Title:

Morphologies of Galaxies with Old Stellar Populations at 
High Redshifts



Alan Stockton
IfA 



Abstract:

The so-called "red-envelope" galaxies at any given redshift are
those that have likely completed essentially all of their star
formation at much higher redshifts.  Luminous examples of such
galaxies at high redshifts are among the very first massive
galaxies to have formed in the universe, and therefore they are
interesting objects for constraining galaxy formation
scenarios.  We have been seeking out these galaxies in the
fields of high-redshift radio sources, which have two important
advantages over general field surveys: (1) radio sources mark
regions of high overdensity, in which processes of galaxy
formation are expected to be accelerated, and (2) we can choose
fields with redshifts that allow the best discrimination
between galaxies with old stellar populations and contaminants,
such as highly reddened starbursts.  After confirming that the
spectral-energy distribution of a galaxy agrees with that
expected for an old stellar population at or near the
radio-source redshift, we obtain either adaptive-optics imaging
or HST NICMOS imaging to determine detailed morphological
parameters for the galaxies.  Most of the objects for which we
have high-resolution imaging appear to be dominated by massive
exponential disks.  Some of the consequences of this finding
will be briefly explored.