Astron. Astrophys. 353, 117-123 (2000)

## 1. Introduction

Modeling the dust and stellar content of spiral galaxies is a very crucial procedure needed for the correct interpretation of the observations. The amount of interstellar dust embedded inside spiral galaxies, the way that dust is distributed within spiral galaxies and also the extinction effects of the dust to the starlight are some of the questions that can be answered by performing radiative transfer modeling of individual spiral galaxies.

One very important thing that needs consideration when doing such analysis is the right choice of the stellar and dust distributions. In particular, the galactic disk is a quite complex system, where stars and dust are mixed together usually in a spiral formation. For this reason, one has to use realistic distributions able to reproduce quite accurately the observations. On the other hand, simple mathematical expressions for these distributions are chosen in order to keep the free parameters to the minimum.

For the distribution of the starlight in the disk of spiral galaxies, the exponential function is very widely in use. This simple mathematical expression is able to describe the distribution of stars in both directions, radially and perpendicular to the disk. Decomposition techniques used by different authors in order to separate the bulge and the disk component strongly support this argument. For galaxies seen face-on (and at moderate inclination angles), radial profile fitting (e.g. Freeman 1970), fitting to azimuthally averaged profiles (e.g. Boroson 1981), as well as ellipse fitting techniques to 2D images (e.g. de Jong 1995) show that the exponential in the radial distance R is a good representation of galactic disks with only small deviations mainly due to the spiral structure of the galaxy (see Serna 1997). Other works like those of Shaw & Gilmore (1989) and de Grijs (1997) dealing with modeling of edge-on galaxies support the idea that exponential functions are good representations also for the z (vertical to the disk) direction.

Performing radiative transfer modeling of edge-on galaxies, Xilouris et al. (1997, 1998, 1999) found that exponential functions for the luminosity density of the stars in the disk as well as for the extinction coefficient give an excellent description of the observations. The advantage of modeling galaxies in the edge-on configuration is that the integration of light along the line of sight is able to cancel out most of the structure of the galaxies (i.e. spiral structure) and therefore allows for simple functions such as exponentials to give good representation of the observations. Thus, although in the face-on configuration a large variation between arm and interarm regions might be present for both the stars and the dust (White & Keel 1992, Corradi et al. 1996, Beckman et al. 1996, Gonzalez et al. 1998), in the edge-on case an average description of the galaxy characteristics can be obtained quite accurately. We are going to investigate the validity of this argument by comparing the exponential distributions with more realistic distributions which include spiral structure.

In Sect. 2 we describe the method that we use to address this problem and in Sect. 3 we present the results of our calculations. Finally, in Sect. 4 we summarize our work.

© European Southern Observatory (ESO) 2000

Online publication: December 8, 1999