Dust in Hot Plasma of Nearby Dusty Elliptical Galaxies Observed with the Spitzer Space Telescope

We report on mid- and far-IR Spitzer observations of 7 nearby dusty elliptical galaxies by using the Multiband Imaging Photometer for Spitzer (MIPS) and Infrared Spectrograph (IRS). Our sample galaxies are known to contain excessive amounts of interstellar dust against sputtering destruction in hot plasma filling the interstellar space of elliptical galaxies. In order to study the origin and the properties of the excess dust in the hot plasma, we selected galaxies with a wide range of X-ray luminosities, but similar optical luminosities for our Spitzer Guest Observers (GO1) program. The 7 galaxies were detected at the MIPS 24$\mu$m, 70$\mu$m, and 160$\mu$m bands; the far- to mid-IR flux ratios of relatively X-ray-bright elliptical galaxies are lower than those of X-ray-faint galaxies. From the IRS spectra, polycyclic aromatic hydrocarbon (PAH) emission features were detected significantly from 5 of the 7 galaxies; the emission intensities are weaker as the X-ray luminosity of the galaxy is larger. We have found a correlation between the far- to mid-IR flux ratio and the equivalent width of the PAH emission feature. We have obtained an apparent spatial correspondence between the mid-IR and X-ray distributions in the outer regions for the three X-ray-brightest galaxies in our sample. Possible interpretations for our observational results are discussed.