There has been a long period of time over which it has been perfectly acceptable to write papers on investigations into the nature of the dark matter halos of field galaxies that begin with a statement along the lines of "Although modern theories of galaxy formation posit that all large galaxies reside within massive halos of dark matter, the characteristic properties of those halos (e.g., mass, radial extent, and shape) are not well-constrained by the current observations". That time is now coming to an end. The wealth of data that has been acquired in recent years is truly beginning to place strong, direct constraints on the dark matter halos of field galaxies.
Weak lensing and satellite dynamics have proven themselves to be excellent probes of the gravitational potentials of large, bright galaxies on physical scales r 100 h-1 kpc. While one might be skeptical and discount the results that come from one technique or the other, the fact that both are yielding consistent constraints cannot be ignored. Both weak lensing and satellite dynamics lead to statistical constraints on the halo population as a whole, rather than constraints on any one particular galaxy halo, and it is especially the acquisition of extremely large data sets that has allowed these techniques to begin to fulfill their promise of mapping out the gravitational potentials associated with large, massive halos. Weak lensing and satellite dynamics have inherent advantages and disadvantages, but since their systematic errors and selection biases are completely uncorrelated, they are extremely complementary to each other. At least at the moment, when strong constraints are only just beginning to emerge from each technique, this complementarity is very reassuring.
Based upon my own critical, and hopefully unbiased, reading of the recent literature, I think it is fair to say that, both individually and in combination, weak lensing and satellite dynamics are pointing toward the following scenario for the nature of large, bright field galaxies and their halos:
The dark matter halos are well-characterized by NFW-type objects in terms of their gravitational properties. The dynamics of satellite galaxies strongly prefer NFW halos to isothermal halos.
The virial masses that are inferred for large field galaxies are in good agreement with the predictions for galaxy-mass halos in the context of cold dark matter. Specifically, the virial mass of the halo of an "average" L* galaxy is in the range (8 - 10) × 1011h-1 M when NFW profiles are fit to the data.
There are clear differences in the depths of the potential wells of the halos that surround galaxies of differing morphology and differing intrinsic luminosity. Specifically, the virial masses of the halos of L* ellipticals exceed those of L* spirals by a factor of at least 2. The actual value of the mass excess depends upon details of the data and its analysis. In addition, the virial masses of the halos of high luminosity galaxies exceed those of low luminosity galaxies. Again, however, the amount by which they differ depends upon details of the data and its analysis.
Averaged over all galaxies with L L*, the mass-to-light ratio computed on scales larger than the optical radii of the galaxies is, at most, weakly-dependent upon the luminosity of the galaxy. At the 2 level, the mass-to-light ratio of the average galaxy with L L* is consistent with a constant value.
The dark matter halos are flattened, rather than spherical, and the degree of flattening on large scales (~ 100 kpc to ~ 200 kpc) is consistent with the predictions of cold dark matter.
It is worth noting that the above list comes from quite a diverse set of data. In particular, the data are spread over a wide range in redshift. With the exception of preliminary data from DEEP2, the satellite dynamics studies have median redshifts of zmed ~ 0.07. The weak lenses in the SDSS data have a median redshift of zmed ~ 0.16 and the weak lenses in the RCS and COMBO-17 data have considerably higher redshifts, zmed ~ 0.4. Since it is clear that the field galaxy population has evolved since z ~ 0.5, it is not entirely fair to lump the results from all of these studies together, and I think the big challenge to the weak lensing community in particular will be to eventually place constraints on the evolution of field galaxies and their halos from, say, z ~ 1 to the present.
Nevertheless, I think we have reached a particularly gratifying time in which we are really being able to measure some of the fundamental properties of dark mater halos on physical scales that extend well beyond the visible images of the galaxies at their centers. A remarkably consistent picture of the large-scale gravitational properties of the halos is emerging from the observations and, at least for now, that picture seems entirely in accord with a cold dark matter universe.
I am deeply indebted to Henk Hoekstra, Martina Kleinheinrich, and Erin Sheldon for their help with the preparation of numerous figures at a time when they all had much more important things do to, and to Tom Peterson, without whose indulgence and encouragement this article would probably never have been written. Support under NSF contracts AST-0098572 and AST-0406844 is also gratefully acknowledged.