Spitzer Space Telescope - General Observer Proposal #40810


Young or Old Elliptical Galaxies? A Conflict of Age-Dating Methods

Principal Investigator: Joel Bregman
           Institution: University of Michigan

     Technical Contact: Joel Bregman, University of Michigan

Co-Investigators: 
Jesse Bregman, NASA-Ames Research Center                              
Pasquale Temi, SETI                                                   

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
 Observing Modes: IrsStare 
  Hours Approved: 7.7 

Abstract:
We developed a stellar population age diagnostic from mid-IR spectra, because as
a population ages, the 10um silicate excess from stellar mass loss decreases
relative to the shorter wavelength continuum. Our IRS program showed that all 29
nearby ellipticals are old systems (10 Gyr), but our ages are in conflict with
the optical absorption line age determinations, which show a sizeable fraction
of these same galaxies to have ages of only 2-6 Gyr. Either result has important
implications for galaxy formation scenarios. However, one of these age-dating
schemes is seriously flawed, so here we test the mid-IR method to determine if
it is capable of correctly identifying an early-type galaxy with a known young
stellar component. We propose to observe a handful of the nearest E+A galaxies,
systems that had a major starburst during the past Gyr, but with no current star
formation. Not only will this provide a critical test of our age-dating scheme,
it will be the first mid-IR spectra of this important class of galaxies.