The Physical Characteristics of the Gas in the Disk of Centaurus A Using the Herschel Space Observatory

We search for variations in the disk of Centaurus A of the emission from atomic fine structure lines using Herschel PACS and SPIRE spectroscopy. In particular, we observe the [C II](158 mu m), [N II](122 and 205 mu m), [O I](63 and 145 mu m), and [O III](88 mu m) lines, which all play an important role in cooling the gas in photo-ionized and photodissociation regions (PDRs). We determine that the ([C II]+[O I](63))/F-TIR line ratio, a proxy for the heating efficiency of the gas, shows no significant radial trend across the observed region, in contrast to observations of other nearby galaxies. We determine that 10%-20% of the observed [C II] emission originates in ionized gas. Comparison between our observations and a PDR model shows that the strength of the far-ultraviolet radiation field, G(0), varies between 10(1.75) and 10(2.75) and the hydrogen nucleus density varies between 10(2.75) and 10(3.75) cm(-3), with no significant radial trend in either property. In the context of the emission line properties of the grand-design spiral galaxy M51 and the elliptical galaxy NGC 4125, the gas in Cen A appears more characteristic of that in typical disk galaxies rather than elliptical galaxies.