Radio-selected Galaxies in Very Rich Clusters at [CLC][ITAL]z[/ITAL][/CLC] ≤ 0.25. II. Radio Properties and Analysis

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 (Morrison et al. 2002). These radio data sample the ultra-faint (L1.4 ≥ 2 × 10 22 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 (starburst and active galactic nuclei) 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 ≥ 5M⊙) & 5 M⊙yr −1 . We have found that the excess number of low-luminosity ‘starburst’ radio-selected galaxies (SBRG) found by Owen et al. (1999) in Abell 2125 is not indicative of other rich clusters in our sample. The average fraction of SBRG is h fSBRG i = 0.022 ± 0.003. The A2125 fraction is fSBRG = 0.09 ± 0.03 which is significantly different from the sample average at a > 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 ICM. The mid-infrared ISOCAM results for A1689’s radio galaxy population suggests 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 SBRG have a core radius of 0.40 ± 0.08 Mpc which is > 3× larger than the HLRG core radius. In addition, 48% of the SBRGs have colors that are bluer than a typical Sab galaxy compared to 4% for the HLRGs. The average absolute magnitude for the SBRG’s is h MR i = −21.93 ± 0.05, while for the HLRG’s it is h MR i = −22.33 ± 0.07, indicating that the SBRG 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. Subject headings: galaxies: evolution — galaxies: clusters: Abell — galaxies: starburst — radio continuum: galaxies