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Radio-selected Galaxies in Very Rich Clusters at z ≤ 0.25. II. Radio Properties and Analysis

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 M⊙) ≳5 M⊙ 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 M⊙ yr-1 of star formation.