The Stellar Population Histories of Elliptical
Galaxies
A Ph.D. Dissertation by Scott C. Trager
Board of Studies in Astronomy and Astrophysics,
UCO/Lick Observatory, University of California, Santa
Cruz
This dissertation sets out to probe the stellar population histories
of local field and distant cluster elliptical galaxies.
Absorption-line strengths of the centers of 381 early-type galaxies
and 38 globular clusters measured from the Lick Image Dissector
Scanner (Lick/IDS) are presented. Error estimation and corrections
for velocity-dispersion broadening are described in detail. Monte
Carlo simulations show that the Lick/IDS data are not accurate enough
to infer ages and abundances of individual ellipticals with
confidence.
The excellent data of González (1993) are therefore used to infer
the stellar population ages and abundances of the centers of local
field ellipticals. Elliptical galaxy nuclei follow three relations in
this sample. (1) The t-Z relation. Elliptical nuclei
have an age-abundance relation at fixed velocity dispersion 
that follows the Worthey (1994) ``3/2 rule.''
Ellipticals therefore have fixed color and metal-line strengths at
fixed . (2) The -Z relation. The
abundance zeropoint of the t-Z relation increases with
increasing . Taken together, (1)
and (2) predict scaling relations like the Mg 
- and color-magnitude relations. (3) The
-[Mg/Fe] relation. The abundance ratio [Mg/Fe] increases with
increasing , as the
-Z relation for Mg has twice the slope of the
-Z relation for Fe. Relations (1)-(3) can be expressed as a
pair of planes in t-Z- space, one
for Fe and one for Mg, with similar age dependences but different -dependences. Scenarios for the possible origins of
these relations are presented.
Absorption-line strengths of eighteen early-type galaxies in two rich
clusters at z=0.41 (CL0939+4713) and z=0.76
(CL1322+3027) have been measured from Keck LRIS spectra. The
Balmer-line strengths of ellipticals at z=0.41 are consistent
with passive evolution of local field ellipticals but seem too
metal-rich. Both Balmer- and metal-line strengths of ellipticals at
z=0.76 are consistent with passive evolution of local field
ellipticals.
Spectra of four z>3 objects discovered serendipitiously are
presented. They are small ( 
kpc), bright ( 
- 
), lumpy, and are most
likely gravitationally lensed. They are metal-poor ( 
), mildly dusty, and have high star-formation rates ( 
). A model for their evolution is presented. It is suggested that
they are the progenitors of the Population II component of local
spheroids.