© CAMBRIDGE UNIVERSITY PRESS 1999

1.4.4 Estimates on Galaxy Halo Scales

A classic paper by Little & Tremaine (1987) had argued that the available data on the Milky Way satellite galaxies required that the Galaxy's halo terminate at about 50 kpc, with a total mass of only about 2.5 x 10¹¹ M. But by 1991, new data on local satellite galaxies, especially Leo I, became available, and the Little-Tremaine estimator increased to 1.25 x 10¹² M. A recent, detailed study finds a mass inside 50 kpc of (5.4 ± 1.3) x 10¹¹ M (Kochanek 1996a). Work by Zaritsky et al. (1993) has shown that other spiral galaxies also have massive halos. They collected data on satellites of isolated spiral galaxies, and concluded that the fact that the relative velocities do not fall off out to a separation of at least 200 kpc shows that massive halos are the norm. The typical rotation velocity of ~ 200-250 km s^-1 implies a mass within 200 kpc of ~ 2 x 10¹² M. A careful analysis taking into account selection effects and satellite orbit uncertainties concluded that the indicated value of ₀ exceeds 0.13 at 90% confidence (Zaritsky & White 1994), with preferred values exceeding 0.3. Newer data suggesting that relative velocities do not fall off out to a separation of ~ 400 kpc (Zaritsky et al. 1997) presumably would raise these ₀ estimates.

However, if galaxy dark matter halos are really so extended and massive, that would imply that when such galaxies collide, the resulting tidal tails of debris cannot be flung very far. Therefore, the observed merging galaxies with extended tidal tails such as NGC 4038 / 39 (the Antennae) and NGC 7252 probably have halo:(disk+bulge) mass ratios less than 10:1 (Dubinski, Mihos, & Hernquist 1996), unless the stellar tails are perhaps made during the collision process from gas that was initially far from the central galaxies (J. Ostriker, private communication, 1996); the latter possibility can be checked by determining the ages of the stars in these tails.

A direct way of measuring the mass and spatial extent of many galaxy dark matter halos is to look for the small distortions of distant galaxy images due to gravitational lensing by foreground galaxies. This technique was pioneered by Tyson et al. (1984). Though the results were inconclusive (Kovner & Milgrom 1987), powerful constraints could perhaps be obtained from deep HST images or ground-based images with excellent seeing. Such fields would also be useful for measuring the correlated distortions of galaxy images from large-scale structure by weak gravitational lensing; although a pilot project (Mould et al. 1994) detected only a marginal signal, a reanalysis detected a significant signal suggesting that _{0 8} ~ 1 (Villumsen 1995). Several groups are planning major projects of this sort. The first results from an analysis of the Hubble Deep Field gave an average galaxy mass interior to 20 h^-1 kpc of 5.9^+2.5_-2.7 x 10¹¹ h^-1 M (Dell'Antonio & Tyson 1996).