Computer Science
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50728b&link_type=abstract
Spitzer Proposal ID #50728
Computer Science
Scientific paper
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.
Bregman Jesse
Bregman Joel
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