Deep submillimeter images of NGC 7331; dust at the periphery of spiral disks

¹ Department of Physics & Astronomy, University of Wales, PO Box 913, Cardiff CF2 3YB, UK
² DEMIRM, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
³ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany

Corresponding author: P. B. Alton, paul.alton@astro.cf.ac.uk

Received: 11 September 2000
Accepted: 29 November 2000

Abstract

We present deep 450 and m SCUBA images of the nearby spiral galaxy NGC 7331. Using the submillimeter emissivity inferred from COBE observations of Milky Way dust, we convert our SCUBA images into maps of optical depth. The opacity derived in this way is quite low at the visible limit of NGC 7331 ( at the R₂₅ radius for the disk seen face-on). In a similar fashion, we exploit SCUBA and ISOPHOT images of a further 10 galaxies and, collectively, these data indicate -0.2 at the R₂₅ radius. Our constraints on disk opacity are fed into a simulation of how light emanating from high redshifts is attenuated by foreground spirals. In making this calculation, we consider the possibility that galactic disks may have also contained different dust masses in the past. We estimate that less than 10% of the light emitted by Hubble Deep Field galaxies fails to reach the B-band observer due to intervening spirals.