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Astron. Astrophys. 356, 795-807 (2000)

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1. Introduction

The introduction of submillimeter (submm), imaging arrays such as SCUBA (Submillimeter Common User Bolometer Array; Holland et al. 1999), heralds a revolution in our understanding of the dust properties of external spiral galaxies. For the first time, we can map cold (15-20K) interstellar grains with high sensitivity and good spatial resolution ([FORMULA]) (Chini et al. 1995; Hughes et al. 1997; Clements et al. 1993). Previous studies of radiation emitted by dust have relied heavily on measurements carried out by the Infrared Astronomical Satellite (IRAS) at 60 and [FORMULA]m. These wavebands are sensitive primarily to warm ([FORMULA] 30K) dust, and as such, are unlikely to give an accurate impression of either the true quantity or distribution of interstellar grains in external disks (Alton et al. 1998a; Devereux & Young 1990). Furthermore, satellite missions such as IRAS, and the recently completed Infrared Space Observatory (ISO) project, are constrained at present to using small mirrors (0.6-m c.f. SCUBA 15-m dish). This limits their resolving capability to [FORMULA] in the far-infrared (FIR) which is only adequate for probing disk structure in the very closest spiral galaxies (Alton et al. 1998b).

One of the first studies of nearby galaxies, using SCUBA, was carried out by Alton et al. (1998c) who mapped the edge-on galaxy NGC 891 at 450 and [FORMULA]m (see also Israel et al. 1999). By carefully comparing the submm emission over the same region as the FIR flux ([FORMULA]), they found that this galaxy contains about an order of magnitude more dust than would have been inferred from IRAS observations alone. This same conclusion was reached by Xilouris et al. (1999) who fitted the optical and near-infrared surface photometry of both NGC 891 and 6 other nearby, edge-on spirals with a sophisticated radiation transfer simulation. Their model, which takes account of both scattering and absorption by interstellar grains, only produces a self-consistent solution across several wavebands if the extinction lane harbours about 10 times more dust than might be expected from IRAS measurements.

In this paper, we relate the extinction modelling carried out for NGC 891 (Xilouris et al. 1998; hereafter XAD) with the corresponding submm images acquired from SCUBA (Alton et al. 1998c; hereafter ABR). In doing so, we address some important issues, namely:

  1. whether the submm emission, which accounts for the bulk of grain material in NGC 891, correlates closely with the dust evident in the extinction lane (Sect. 3);

  2. in the case of a positive correlation, whether the submm emissivity in NGC 891 (or, equivalently, the grain mass-absorption coefficient) is comparable to that recorded for our own galaxy (Sect. 4);

  3. given that we recover most of the dust mass from our submm images, how the gas-to-dust ratio varies along the major axis of NGC 891(Sect. 5);

  4. how the submm emission is distributed with respect to the neutral gas phases (HI,H2) and the stars in the disk (Sect. 5). This information will prove crucial to the growing number of submm surveys, where sources are poorly resolved subsidary observational data are limited.

We begin by summarizing the salient results from XAD and ABR (Sect. 2 and Sect. 3) before seeking to relate the submm to the extinction dust lane.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000