarthy, Spinrad, & van Breugel 1995; Chambers, Miley, & van Breugel 1987; de Ko et al. 1996). (2) We Ðnd candidates for optical synchrotron hot spots in 3C and 3C 380 and an optical jet in 3C 346. These results are discussed further in Paper II. We would like to thank Dr. Robert Laing for supplying us with information on (unpublished) core positions of several sources. G. K. M. acknowledges support from a European Union research grant, a NATO research grant, and a Programme Subsidy from the Netherlands Organisation for Pure Research (NWO). C. P. O. and S. A. 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