Nature of Faint Radio Sources in GOODS-North and GOODS-South Fields. I. Spectral Index and Radio–FIR Correlation

Abstract

We present the first results from the deep and wide 5 GHz radio observations of the Great Observatories Origins Deep Survey (GOODS)-North (sigma = 3.5 mu Jy beam(-1), synthesized beam size theta = 1.'' 47 x 1.'' 42, and 52 sources over 109 arcmin(2)) and GOODS-South (sigma = 3.0 mu Jy beam(-1), theta = 0.'' 98 x 0.'' 45, and 88 sources over 190 arcmin(2)) fields using the Karl G. Jansky Very Large Array. We derive radio spectral indices alpha between 1.4 and 5 GHz using the beam-matched images and show that the overall spectral index distribution is broad even when the measured noise and flux bias are considered. We also find a clustering of faint radio sources around alpha = 0.8, but only within S-5 GHz < 150 mu Jy. We demonstrate that the correct radio spectral index is important for deriving accurate rest-frame radio power and analyzing the radio-FIR correlation, and adopting a single value of alpha = 0.8 leads to a significant scatter and a strong bias in the analysis of the radio-FIR correlation, resulting from the broad and asymmetric spectral index distribution. When characterized by specific star formation rates, the starburst population (58%) dominates the 5 GHz radio source population, and the quiescent galaxy population (30%) follows a distinct trend in spectral index distribution and the radio-FIR correlation. Lastly, we offer suggestions on sensitivity and angular resolution for future ultra-deep surveys designed to trace the cosmic history of star formation and AGN activity using radio continuum as a probe.