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

¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
e-mail: thomas.hughes@ugent.be
² Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
³ UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁴ Laboratoire d’Astrophysique de Marseille, Université d’Aix-Marseille and CNRS, UMR 7326, 13388 Marseille Cedex 13, France
⁵ Centre for Cosmology, Department of Physics and Astronomy, University of California, Irvine CA 92697, USA
⁶ Institut für theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle Str. 2, 69120 Heidelberg, Germany
⁷ Department of Physics & Astronomy, University of Sussex, Brighton BN1 9QH, UK
⁸ CEA, Laboratoire AIM, Université Paris VII, IRFU/Service d’Astrophysique, Bât. 709, Orme des Merisiers, 91191 Gif-sur-Yvette, France
⁹ Infrared Processing and Analysis Centre, California Institute of Technology, MS 100-22, Pasadena CA 91125, USA
¹⁰ Istituto di Astrofisica e Planetologia Spaziali, INAF-IAPS, via Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 1 August 2014
Accepted: 21 October 2014

Abstract

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)/F_TIR 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 ([Cii]+[Oi]63)/F_TIR but constant ([Cii]+[Oi]63)/F_PAH with increasing FIR colour suggests that polycyclic aromatic hydrocarbons (PAHs) may become important for gas heating in the central regions. We compare the observed flux of the FIR cooling lines and total IR emission with the predicted flux from a PDR model to determine the gas density, surface temperature and the strength of the incident far-ultraviolet (FUV) radiation field, G₀. Resolving details on physical scales of ~0.6 kpc, a pixel-by-pixel analysis reveals that the majority of the PDRs in NGC 891’s disc have hydrogen densities of 1 < log (n/ cm^-3) < 3.5 experiencing an incident FUV radiation field with strengths of 1.7 < log G₀< 3. Although these values we derive for most of the disc are consistent with the gas properties found in PDRs in the spiral arms and inter-arm regions of M 51, observed radial trends in n and G₀ are shown to be sensitive to varying optical thickness in the lines, demonstrating the importance of accurately accounting for optical depth effects when interpreting observations of high inclination systems. Increasing the coverage of our analysis by using an empirical relationship between the MIPS 24 μm and [Nii] 205 μm emission, we estimate an enhancement of the FUV radiation field strength in the far north-eastern side of the disc relative to the rest of the disc that coincides with the above-average star formation rate surface densities and gas-to-dust ratios. However, an accurate interpretation remains difficult due to optical depth effects, confusion along the line-of-sight and observational uncertainties.