2012MNRAS.425.1934F

We present a detailed study of the Circinus galaxy, investigating its star formation, dust and gas properties, both in the inner and outer disc. To achieve this, we obtained high-resolution Spitzer mid-infrared images with the IRAC (3.6, 5.8, 4.5 and 8.0µm) and MIPS (24 and 70µm) instruments and sensitive HI data from the Australia Telescope Compact Array and the 64-m Parkes telescope. These were supplemented by CO maps from the Swedish-ESO Submillimetre Telescope. 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 A_V= 2.1mag from the spectral energy distribution of the Circinus galaxy and total stellar and gas masses of 9.5 x 10¹⁰ and 9 x 10⁹M_☉, respectively. Using various wavelength calibrations, we find obscured global star formation rates between 3 and 8M_☉/yr. Star-forming regions in the inner spiral arms of Circinus, which are rich in HI, are beautifully unveiled in the Spitzer 8 µm image. The latter is dominated by polycyclic aromatic hydrocarbon (PAH) emission from heated interstellar dust. We find a good correlation between the 8µm emission in the arms and regions of dense HI gas. The (PAH 8µm)/24µm surface brightness ratio shows significant variations across the disc of Circinus.