Galaxy Population in a Cluster of Galaxies around the Radio Galaxy 3C 324 at $z=1.2$

Abstract

Abstract We discuss the properties of galaxies around the radio galaxy 3C 324 at $z=1.2$ based on $BVRIK^\prime$ multi-band imaging data. We have applied a photometric-redshift technique to objects in the 3C 324 field, and identified 35 objects as plausible cluster members. We have found that red and luminous members are concentrated in a small region enclosed by a circle of 40$^\prime$ radius (0.33Mpc at $z=1.2$ for $\Omega_0 = 0.3, \lambda_0 = 0.7, H_0 = 70 \,\mathrm{km} \,\mathrm{s}^{-1} \,\mathrm{Mpc}^{-1}$ cosmology) from the 3C 324 galaxy. The 3C 324 cluster is probably much more compact in size compared with the local clusters. We constructed a $K^\prime$-band luminosity function of the cluster members and fitted a Schechter function, and found the characteristic magnitude to be $K^{\ast\prime}_\mathrm{AB} = 20.2 \pm 0.6$. This value is consistent with the extrapolation of the pure passive evolution seen for $z \lt 1$ clusters. We have identified eight bright galaxies which form a red color–magnitude sequence. The slope of the sequence is consistent with the passive evolution model down to $K^{\prime}_\mathrm{AB} \lt 22$; we also found that there is no clear age variation in these bright red galaxies. However, seven out of these eight galaxies exhibit a significant excess in the rest UV light with respect to the passive evolution model. This may suggest that the massive early-type galaxies in this high-redshift cluster are still forming stars to some extent. We have confirmed a truncation of the color–magnitude sequence at $K^{\prime}_\mathrm{AB}\sim22$; faint passively-evolving galaxies may not yet be present in this cluster at $z \sim 1.2$. The overall color distribution of the cluster members, selected by the photometric redshift technique, is found to be very broad. We derived the fraction of blue galaxies in this cluster following a definition of Butcher and Oemler (1984, AAA 038.160.068), and obtained $f_\mathrm{B} = 0.39 \pm 0.28$, which is higher than that for $z \lt 1$ clusters. This indicates that the star-formation activity of this cluster is, on the average, higher than that of lower redshift counterparts.