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Astron. Astrophys. 325, 135-143 (1997)

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1. Introduction

The amount and distribution of dust in spiral galaxies remains an unsolved problem. For its solution, two approaches appear to be promising. The large N approach and the small N one.

The large N approach involves the statistical study of a large number of galaxies. Most of these studies examine how particular photometric parameters (such as surface brightness, colour, diameter, etc.) depend on the inclination angle of the galactic disk. Examples of such studies are those of Holmberg (1958), Heidmann et al. (1972), Valentijn (1990; 1994), Choloniewski (1991), Cunow (1992), Han (1992), Huizinga & van Albada (1992), Giovanelli et al. (1994; 1995) and Xu & Buat (1995). Because N is large, this approach is extremely useful for general conclusions concerning the opacity of spiral galaxies. However, the relatively simple galaxy models used in statistical studies, as well as possible selection effects that may bias the data sample (see Davies et al. 1993), can sometimes lead to wrong results.

The small N approach involves detailed study of the nearby spiral galaxies, which are well resolved. For this kind of approach, direct measurements of opacity, mainly using pairs of galaxies (Keel 1983; Andredakis & van der Kruit 1992; White & Keel 1992) as well as detailed modelling of galaxies (Kylafis & Bahcall 1987; Phillipps et al. 1991; Bosma et al. 1992; Byun 1993; Emsellem 1995; Beckman et al. 1996) are the most common ways to attack the problem. With the detailed model treatment, one has the advantage of deriving the parameters that describe the distribution of stars and dust in an accurate way. We believe that the small N approach will help a lot our understanding of the dust distribution in some galaxies and therefore in the minimization of the errors in the statistical studies (see also Byun et al. 1994). Several attempts have been reported so far to model the radiative transfer in galaxies (Kylafis & Bahcall 1987; Bruzual et al. 1988; Disney et al. 1989; Witt et al. 1992; Di Bartolomeo et al. 1995; Xu & Buat 1995; Bianchi et al. 1996; Corradi et al. 1996).

We have initiated a program to study the distribution of dust in galaxies using the small N approach. We will start with edge-on galaxies and then proceed to study less inclined ones. Edge-on galaxies have several advantages for this kind of work. One is that, in this view of a galaxy, one can easily separate the three main components of the galaxy (i.e., the stellar disk, the dust and the bulge). Another is that a lot of the details of a galaxy that are evident when the galaxy is seen face-on (e.g., spiral arms), are smeared out to a large degree when the galaxy is seen edge-on. Thus, a simple model with relatively few parameters can be used for the distribution of stars and dust in the galaxy. A third advantage is that the dust is very prominently seen in the dust lane.

The first galaxy of our study is UGC 2048 (NGC 973) and the method we use is that of Kylafis & Bahcall (1987).

In Sect. 2 we describe the observations, in Sect. 3 we give the data reduction procedure, in Sect. 4 we present the model that we have used, in Sect. 5 we fit the model to the real data and we calculate the total amount of dust in the galaxy and in Sect. 6 we draw our conclusions.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998