Radio-selected Galaxies in Very Rich Clusters at z<=0.25. II. Radio Properties and Analysis
Owen, Frazer N.; Morrison, Glenn E.
United States
Abstract
We report on the properties of radio-selected galaxies within 30 very rich Abell clusters with z<~0.25. The radio, optical, and X-ray data for these clusters were presented in Paper I. These radio data sample the ultrafaint (L1.4>=2×1022 W Hz-1) radio galaxy population with MR<=-21 using the well-known FIR-radio correlation to link the radio with ongoing star formation within individual cluster galaxies. Spectroscopic redshifts exist for ~96% of the optical identifications. These radio-selected galaxies reveal the ``active'' galaxy population (starbursts and active galactic nuclei [AGNs]) within these rich cluster environments that can be identified regardless of their level of dust obscuration. These new radio data provide the largest sample to date of low-luminosity radio galaxies within rich cluster environments, allowing an unbiased search for dusty starbursting galaxies. For all clusters in our sample, we are sensitive to star formation rates (M>=5 Msolar) >~5 Msolar yr-1. We have found that the excess number of low-luminosity ``starburst'' radio-selected galaxies (SBRGs) found previously by Owen et al. in Abell 2125 is not indicative of other rich clusters in our sample. The average fraction of SBRGs is <fSBRG>=0.022+/-0.003. The A2125 fraction is fSBRG=0.09+/-0.03, which is significantly different from the sample average at a greater than 99.99% confidence level. Both A1278 and A1689 are slightly different from the rest of the sample at ~90% confidence level. The bimodal structure of both the X-ray brightness distribution and optical adaptively smoothed images of A1278 and A2125 suggests that ongoing cluster-cluster mergers may be enhancing this SBRG population. The A1689 excess low-luminosity (and high-luminosity) radio galaxy population may be due to interaction with the intracluster medium. The mid-infrared ISOCAM results for A1689's radio galaxy population suggest that the radio emission for both low- and high-luminosity radio galaxies is AGN in origin except for one radio galaxy. There is a significant spatial distribution difference between the low- and high-luminosity (HLRG) radio-selected populations. The SBRGs have a core radius of 0.40+/-0.08 Mpc, which is greater than 3 times larger than the HLRG core radius. In addition, 48% of the SBRGs have colors that are bluer than a typical Sab galaxy, compared with 4% for the HLRGs. The average absolute magnitude for the SBRGs is <MR>=-21.93+/-0.05, while for the HLRGs it is <MR>=-22.33+/-0.07, indicating that the SBRGs are less optically luminous than their HLRG counterparts. The HLRGs seem to be a subclass of the cluster's massive red elliptical population, while the SBRGs have a projected radial distribution more like the blue spiral population. Our results indicate that most of the SBRGs are probably gas-rich disk galaxies undergoing >~5 Msolar yr-1 of star formation.