Faint, Evolving Radio AGN in SDSS Luminous Red Galaxies

TL;DR

This study detects and analyzes faint radio AGN in Luminous Red Galaxies from SDSS, revealing their evolving properties and potential cosmic downsizing, with implications for galaxy evolution and AGN activity over redshift.

Contribution

It introduces a stacking method to study faint radio AGN in LRGs and uncovers their evolving radio-optical luminosity relation and cosmic evolution.

Findings

Radio luminosity scales with optical luminosity, decreasing with redshift.

Significant cosmic evolution of low-luminosity AGN in LRGs from z=0.2 to 0.7.

Increased AGN heating in massive ellipticals at higher redshifts.

Abstract

We detect and study the properties of faint radio AGN 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 microJy, 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_1.4 GHz ~ L_opt^(beta), where beta appears to decrease from beta ~ 1 at z = 0.4 to beta ~ 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 AGN, experiences significant cosmic evolution between z = 0.2 and z = 0.7. This implies 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. (Abridged)