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 \prime } radius ( 0.33 Mpc at z = 1.2 for \Omega _ { 0 } = 0.3 , \lambda _ { 0 } = 0.7 , H _ { 0 } = 70 km s ^ { -1 } 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 fit a Schechter function , and found the characteristic magnitude to be K ^ { \prime \ast } _ { AB } = 20.2 \pm 0.6 .
This value is consistent with the extrapolation of the pure passive evolution seen for z < 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 } _ { AB } < 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 } _ { AB } \sim 22 ; 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 ) , and obtained f _ { B } = 0.39 \pm 0.28 , which is higher than that for z < 1 clusters .
This indicates that the star-formation activity of this cluster is , on the average , higher than that of lower redshift counterparts .