Spitzer Space Telescope - General Observer Proposal #50728


Hidden Star Formation in Elliptical Galaxies? 

Principal Investigator: Jesse Bregman
           Institution: NASA Ames Research Center

     Technical Contact: Jesse Bregman, NASA Ames Research Center

Co-Investigators: 
Joel Bregman, University of Michigan                                  

Science Category: nearby galaxies (z<0.05, v_sys<15,000 km/s) 
 Observing Modes: IrsStare 
  Hours Approved: 13.1 

Abstract:
In our Spitzer Cycle-1 study of normal early-type galaxies, we obtained low
resolution 5-21 um spectra that showed an old population with low mass stars
shedding gas as they evolve. Surprisingly, about a third of these galaxies also
showed forbidden line emission from NeIII (15.55 um) sometimes accompanied by
NeII (12.81 um) and/or SIII (18.7 um) lines (we did not observe using LL1, so
did not have a chance to observe the SIII 33.5 um line). We see two likely
possible origins for the emission lines. The first scenario is that there is no
recent star formation, and all the heating comes from old stars (i.e., the cores
of planetary nebulae and post-AGB stars), while in the second scenario, there
has been recent star formation and the ionizing radiation from these stars is
producing the observed line emission. In the first case, we expect the gas
density to be approx. 30-100 cm-3, while in the second case, the density could
be approx. 100x higher. Since the ratio of the SIII lines is sensitive to the
density in the range expected for these two cases, we propose to use the IRS
high resolution modes to measure the density in these galaxies, providing a
discriminator between these two possibilities. We will also obtain a good
measure of the hardness of the ionizing radiation by measuring the ratio of
NeIII/NeII. These data will lead to one of two results for normal ellipticals:
it will determine the duty cycle of star formation and the star formation rate;
or it will place a limit on the frequency of even low level star formation
activity.