Spitzer Space Telescope - General Observer Proposal #3419


Breaking the Age-Metallicity degeneracy in local early-type galaxies:
clues about the formation and evolution of spheroids.

Principal Investigator: Alessandro Bressan
           Institution: INAF Padova Astronomical Observatory

Co-Investigators: 
Pasquale Panuzzo, Imperial College, UK & INAF Astronomical Observato  
Roberto Rampazzo, INAF Astronomical Observatory of Padova, Italy      
Lucio Buson, INAF Astronomical Observatory of Padova, Italy           
Marcel Clemens, INAF Astronomical Observatory of Padova, Italy        
Gian Luigi Granato, INAF Astronomical Observatory of Padova, Italy    
Laura Silva, INAF Astronomical Observatory of Trieste, Italy          
Luigi Danese, SISSA, Italy                                            

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
 Observing Modes: IrsStare 
  Hours Approved: 23.0 

Abstract:
We propose to obtain Spitzer IRS (SL1, SL2 and LL2) high signal to
noise (S/N~40) observations in the spectral region from 5.2 to 21.3
micron, of a sample of early-type galaxies in Virgo cluster and in the
field.  We present a new method that, fully exploiting the spatial
resolution and sensitivity of Spitzer Space Telescope, seeks for the
signatures of younger and/or more metal rich stellar populations,
through the presence of the 9.7 micron silicate feature of their
Asymptotic Giant Branch (AGB) stars.  By combining such mid-infrared
observations with optical observations, we are able to break the
"age-metallicity" degeneracy that affects any estimate of age and
metallicity in such galaxies.  The 9.7 micron feature has been already
detected by ISO in a few early type galaxies -we show the case of
NGC1399-, though without the required spatial resolution and high
signal to noise.  We propose to apply this method to early type
galaxies populating two different environments:  the Virgo cluster and
the local field.  Spitzer Space Telescope will allow, for the first
time, a ranking of these galaxies in age and metallicity unbiased by
the effect of "age-metallicity" degeneracy, and thus will cast light
on the processes of formation of spheroids in these two markedly
different environments.