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Session 47 - Old Stellar Populations Beyond The Milky Way - II.
Oral session, Wednesday, June 11
North Main Hall A,
The reddest and most luminous galaxies form a ridge line in color-magnitude diagrams at bright apparent magnitudes, referred to as the ``red envelope.'' At sufficiently faint apparent magnitudes, this envelope is expected to shift bluewards or disappear because one is viewing back to redshifts where today's galaxies with old, red stellar populations are seen at younger ages when they are bluer and brighter. In a simple scenario where early-type galaxies form all of their stars at an early time and then evolve ``passively,'' the demise of the red envelope is sensitive to the epoch and coevality of early-type galaxy formation. However, subsequent bursts of star formation or galaxy merging will alter this simple picture. The near-infrared is ideal for tracking the red envelope because colors can be modeled reliably to high redshift and are relatively insensitive to small bursts of star formation. We present an analysis of the red envelope observed in deep J and K band imaging from the Keck 10m and KPNO 4m telescopes. The numbers of red galaxies consistent with old stellar populations to K = 23 are compared to expectations for various formation epochs. We find a dearth of systems corresponding to L > L ellipticals at z>1. We compare these findings with a summary of other evidence for ``passive'' luminosity and color evolution in early type galaxies in clusters and the field.
