Surface photometry of edge-on spiral galaxies. I - A model for the three-dimensional distribution of light in galactic disks.

In this paper we investigate the three-dimensional distribution of light in galactic disks as a first step towards a comprehensive study of the structure of edge-on spiral galaxies. The model has the feature of being isothermal in z at all radii with a scale parameter z0 and has an exponential dependence of surface brightness upon R with a scalelength h. When we apply this model to previously published photometry of NGC 4244 and 5907 (van der Kruit, 1979), edge-on spiral galaxies without an appreciable bulge, we find:

1. To an excellent approximation the scale parameter z₀ is independent of R.

2. The radial dependence requires a cut-off at a radius R_max of a few scale-lengths h. This cut-off is very sharp with an e-folding less than about 1 kpc. This sets an upper limit of about 10 km s^-1 to the radial velocity dispersion in the stars near R_max.

3. Comparison to the solar neighbourhood shows that we are fitting the self-gravitating intermediate population II and disk population.

This self-gravitating, locally-isothermal, truncated, exponential disk model has a number of interesting features - in particular in connection with finding (1) - which are discussed at the end of the paper. The predicted radial dependence of the velocity dispersion 2_z>^1/2 upon R is a basic characteristic of these disks, which is in principle open to observational verification.

Application of our model disk to NGC 4565 and subtracting it from the observed surface brightness distribution reveals the light distribution in its prominent bulge and spheroid. In contrast to NGC 4244 and 5907 there is evidence for a "thick disk" in Burstein's (1979c) sense; however we infer from the considerable flattening of the spheroid isophotes with radius that this luminosity is presumably nothing other than the halo population responding to the gravitational field of the disk.