HUBBLE SPACE TELESCOPE Imaging of the Host Galaxies of High‐Redshift Radio‐loud Quasars

Abstract

We present rest-frame UV and Lyα images of spatially resolved structures (hosts) around five high-redshift radio-loud quasars obtained with the WFPC2 camera on the Hubble Space Telescope (HST). The quasars were imaged with the PC1 through the F555W (V-band) filter, which at the redshifts of the quasars (2.1 < z < 2.6) have central wavelengths of λrest ≈ 1500-1800 A, and at rest-frame Lyα using appropriately chosen narrowband filters with the WFC2. The objects were selected from ground-based imaging surveys. Those had shown that many radio-loud quasars at high redshift have prominent host galaxies that appeared to have properties similar to those of high-redshift radio galaxies. Our HST observations allow a more detailed investigation of quasar host morphologies and a comparison with similar HST studies of radio galaxies by others. Using several methods to measure and quantify the host properties we find that all five quasars are extended and that this fuzz contains ≈5%-40% of the total continuum flux and 15%-65% of the Lyα flux within a radius of about 15. The rest-frame UV luminosities of the hosts are log λPλ ≈ 11.9-12.5 L☉ (assuming no internal dust extinction), comparable to the luminous radio galaxies at similar redshifts and a factor 10 higher than both radio-quiet field galaxies at z ~ 2-3 and the most UV-luminous low-redshift starburst galaxies. The Lyα luminosities of the hosts are log LLyα ≈ 44.3-44.9 ergs s-1, which are also similar to the those of luminous high-redshift radio galaxies and considerably larger than the Lyα luminosities of high-redshift field galaxies. To generate the Lyα luminosities of the hosts would require roughly a few percent of the total observed ionizing luminosity of the quasar. The UV continuum morphologies of the hosts appear complex and knotty at the relatively high surface brightness levels of our exposures (about 24 V mag arcsec-2). In two quasars we find evidence for foreground galaxies that confuse the host galaxy morphologies and that are responsible for some of the perceived radio/optical misalignments observed in ground-based imaging data. We do find good alignment between the extended Lyα and the radio sources, strong evidence for jet-cloud interactions in two cases, again resembling radio galaxies, and what is possibly the most luminous radio-UV synchrotron jet in one of the hosts at z = 2.110. We discuss the significance of jet-cloud collisions in radio-loud quasars and their influence on radio morphologies in the framework of simple orientation-based quasar/radio galaxy unification schemes. Our observations suggest that the host galaxies of radio-loud steep-spectrum quasars are similar to those of radio galaxies and strengthen previous conclusions based on ground-based data that both types of objects are probably members of the same parent population.