Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers in MS 1054-03 (z = 0.83)1,

Abstract

We present follow-up spectroscopy of the galaxy cluster MS 1054-03 (z=0.83) confirming that at least six of the nine merging galaxy pairs identified by van Dokkum et al. in 1999 are indeed bound systems: they have projected separations of R_s<10 h^-1 kpc and relative line-of-sight velocities of Δv<165 km s^-1. For the remaining three pairs, we were unable to obtain redshifts of both constituent galaxies. To identify a more objective sample of merging systems, we select bound red galaxy pairs (R_s<=30 h^-1 kpc, Δv<=300 km s^-1) from our sample of 121 confirmed cluster members: galaxies in bound red pairs make up 15.7%+/-3.6% of the cluster population. The (B-K_s) color-magnitude diagram shows that the pair galaxies are as red as the E/S0 members and have a homogeneous stellar population. The red pair galaxies span a large range in luminosity and internal velocity dispersion, to include some of the brightest, most massive members (L>L^*, σ_1D>200 km s^-1) these bound galaxy pairs must evolve into E/S0 members by z~0.7. These results, combined with MS 1054's high merger fraction and reservoir of likely future mergers, indicates that most, if not all, of its early-type members evolved from (passive) galaxy-galaxy mergers at z<~1.

Based on observations obtained at the W. M. Keck Observatory, which is operated jointly by Caltech and the University of California.

Based on observations obtained at the European Southern Observatory, Paranal, Chile.