The Nature of Radio Emission from Distant Galaxies

Abstract

I describe an observational program aimed at understanding the radio emission from distant, rapidly evolving galaxy populations. These observations were carried out at 1.4 and 8.5 GHz with the VLA centered on the Hubble Deep Field. Further MERLIN observations of the HDF region at 1.4 GHz provided an angular resolution of 0.2" and when combined with the VLA data produced an image with an unprecedented rms noise of 4 $\mu$Jy. All radio sources detected in the VLA complete sample are resolved with a median angular size of 1-2". The differential count of the radio sources is marginally sub-Euclidean ($\gamma$ = -2.4 $\pm$ 0.1) and fluctuation analysis suggests nearly 60 sources per arcmin$^2$ are present at the 1 $\mu$Jy level. A correlation analysis indicates spatial clustering among the 371 radio sources on angular scales of 1- 40 arcmin. Optical identifications are made primarily with bright (I = 22) disk systems composed of irregulars, peculiars, interacting/merging galaxies, and a few isolated field spirals. Available redshifts span the range 0.2 - 3. These clues coupled with the steep spectral index of the 1.4 GHz selected sample are indicative of diffuse synchrotron radiation in distant galactic disks. Thus the evolution in the microjansky radio population is driven principally by star-formation. I have isolated a number of optically faint radio sources (about 25% of the overall sample) which remain unidentified to I = 26-28 in the HDF and flanking optical fields. Several of these objects have extremely red counterparts and constitute a new class of radio sources which are candidate high-z dusty protogalaxies.