Annu. Rev. Astron. Astrophys. 1994. 32:
115-52 Copyright © 1994 by Annual Reviews. All rights reserved |
Total Masses and Related Quantities
The study of total masses has been approached both through the measurement of global HI profile widths and through the detailed study of rotation curves. In an early work, Brosche (1971) noted that the maximum rotational velocity decreased along the spiral sequence from Sa to Im as if the morphological sequence could be understood as a sequence of angular momentum at constant mass. Today the issue appears more complicated. The masses and mass-to-light ratios of galaxies were the subject of a review by Faber & Gallagher (1979) to which the reader is referred for a general discussion.
Similarly to Faber & Gallagher, we have examined the total mass within the optical radius by using the 21 cm line width as the indicator of rotational velocity. While their analysis made use of 121 disk galaxies, the present sample contains mass estimates for over 3000 objects. Figures 2 and 3 include the distribution of properties relevant to mass: MT, the total mass surface density T, and the total mass to light ratio MT/LB.
As with luminosity and size we find, in Figure 2c, a small systematic variation of total mass among the classical spirals followed by a pronounced decrease for the dwarf systems. It is important to note that the three parameters L, R, and M are strongly coupled through the Tully-Fisher relations (Tully & Fisher 1977), i.e., both R and V vary with L. Unfortunately our understanding of the Tully-Fisher relations is too poor to point to the fundamental parameter(s).
TOTAL MASS-TO-LIGHT RATIO
ROTATION CURVES
In studying the dark matter distribution, HI rotation curves are more
suitable given that they generally extend far beyond the point where
H is detected, and thus better
constrain the halo
distribution. Most recently,
Broeils (1992)
has combined new
observations with those available in the literature to look at
morphologial dependences in the mass and light distributions and the
applicability of dark matter and alternative models. He sees a clear
indication that the dark matter component increases for the latest
types, but along the spiral sequence evidence for significant
morphological trends is lacking, partly due to the still-small number
of objects mapped.
With the discovery of two galaxies with declining rotation curves,
Casertano and van
Gorkom (1991)
have found correlations between the
peak circular velocity of a galaxy, its central surface brightness and
the slope of the outer rotation curve. As in Broeils' sample, they do
not sample the full range of the spiral sequence.
Kent (1984)
has pointed out that Sa galaxies may require no dark matter within the
optical disk, but few Sa galaxies have been mapped in HI, primarily
because their lower characteristic HI surface densities make
synthesis observations more difficult. The current observations
suggest two conclusions:
MASS SURFACE DENSITY
1.
Most dwarf galaxies show rotation curves that are still rising at
their last measured point.
2.
Classical spirals show rotation curves whose outer portions are
rising, flat or falling.