Study of the ISM in Early-type Galaxies: the Infrared View

Grant #: NNA06CB62A
Senior Scientist: Cristina Dalle Ore

The purpose of our proposed research is to study the ISM of Elliptical galaxies using proprietary and archival data taken with  the Spitzer Space Telescope.


The proposed project is split into two separate tasks :

1) The 5-14µm spectra of elliptical galaxies are dominated by the stellar light of old mass-losing giants and AGB stars.  Studies of IRS spectra and 3-24µm broad-band data show that the mid-IR light is dominated by an old stellar population (~10 Gyr).  However, a small fraction of these ordinary elliptical galaxies exhibit PAH emission, raising the possibility that low-level star formation occurs occasionally.  As a measure of star formation, PAH emission can be more sensitive than optical absorption line diagnostics because the brightest PAH features do not compete with a strong stellar continuum.  Our goals are to determine the fraction of elliptical galaxies with star formation and to measure their star formation rates.  To accomplish this, we will  use Spitzer observations to determine whether this PAH emission is indeed due to star formation or if it merely reflects the presence of an inert interstellar component. 


2) We aim to address the amount, the origin and the spatial distribution of interstellar dust in ellipticals. The high sensitivity and good spatial resolution of the far infrared photometer on board of SPITZER allow, for the first time, an accurate investigation of these issues. SPITZER observations will discriminate among a variety of proposed scenarios. Asymmetric, extended FIR emission would favor a merger origin of the dust, while regular, concentrated FIR images are expected if the dust is produced internally by galactic stars. In the first case these observations may provide an estimate of the frequency of (minor) merging events for giant elliptical galaxies.


We will also study the physical interaction of dust with the hot ISM. The FIR luminosity from the central regions may help unravel the mystery of “non-cooling” cooling flows. The data will be interpreted using detailed theoretical models for interstellar dust emission from ellipticals developed by our group. 


This program is comprised of observational and theoretical projects and the work is done in collaboration with observational and theoretical astronomers both at NASA Ames and at the University of California Santa Cruz, and the University of Michigan, Ann Arbor.