Astron. Astrophys. 331, 894-900 (1998)

## 3. Model

NGC 891 (often quoted to be very similar to our own Galaxy) is an Sb Hubble type galaxy, seen almost edge-on. The relatively small distance (9.5 Mpc) to this galaxy, gives us an advantage of high-resolution imaging, revealing many details of the distribution of light and dust.

Considered to be an example of a typical spiral galaxy, NGC 891 is expected to consist of a central bulge (mainly populated by old stars) and a disk formed by a mixture of stars and interstellar dust and gas, organized in spiral formations. The projection of such a system on the sky, with the plane of the disk almost parallel to our line of sight, results in a surface brightness distribution like that shown in Fig. 1. Neglecting for the moment the small scale structure and clumpiness, one can distinguish three main components in the projected image. A stellar disk, a dust lane (located along the major axis of the disk) and a bulge in the central region of the galaxy. Even though the detailed spiral structure of the galaxy is not known and may be quite complicated on small scales, it is possible to use simple mathematical functions to obtain an "on average" description of the galaxy. The successful modelling that was done on UGC 2048 (Paper I), encourages us to do the same to NGC 891. It is obvious though that studying galaxies in various orientations, which is something we intend to do, will allow for a better comprehension of the general opacity problem.

 Fig. 1. An 80 min exposure of NGC 891 in the B-band.

Thus, for the stellar distribution in the disk, we use exponential functions in both the radial and the vertical direction with respect to the plane of the disk i.e.,

Here R and z are the cylindrical coordinates, is the stellar emissivity at the center of the disk and and are the scalelength and scaleheight respectively of the stars in the disk. The central surface brightness of such a distribution, assuming that the galaxy is seen edge-on is given by

For a detailed description of all the parameters, the reader should refer to Sect. 4 of Paper I.

For the stellar emissivity in the bulge, both the Hubble profile (Hubble 1930) and the law (de Vaucouleurs 1953) provide an excellent fit. As mentioned in Paper I, the parameters describing the stellar disk are not affected significantly by the type of bulge used. Thus, we use the Hubble profile, the emissivity of which is given by

where is the emissivity of the bulge at the center and

with being the effective radius of the bulge and a and b the semi-major and semi-minor axis respectively. The central value of the bulge surface brightness, if the model galaxy is seen edge-on, is

The total stellar emissivity is then given by

For the extinction coefficient we use a double exponential law, namely

where is the extinction coefficient at wavelength at the center of the disk and and are the scalelength and scaleheight respectively of the dust. The central optical depth of the model galaxy seen face-on is

The model that we have used is that described in KB87 (see also Paper I). A Henyey-Greenstein phase function has been used for the scattering of the dust (Henyey & Greenstein 1941). The values for the anisotropy parameter g and the albedo have been taken from Bruzual et al. (1988). Our task is to find those values of the parameters in Eqs. (1) - (8) which create images of the model galaxy as close as possible to the images of the observed galaxy.

© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998