Last updated: July-7-99

Masses of Galaxies

The methods for determining masses of galaxies and the resulting mass-to-light ratios are reviewed in [Tri87]. In many instances the only physics needed is the simple dimensional argument based on M ~ V 2 R / G, where velocity support at a given radius is equated to the mass internal to that point. A variety of means of obtaining indicative velocities are listed below.

(1) Rotation Curves

Many rotation curves are flat or even rising out to large radii. Extending beyond the optical limits for measuring rotation, HI rotation curves suggest M / L ~ 20-30 with some 75% of the matter apparently contained in a dark halo [vanA85], [Bah85], [Car85], [Kent86]. Reviews of the gas velocity fields are found in [Kruit78], [FG79], [Rubin79a], [Rubin79b], [Rub82], [Bos81], [Bos82].

(2) Line-Width of the 21-cm HI line

A large number of HI rotation curves can be found in [vanA85]

(3) Velocity Dispersion of the Stars

Reviews of absorption-line kinematics in galaxies can be found in [Cap79], [Bert81] and [Illing81].

(4) Planetary Nebulae and Globular Clusters

Both planetary nebulae and globular clusters each act as point-source probes of the halo potential in Population II rich systems. Globular clusters appear to have larger velocity dispersions at the same radius as compared to the unresolved field population [Hes86, Mou86]; projection effects may be involved as well as true dynamical differences.

(5) Kinematics of Companion Galaxies

Companion galaxies can probe the outer halos of parent galaxies; however satellite-rich systems are relatively uncommon. A rare example is NGC 720 [Dre86] whivch has six physical companions.

(6) The Local Group

Using the virial theorem, the mass of our Local Group has been variously estimated to be in the range 2-7 x 1012 solar masses, [Hum55], [Mis85] with the Milky Way contributing about 10% of this mass out to a radius of 20 kpc.

(7) Binary Galaxies

Velocity differences and projected separations have been statistically studied for pairs of galaxies [Page52], [Page62], [Bla84], [Kara85], [La84], [Pet79], [Sch87], [Whi83]. Under a variety of assumptions regarding the intrinsic eccentricity of the orbits the enclosed total mass can vary more than a factor of two. Removal of physically associated but unbound pairs, interlopers, and accidental projections are problematic.

(8) Velocity Dispersion in Groups and Clusters

Small groups and clusters have derived virial masses mear to 1013 solar masses [BB61], [Gel83], [Bar84], [Bar85], [Bri86], [Car85]; however, careful attention needs to paid to selecting only those systems that are bound [KK82], [VB86], [MT74].

(9) Superclusters

If bound, superclusters are typically required to have 1016-17 solar masses contained within fiducial radii of 30-50 Mpc [Abe61]. This results in calculated values of M / L in the range 100-300 [Dek80], [Ford81], [Har81], [Pee86].


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