A resolved analysis of cold dust and gas in the nearby edge-on spiral NGC 891^⋆,⋆⋆

¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281-S9, 9000 Gent, Belgium
e-mail: thomas.hughes@ugent.be
² School of Physics &, Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
³ Department of Physics & Astronomy, McMaster University, Hamilton L8S 4M1, Canada
⁴ Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
⁵ UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁶ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁸ Aix-Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille, UMR 7326, 13388 Marseille, France
⁹ Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
¹⁰ European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching bei München, Germany
¹¹ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot DAPNIA/Service d’Astrophysique, Bat. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
¹² Istituto di Astrofisica e Planetologia Spaziali, INAF-IAPS, via Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 13 December 2013
Accepted: 21 February 2014

Abstract

We investigate the connection between dust and gas in the nearby edge-on spiral galaxy NGC 891, a target of the Very Nearby Galaxies Survey. High resolution Herschel PACS and SPIRE 70, 100, 160, 250, 350, and 500 μm images are combined with JCMT SCUBA 850 μm observations to trace the far-infrared/submillimetre spectral energy distribution (SED). Maps of the Hi 21 cm line and CO(J = 3−2) emission trace the atomic and molecular hydrogen gas, respectively. We fit one-component modified blackbody models to the integrated SED, finding a global dust mass of (8.5 ± 2.0) × 10⁷ M_⊙ and an average temperature of 23 ± 2 K, consistent with results from previous far-infrared experiments. We also fit one-component modified blackbody models to pixel-by-pixel SEDs to produce maps of the dust mass and temperature. The dust mass distribution correlates with the total stellar population as traced by the 3.6 μm emission. The derived dust temperature, which ranges from approximately 17 to 24 K, is found to correlate with the 24 μm emission. Allowing the dust emissivity index to vary, we find an average value of β = 1.9 ± 0.3. We confirm an inverse relation between the dust emissivity spectral index and dust temperature, but do not observe any variation of this relationship with vertical height from the mid-plane of the disc. A comparison of the dust properties with the gaseous components of the ISM reveals strong spatial correlations between the surface mass densities of dust (Σ_dust) and the molecular hydrogen (Σ_H2) and total gas surface densities (Σ_gas). These observations reveal the presence of regions of dense, cold dust that are coincident with peaks in the gas distribution and are associated with a molecular ring. Furthermore, the observed asymmetries in the dust temperature, the H₂-to-dust ratio and the total gas-to-dust ratio hint that an enhancement in the star formation rate may be the result of larger quantities of molecular gas available to fuel star formation in the NE compared to the SW. Whilst the asymmetry likely arises from dust obscuration due to the geometry of the line-of-sight projection of the spiral arms, we cannot exclude that there is also an enhancement in the star formation rate in the NE part of the disc.