Spectroscopic Confirmation of Multiple Red Galaxy - Galaxy Mergers in Ms 1054

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. (1999) are indeed bound systems: they have projected separations of Rs < 10h −1 kpc and relative line-of sight velocities of ∆v < 165 km s. 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 (Rs ≤ 30h −1 kpc, ∆v ≤ 300 km s) 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 −Ks) 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, σ1D > 200 km s ); these bound galaxy pairs must evolve into E/S0 members by z ∼ 0.7. These results combined with MS1054’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. Subject headings: galaxies: clusters: individual (MS 1054-03) – galaxies: elliptical and lenticular, cD – galaxies: fundamental parameters – galaxies: evolution