AAS 205th Meeting, 9-13 January 2005
Session 110 Active Galaxies
Poster, Wednesday, January 12, 2005, 9:20am-6:30pm, Exhibit Hall

## [110.10] Decomposing ULIRGs: Quantifying the PAH and dust components in Spitzer/IRS spectra

J.A. Marshall, T.L. Herter, V. Charmandaris, H.W.W. Spoon (Cornell University), L. Armus (Caltech Spitzer Science Center)

We have developed a method to decompose mid-infrared spectra into a single PAH component and several silicate-graphite dust components, performing a least-squares fit to obtain the temperature and optical depth of each. Here, we use this method to analyze the set of ultraluminous infrared galaxy (ULIRG) spectra obtained with the IRS onboard the Spitzer Space Telescope as part of the GTO program. ULIRGs are defined as galaxies with infrared (8-1000 \mu m) luminosities in excess of 1012 LSun. This energy is produced by both AGN and starburst activity, with the fraction of energy produced by each process varying from galaxy to galaxy. The parameters returned by the fit procedure reveal the physical composition of ULIRGs but do not necessarily constrain their geometric structure. We find that the luminosity of the PAH component increases and the ratio of the PAH to dust continuum luminosity decreases as the total mid-infrared luminosity of a galaxy increases.

Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.