Insights into gas heating and cooling in the disc of NGC 891 from Herschel far-infrared spectroscopy⋆⋆⋆⋆⋆⋆

We present Herschel PACS and SPIRE spectroscopy of the most important far-infrared cooling lines in the nearby, edge-on spiral galaxy, NGC 891: [Cii] 158 μm, [Nii] 122, 205 μm, [Oi] 63, 145 μm, and [Oiii] 88 μm. We find that the photoelectric heating efficiency of the gas, traced via the ([Cii]+[Oi]63)/FTIR ratio, varies from a mean of 3.5 × 10-3 in the centre up to 8 × 10-3 at increasing radial and vertical distances in the disc. A decrease in …