Andrew Bunker

Abstract:

We present a catalog of 74 galaxies detected serendipitously during a campaign of spectroscopic observations of the Hubble Deep Field North (HDF) and its environs. Among the identified objects are five candidate Lyα emitters at z ≳ 5, a galaxy cluster at z = 0.85, and a Chandra source with a heretofore undetermined redshift of z = 2.011. We report redshifts for 25 galaxies in the central HDF, 13 of which had no prior published spectroscopic redshift. Of the remaining 49 galaxies, 30 are located in the single-orbit HDF flanking fields. We discuss the redshift distribution of the serendipitous sample, which contains galaxies in the range 0.10 < z < 5.77 with a median redshift of z = 0.85, and we present strong evidence for redshift clustering. By comparing our spectroscopic redshifts with optical/IR photometric studies of the HDF, we find that photometric redshifts are in most cases capable of producing reasonable predictions of galaxy redshifts. Finally, we estimate the line-of-sight velocity dispersion and the corresponding mass and expected X-ray luminosity of the galaxy cluster, we present strong arguments for interpreting the Chandra source as an obscured active galactic nucleus, and we discuss in detail the spectrum of one of the candidate z ≳ 5 Lyα emitters.

Abstract:

(Abridged) This is the second in a series of three papers which present and interpret basic observational data on the 6C* 151-MHz radio sample: a low-frequency selected sample which exploits filtering criteria based on radio properties (steep spectral index and small angular size) to find radio sources at redshift z > 4 within a 0.133sr patch of sky. We present results of a programme of optical spectroscopy which has yielded redshifts in the range 0.5 < z < 4.4 for the 29 sources in the sample, all but six of which are secure. We find that the fil tering criteria used for 6C* are very effective in excluding the low-redshift, low-luminosity radio sources: the median redshift of 6C* is z~1.9 compared to z~1.1 for a complete sample matched in 151-MHz flux density. By combining the emission-line dataset for the 6C* radio sources with those for the 3CRR, 6CE and 7CRS samples we establish that z > 1.75 radio galaxies follow a rough proportionality between Lyalpha- and 151 MHz-luminosity which, like similar correlations seen in samples of lower-redshift radio sources, are indicative of a primary link between the power in the source of the photoionising photons (most likely a hidden quasar nucleus) and the power carried by the radio jets. We argue that radio sources modify their environments and that the range of emission-line properties seen is determined more by the range of source age than by the range in ambient environment. This is in accord with the idea that all high-redshift, high-luminosity radio sources are triggered in similar environments, presumably recently collapsed massive structures.

Abstract:

In this paper, the third and final of a series, we present complete K-band imaging and some complementary I-band imaging of the filtered 6C* sample. We find no systematic differences between the K-z relation of 6C* radio galaxies and those from complete samples, so the near-infrared properties of luminous radio galaxies are not obviously biased by the additional 6C* radio selection criteria (steep spectral index and small angular size). The 6C* K-z data significantly improve delineation of the K-z relation for radio galaxies at high-redshift (z >2). Accounting for non-stellar contamination, and for correlations between radio luminosity and stellar mass, we find little support for previous claims that the underlying scatter in the stellar luminosity of radio galaxies increases significantly at z >2. In a particular spatially-flat universe with a cosmological constant, the most luminous radio sources appear to be associated with galaxies with a luminosity distribution with a high mean (~5 Lstar), and a low dispersion (sigma ~ 0.5 mag) which formed their stars at epochs corresponding to z >~2.5. This result is in line with recent sub-mm studies of high-redshift radio galaxies and the inferred ages of extremely red objects from faint radio samples.