Annu. Rev. Astron. Astrophys. 1994. 32: 115-52
Copyright © 1994 by . All rights reserved

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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 sigmaT, 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 As seen in Figure 2d, median values of MT/LB are essentially constant at a value near five over the entire type range S0-Im. Although the trends of LB, Rlin and MT are each in the same sense over this range, it still comes as a surprise to these authors to find neither a type dependence nor a distance dependence (Figure 1) in MT/LB. In particular, a type dependence has been found frequently in analysis of much smaller samples, though the range in slopes found in these determinations includes both positive and negative values, as well as a zero slope (Meisels 1983).

ROTATION CURVES Recent studies of the mass distribution within galaxies have employed the detailed rotation curves obtained either from the Halpha or HI 21 cm line. Rubin (1991) presents a summary of the results particularly of the optical studies that look for environmental effects. Broeils (1992) has examined the HI rotation curves and optical surface photometry of 23 spiral and irregular galaxies to look for the morphological dependences in the distribution of mass and light. An extensive bibliography of published rotation curves is available in Corradi & Capaccioli (1991).

In studying the dark matter distribution, HI rotation curves are more suitable given that they generally extend far beyond the point where Halpha 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:

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.

MASS SURFACE DENSITY As evident in Figure 3c, the total mass surface density sigmaT shows a clear decrease along the spiral sequence. The trend is slow but monotonic from S0/a to Sc with a steepening toward the dwarfs.

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