Tightness of the color-magnitude relation of elliptical galaxies and the epoch of major galaxy merging

Abstract

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.

25 pages, 6 figures, ApJ in press