FAINT, EVOLVING RADIO ACTIVE GALACTIC NUCLEI IN SDSS LUMINOUS RED GALAXIES

We detect and study the properties of faint radio active galactic nuclei (AGNs) in luminous red galaxies (LRGs). The LRG sample comprises 760,000 objects from a catalog of LRG photometric redshifts constructed from the Sloan Digital Sky Survey (SDSS) imaging data, and 65,000 LRGs from the SDSS spectroscopic sample. These galaxies have typical 1.4 GHz flux densities in the 10s-100s of μJy, with the contribution from a low-luminosity AGN dominating any contribution from star formation. To probe the radio properties of such faint objects, we employ a stacking technique whereby FIRST survey image cutouts at each optical LRG position are sorted by the parameter of interest and median-combined within bins. We find that median radio luminosity scales with optical luminosity (L opt) as L 1.4GHz ∝ L βopt, where β depends on the redshift being probed. Above z ≅ 0.4, β appears to decrease from β ≅ 1 at z = 0.4 to β ≅ 0 at z = 0.7, a result which could be indicative of AGN cosmic downsizing. We also find that the overall LRG population, which is dominated by low-luminosity AGNs, experiences significant cosmic evolution between z = 0.2 and z = 0.7. A simultaneous fit to untangle the redshift and luminosity dependences yields redshift evolution of the form L 1.4GHz ∝ (1 + z)3.15±0.07, implying a considerable increase in total AGN heating for these massive ellipticals with redshift. By matching against the FIRST catalog, we investigate the incidence and properties of LRGs associated with double-lobed (FR I/II) radio galaxies.