Tightness of the color-magnitude relation of elliptical galaxies and the   epoch of major galaxy merging
  We investigate a one-zone chemophotometric evolution model of disk-disk galaxy mergers in order to clarify whether or not galaxy mergers with the widely spread merging epoch can reproduce reasonably well the observed small scatter of the color-magnitude (C-M) relation in cluster ellipticals at low and intermediate redshift (z<1). We consider that merger progenitor disks begin to consume interstellar gas at moderate rate from z ~ 5 and then merge to form an elliptical with the secondary starburst at z=z_merge. We find that even if the epoch of galaxy merging is rather extended (0.3<z_merge<3.0), the dispersion in the rest-frame U-V color among galaxy mergers is well within the observed one (~0.05 mag at z = 0). We also find that the z_merge is required to be within a certain range to keep the observed C-M relation tight at a given z. For example, the required range of z_merge in galaxy mergers between Sa disks is 1.3<z_merge<3.0 for cluster ellipticals at z = 0.895, 0.9<z_merge<3.0 for z = 0.55, and 0.3<z_merge<3.0 for z = 0. The main reason for the derived small scatter is that younger stellar populations, which are formed during the secondary starburst of galaxy mergers, are formed preferentially from more metal-enriched interstellar gas. This result reinforces the Worthey's suggestion (Worthey et al. 1996) that the age-metallicity conspiracy, which means that younger stellar populations are preferentially more metal-enriched, can operate to keep the tight C-M relation. These numerical results imply that the observed small scatter in the C-M relation at low and intermediate redshift (z < 1) does not necessarily require the coevality of elliptical galaxies in clusters or their formation at high z, which has been conventionally believed in the classical passive evolution picture. 
Yasuhiro Shioya and Kenji Bekki