Deep WFPC2 and Ground-based Imaging of a Complete Sample of 3C Quasars and Galaxies

We present the results of an HST and ground-based imaging study of a complete 3C sample of z ~ 1 sources, including 5 quasars and 5 radio galaxies. We have resolved continuum structure around all of our quasars in the WFPC2 images and in four of the five ground-based K' images. All of the quasars have some optical continuum structure that is aligned with the radio axis. In 3 of these cases, some of this optical structure is most likely due to optical synchrotron radiation, including optical counterparts to two radio jets and one radio lobe. Two quasars have aligned continuum and emission-line structures that are probably not due to beamed optical synchrotron emission. In another quasar, we see a red aligned object that lies 3 arcsec beyond the radio lobe, and may be an unassociated foreground galaxy, but has a remarkable morphological resemblance to the radio lobe itself. The radio galaxies and the quasars in this small sample have similar incidence of alignment, and the optical and K' flux densities are consistent within the high dispersion. The average quasar host galaxy luminosity is equivalent to, or a little fainter than, L*. All components around the quasars have optical-infrared colors that are redder than or similar to the colors of their respective nuclei; this is generally more consistent with a stellar rather than scattered origin for the emission. This study provides qualitative support for the unification of FRII quasars and galaxies.Comment: 69 pages, LaTeX (aaspp4.sty); 10 tables (aj_pt4.sty); 22 figures; accepted to A.J., August 199

Colors and magnitudes of quasar host galaxies at high redshift

We discuss the results so far of a program to image in the rest-frame optical and near-UV the host galaxies of a sample of 5 high redshift (z ~ 2–3) radio-quiet quasars with nuclear luminosities lower than most samples studied at high z. We found that in the rest-frame optical the hosts have luminosities of about L*, comparable to the Lyman break galaxies at similar redshifts. From the rest-frame UV imaging, we can derive optical–UV colors, and find they show a great deal of variation, but may be somewhat redder and less starforming than the LBGs. We are supplementing this work at high z with study of the hosts of z ~ 1 radio-quiet quasars with similar low nuclear luminosities

Deep Spectroscopy in the Field of 3C 212

We present the results of longslit and multiaperture spectroscopy of faint galaxies in the field of the z=1.049 quasar 3C212. We show that an apparently aligned optical feature beyond the NW radio lobe has a redshift z=0.928, and a similarly aligned feature just beyond the SE radio lobe has a redshift z=1.053, quite close to that of the quasar. While the NW optical component is extremely well aligned with the radio jet and has a morphology that is very similar to that of the radio lobe lying 3" interior to it, the fact that we find three other field galaxies with closely similar redshifts indicates that it is most likely a member of an intervening group rather than an unusual example of true alignment with the radio structure. In addition, we have found two galaxies (besides the one near the SE radio lobe) having redshifts close to that of 3C212. We have firm or probable redshifts for 66 out of 82 galaxies we have observed in this field. Comparison with the redshift distribution of faint field galaxies indicates that a large fraction of the remaining 16 galaxies are likely to be at redshifts >1.4. There are at least two low-redshift dwarf starburst galaxies showing low metal abundance and high ionization in our sample; such galaxies are likely to be relatively common in very faint samples, and, under certain conditions, they could be confused with high-redshift objects.Comment: 15 pages, 4 figures, one of which is a separate JPEG plate. To be published in the April 1998 issue of The Astronomical Journa

Radio-optical alignments in a low radio luminosity sample

We present an optically-based study of the alignment between the radio axes and the optical major axes of eight z~0.7 radio galaxies in a 7C sample. The radio galaxies in this sample are ~20-times less radio luminous than 3C galaxies at the same redshift, and are significantly less radio-luminous than any other well-defined samples studied to date. Using Nordic Optical Telescope images taken in good seeing conditions at rest-frame wavelengths just longward of the 4000A break, we find a statistically significant alignment effect in the 7C sample. Furthermore, in two cases where the aligned components are well separated from the host we have been able to confirm spectroscopically that they are indeed at the same redshift as the radio galaxy. However, a quantitative analysis of the alignment in this sample and in a corresponding 3C sample from HST archival data indicates that the percentage of aligned flux may be lower and of smaller spatial scale in the 7C sample. Our study suggests that alignments on the 50-kpc scale are probably closely related to the radio luminosity, whereas those on the 15 kpc scale are not. We discuss these results in the context of popular models for the alignment effect.Comment: 16 pages, 8 figures. Accepted by MNRA

High-Resolution K' Imaging of the z = 1.786 Radio Galaxy 3C 294

We have obtained imaging in the K' band (~I-band rest frame) of the z=1.786 radio galaxy 3C 294 with the 36-element curvature-sensing adaptive optics system Hokupa`a and the Canada-France-Hawaii Telescope. At a resolution of < \~0."15, the galaxy is seen as a group of small but resolved knots distributed over a roughly triangular region ~1."4 across. The interpretation of the structure depends on the location of the nucleus, as indicated by the compact radio core. Its position is uncertain by > ~0."5 (2-sigma) because of uncertainties in the optical astrometry, but our best estimate places it at or near the southern apex of the distribution. If this location is correct, the most likely interpretation is that of a hidden quasar nucleus illuminating dusty infalling dwarf-galaxy-like clumps having characteristic sizes of ~1.5 kpc.Comment: 8 pages. One figure in gif format. Postscript version including fig. (240 kb) available at http://www.ifa.hawaii.edu/~canaguby/preprints.html To appear in ApJ. Letter

Optical spectroscopy of two overlapping, flux-density-limited samples of radio sources in the North Ecliptic Cap, selected at 38 MHz and 151 MHz

We present the results of optical spectroscopy of two flux-density-limited samples of radio sources selected at frequencies of 38 and 151 MHz in the same region around the North Ecliptic Cap, the 8C-NEC and 7C-III samples respectively. Both samples are selected at flux density levels ~20 times fainter than samples based on the 3C catalogue. They are amongst the first low-frequency selected samples with no spectral or angular size selection for which almost complete redshift information has been obtained and they will therefore provide a valuable resource for understanding the cosmic evolution of radio sources and their hosts and environments. The 151-MHz 7C-III sample is selected to have S_151 >=0.5 Jy and is the more spectroscopically complete; out of 54 radio sources fairly reliable redshifts have been obtained for 44 objects. The 8C sample has a flux limit of S_38 >=1.3 Jy and contains 58 sources of which 46 have fairly reliable redshifts. We discuss possible biases in the observed redshift distribution, and some interesting individual objects. Using the 8C-NEC and 7C-III samples in conjunction, we form the first sample selected on low-frequency flux in the rest-frame of the source, rather than the usual selection on flux density in the observed frame. This allows us to remove the bias associated with an increasing rest-frame selection frequency with redshift. We investigate the difference this selection makes to correlations of radio source properties with redshift and luminosity. We show in particular that flux-density-based selection leads to an overestimate of the steepness of the correlation of radio source size with redshift. (abridged)Comment: 22 pages, 10 figures, accepted by MNRA

The stellar, molecular gas and dust content of the host galaxies of two z~2.8 dust obscured quasars

We present optical through radio observations of the host galaxies of two dust obscured, luminous quasars selected in the mid-infrared, at z=2.62 and z=2.99, including a search for CO emission. Our limits on the CO luminosities are consistent with these objects having masses of molecular gas <~10^10 solar masses, several times less than those of luminous submillimeter-detected galaxies (SMGs) at comparable redshifts. Their near-infrared spectral energy distributions, however, imply that these galaxies have high stellar masses (~10^11-12 solar masses). The relatively small reservoirs of molecular gas and low dust masses are consistent with them being relatively mature systems at high-z.Comment: AJ, in pres

A radio-jet -- galaxy interaction in 3C441

Multi-wavelength imaging and spectroscopy of the z=0.708 radio galaxy 3C441 and a red aligned optical/infrared component are used to show that the most striking aspect of the radio-optical ``alignment effect'' in this object is due to the interaction of the radio jet with a companion galaxy in the same group or cluster. The stellar population of the red aligned continuum component is predominately old, but with a small post-starburst population superposed, and it is surrounded by a low surface-brightness halo, possibly a face-on spiral disc. The [OIII]500.7/[OII]372.7 emission line ratio changes dramatically from one side of the component to the other, with the low-ionisation material apparently having passed through the bow shock of the radio source and been compressed. A simple model for the interaction is used to explain the velocity shifts in the emission line gas, and to predict that the ISM of the interacting galaxy is likely to escape once the radio source bow shock has passed though. We also discuss another, much fainter, aligned component, and the sub-arcsecond scale alignment of the radio source host galaxy. Finally we comment on the implications of our explanation of 3C441 for theories of the alignment effect.Comment: 9 pages, 8 figures. Accepted for publication by MNRA