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Astron. Astrophys. 353, L13-L17 (2000)

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1. Introduction

Until recently,the main source of Far-Infrared (FIR) data for spiral galaxies has come from the IRAS satellite. The availability of instruments capable of observing at [FORMULA]m, like those onboard the satellite ISO, has allowed the detection of large amounts of cold dust, much colder than IRAS was able to detect. Spiral galaxies are found to have a dust content similar to the one in the Galaxy (Alton et al. 1998a).

Cold temperatures (T[FORMULA]20K) can be reached by diffuse dust heated by the general interstellar radiation field, while dust close to star-forming region is hotter (T[FORMULA]50K; Whittet 1992). Since diffuse dust is the main contributor to the internal extinction in a galaxy, observations of cold dust help to trace its opacity. Additionally, ISO observations have suggested that the dust distribution is more extended than the stellar disk (Alton et al. 1998a; Bianchi, Davies & Alton 1999b). If this is confirmed, observation of the distant universe may be severely biased, because of the large cross section of the dust disks.

Unfortunately, the poor resolution ([FORMULA]) of the current FIR images does not allow detailed studies of the spatial distribution of dust. Higher resolution can be achieved in the sub-mm, but a high sensitivity is required because of the fainter dust emission. High sensitivity and resolution are both characteristics of the recently developed SCUBA sub-mm camera. Only a few large nearby galaxies have been observed with SCUBA, notably the highly inclined galaxy NGC 7331 (Bianchi et al. 1998) and the edge-on galaxy NGC 891 (Alton et al. 1998b; Israel et al. 1999). The observed dust emission is found to correlate well with the molecular gas phase, dominant in the centre. However, a dust component associated with the atomic gas is needed to explain the dust and gas column density at large galactocentric distance along the major axis of NCG 891 (Alton et al. 1999).

In this Letter we present SCUBA observations of the face-on galaxy NGC 6946. Because the galaxy is larger than the camera field of view, images have been produced with the scan-mapping technique, chopping within the observed field. The observation and the data reduction needed to restore the source signal are described in the next section. The description and the discussion of the results are given in Sect. 3.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999