Gas and Star Formation in the Circinus Galaxy
  We present a detailed study of the Circinus Galaxy, investigating its star
formation, dust and gas properties both in the inner and outer disk. To achieve
this, we obtained high-resolution Spitzer mid-infrared images with the IRAC
(3.6, 5.8, 4.5, 8.0 micron) and MIPS (24 and 70 micron) instruments and
sensitive HI data from the Australia Telescope Compact Array (ATCA) and the
64-m Parkes telescope. These were supplemented by CO maps from the Swedish-ESO
Submillimetre Telescope (SEST). Because Circinus is hidden behind the Galactic
Plane, we demonstrate the careful removal of foreground stars as well as large-
and small-scale Galactic emission from the Spitzer images. We derive a visual
extinction of Av = 2.1 mag from the Spectral Energy Distribution of the
Circinus Galaxy and total stellar and gas masses of 9.5 x 10^{10} Msun and 9 x
10^9 Msun, respectively. Using various wavelength calibrations, we find
obscured global star formation rates between 3 and 8 Msun yr^{-1}. Star forming
regions in the inner spiral arms of Circinus, which are rich in HI, are
beautifully unveiled in the Spitzer 8 micron image. The latter is dominated by
polycyclic aromatic hydrocarbon (PAH) emission from heated interstellar dust.
We find a good correlation between the 8 micron emission in the arms and
regions of dense HI gas. The (PAH 8 micron) / 24 micron surface brightness
ratio shows significant variations across the disk of Circinus.
