Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a....95..105v&link_type=abstract
Astronomy and Astrophysics, vol. 95, no. 1, Feb. 1981, p. 105-115.
Astronomy and Astrophysics
Astrophysics
536
Astronomical Photometry, Galactic Structure, Spiral Galaxies, Star Distribution, Brightness, Isophotes, Milky Way Galaxy
Scientific paper
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 Rmax 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 Rmax.
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 2z1/2 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.
Searle Leonard
van der Kruit Pieter Corijnus
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