M/L_B and Color Evolution for a Deep Sample of M ^sstarf Cluster Galaxies at z ~ 1: The Formation Epoch and the Tilt of the Fundamental Plane

Abstract

We have measured velocity dispersions (σ) for a sample of 36 galaxies with J < 21.2 or M_r < -20.6 mag in MS 1054-03, a massive cluster of galaxies at z = 0.83. Our data are of uniformly high quality down to our selection limit, our 16 hr exposures typically yielding errors of only δ(σ) ~ 10% for L* and fainter galaxies. By combining our measurements with data from the literature, we have 53 cluster galaxies with measured dispersions, and HST/ACS-derived sizes, colors and surface brightness. This sample is complete for the typical L ^sstarf galaxy at z ~ 1, unlike most previous z ~ 1 cluster samples which are complete only for the massive cluster members (>10¹¹ M _sun). We find no evidence for a change in the tilt of the fundamental plane (FP). Nor do we find evidence for evolution in the slope of the color-σ relation and M/L_B -σ relations; measuring evolution at a fixed σ should minimize the impact of structural evolution found in other work. The M/L_B at fixed σ evolves by Δlog₁₀ M/L_B = -0.50 ± 0.03 between z = 0.83 and z = 0.02 or dlog₁₀ M/L_B = -0.60 ± 0.04 dz, and we find Δ(U - V) _z = -0.24 ± 0.02 mag at fixed σ in the rest frame, matching the expected evolution in M/L_B within 2.25 standard deviations. The implied formation redshift from both the color and M/L_B evolution is z _sstarf = 2.0 ± 0.2 ± 0.3(sys), during the epoch in which the cosmic star formation activity peaked, with the systematic uncertainty showing the dependence of z _sstarf on the assumptions we make about the stellar populations. The lack of evolution in either the tilt of the FP or in the M/L-σ and color-σ relations imply that the formation epoch depends weakly on mass, ranging from z _sstarf = 2.3^+1.3 _-0.3 at σ = 300 km s^-1 to z _sstarf = 1.7^+0.3 _-0.2 at σ = 160 km s^-1 and implies that the initial mass function similarly varies slowly with galaxy mass.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555. These observations are associated with programs nos. 9290, 9772, and 9919.

This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.