The three-dimensional distribution of light and mass in disks of spiral galaxies.
Abstract
In this paper the vertical distribution of old disk stars is re-examined in view of the deviations from the isothermal sheet approximation at small distances from the plane. Such deviations are expected on the basis of the observed age - velocity dispersion relation of stars in the solar neighborhood and have been observed recently by Wainscoat (1986) at red and near infrared wavelengths in an edge-on spiral. The extension of an exponential distribution all the way to the plane is argued to be inconsistent with the kinematics of old disk giants near the sun and the theoretical predictions from the age - kinematics relation. A solution intermediate between the exponential and the isothermal distributions is proposed. The relevant equations describing the z-dependence of the velocity dispersion, gravitational force and distribution of isothermal populations of test particles have been derived and compared to the isothermal and exponential case. From this the corrections to the disk surface densities and mass-to-light ratios which are inferred from observed vertical distributions and velocity dispersions of old disk stars and H I are derived. On this basis the best value for the old disk M/L is derived as 6+/-2 M_sun_/L_sun,B_. In some cases this decreases the inferred global stability paramaeter Y of Efstathiou et al. (1982) to values below what is required for stability, but disks may also be stabilized against global modes by the observed large stellar velocity dispersions in the central regions. The conclusion that the amplitude of observed rotation curves cannot be explained by the disk mass distribution alone becomes stronger. The same holds for the evidence for dark matter within the optical boundaries of spiral galaxies and that against the "maximum disk hypothesis". The radial truncation in stellar disks which is observed in edge-on spirals at 4 to 5 exponential scalelengths has also been identified in face-on and moderately inclined disks. In general these are not seen in azimuthally averaged light profiles as a result of the deviations of circular symmetry in the young population. Only some very early type galaxies show no indication of such truncations out to 6 or 7 scalelengths.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- March 1988
- Bibcode:
- 1988A&A...192..117V
- Keywords:
-
- Gas Dynamics;
- Mass To Light Ratios;
- Spiral Galaxies;
- Star Distribution;
- Astronomical Models;
- Density Distribution;
- Mass Distribution;
- Radial Velocity;
- Velocity Distribution;
- Vertical Distribution;
- Astrophysics