Radio Detection of Radio-Quiet Galaxies

We investigate the radio emission of ~185,000 quiescent (optically unclassifiable) galaxies selected from the Sloan Digital Sky Survey (SDSS). By median-stacking Faint Images of the Radio Sky at Twenty Centimeter (FIRST) cutouts centered on the optically-selected sources, we are able to reach flux densities down to the 10s of μJy. The quiescent galaxy sample is composed of two subgroups inhabiting vastly different regimes: those targeted for the SDSS MAIN galaxy sample (~55%) and those targeted for the Luminous Red Galaxy (LRG) sample (~45%). To investigate the star formation of these quiescent galaxies, we calibrate a radio to star-formation rate (SFR) conversion using a third sample of star-forming (SF) galaxies. We confirm a tight power-law dependence for the SF sample, where L _1.4GHz ~ (SFR)^1.37. Comparing this SFR-indicator with indicators in the optical and UV, we derive conflicting SFR estimates for the MAIN sample quiescent galaxies. These radio-derived SFRs intersect those calculated using the 4000 Å break (D4000) around an SFR of 1 M _sun yr^-1 and agree to within a factor of 3 over the range of SFRs. However, we find that the radio-derived SFRs are too high relative to the SFRs estimated for similar populations of galaxies using analysis of UV emission, implying either contamination of the radio by Active Galactic Nuclei (AGNs) or incomplete dust modeling. If AGN activity is dominant in these galaxies, then a relation between AGN radio luminosity and galaxy mass is required to explain the observed trends. For the LRGs, on the other hand, we find the radio luminosity to be relatively high (compared to the SF galaxies) and independent of SFR as derived from D4000, indicating that an AGN component dominates their radio emission. AGN-based radio emission often implies the existence of radio jets, providing evidence of a mechanism for low-level feedback in these quiescent LRGs.