"The Timescales of Galaxy Formation: The Stellar Evidence"
Dr. Scott Trager (OCIW)

A long-held dogma of stellar population studies is that early-type (E
and S0) galaxies are very old, coeval, and homogeneous stellar
systems, varying only in their metallicities, which are controlled by
their mass.  In this talk, I will present evidence to support a
heretical view: early-type galaxies are not (necessarily) very old,
homogeneous stellar systems and they are certainly not coeval.  Using
stellar population models that (for the first time) properly account
for variations in the relative abundances of the alpha and Fe-peak
elements, our group has discovered that ellipticals (and probably S0s)
form a two-parameter family in which metallicity of a galaxy is
controlled by both its mass and the epoch and amount of any recent
star formation (which we measure as "age") and its [alpha/Fe] ratio is
controlled solely by mass.  We find moreover that the distribution of
mass and age of early-type galaxies, which control metallicity and
[alpha/Fe], depends on both environment and morphological type:
cluster ellipticals are (nearly) uniformly old, independent of mass,
while field ellipticals and S0s in all environments have a wide range
in age at any given mass.  These trends appear to be confirmed by
direct observation of the evolution of early-type galaxies.  The
restriction of early-type galaxies to a single two-dimensional surface
in the four-dimensional space of mass, age, metallicity, and
[alpha/Fe] has profound implications for the formation mechanisms of
these objects: many early-type galaxies formed their stars over
extended timescales, but not necessarily from gas acquired in mergers.
I will conclude with a brief discussion of an on-going survey
extending this work to the bulges of local spiral galaxies.