Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982a%26a...110...61v&link_type=abstract
Astronomy and Astrophysics, vol. 110, no. 1, June 1982, p. 61-78.
Astronomy and Astrophysics
Astrophysics
333
Astronomical Photometry, Brightness Distribution, Disk Galaxies, Mass Distribution, Spiral Galaxies, Star Distribution, Astronomical Models, Milky Way Galaxy, Surface Properties
Scientific paper
We present surface photometry of the edge-on spiral galaxies NGC 5907, 5023, 4217, and 4013, and fit the disks of these systems to the three-dimensional model light distribution proposed by van der Kruit and Searle (l98la). We find good agreement and confirm that 1. all disks have a relatively sharp cut-off in the radial direction, 2. all disks have a thickness independent of radius, 3. the colours indicate that the model describes the distribution of an old disk population.
We show that the Galaxy is similar to the systems observed only if the radial scalelength in its disk is about 5 kpc and H 75km s-1 Mpc-1. If our disk model applies to the Galaxy this is also the scalelength obtained from a discussion of the kinematics of common disk stars in the solar neighbourhood.
We discuss conjectures by Fall (1980) that the constancy of disk thickness may result from the secular acceleration of old disk stars and that the sharp edges may result from an abrupt transition between stability and instability in the primordial gaseous disk. With regard to the latter, an application of the Goldreich and Lynden-Bell criterion to our disks shows that the primordial disks are indeed unstable in the inner region and stable beyond Rmax, but that the criterion fails to provide a detailed prediction of Rmax itself.
In an appendix we present calculations of the rotation curve and of the force in the z-direction Kz at various distances from the centre in our disk model, using a previous result based on the thickness of the H i-layer in NGC 891 that (M/L)old disk is constant throughout. We discuss the accuracy and use of simple analytic approximations. An application of these results shows that all galaxies studied by us have probably a halo component containing half or more of the total mass within the edge of the disk at Rmax.
Searle Leonard
van der Kruit Pieter Corijnus
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