Spectroscopic confirmation of multiple red galaxy-galaxy mergers in MS 1054-03 (z = 0.83)

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 < 10h(-1) kpc and relative line-of-sight velocities of Delta nu < 165 kms(-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,Delta nu <= 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 color-magnitude diagram shows that the pair galaxies are as red as the E/S0 members and have a s 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*,sigma(1D) > 200 km s(-1)); these bound 1D galaxy pairs must evolve into E/S0 members by z similar to 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.