We report on mid- and far-IR Spitzer observations of 7 nearby dusty
elliptical galaxies by using the Multiband Imaging Photometer (MIPS) and
Infrared Spectrograph (IRS). Our sample galaxies are known to contain an
excessive amount 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 luminos
ities for our Spitzer Guest Observers (GO1) program. The 7 galaxies are
detected at the MIPS 24 um, 70 um, and 160 um 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 are detected significantly from 5 of the 7 galaxies; t
he 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 apparent
spatial correspondence between 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.Comment: 25 pages, 7 figures, accepted for publication in Publications of the
Astronomical Society of Japa
