How are Bright Galaxies Embedded within their Dark Matter Halos?

Recent studies suggest that the orientations of large, bright galaxies within their dark matter halos are related to the morphologies of the galaxies themselves. Elliptical galaxies tend to be oriented such that, in projection on the sky, the mass (i.e., the dark matter) is well-aligned with the luminous galaxy. On the other hand, disk galaxies tend to be oriented such that their angular momentum vectors are aligned with the angular momenta of their halos. This results in a misalignment of mass and light, and has important implications for future studies that seek to measure the shapes of the dark matter halos that surround disk galaxies. 1. Bright Galaxies and Dark Matter Halos The idea that large, bright galaxies reside within massive, invisible halos of dark matter is well-accepted. In fact, if the currently-favored theory of structure formation, known as Cold Dark Matter (CDM), is correct, then the dark matter halos of galaxies like our own Milky Way are expected to completely dwarf the visible galaxies that reside within them. High-resolution computer simulations of CDM universes suggest that the dark matter halos of large, bright galaxies should extend to radii that are at least a factor of 10 larger than the radii of the visible galaxies (see, e.g., [1] and references therein). In addition, the simulations suggest that the dark matter halos should be 100 to 200 times more massive than the sum total of the visible material contained within the galaxies (i.e., the stars, the gas, and the dust). For all intents and purposes, then, the “mass” of a galaxy is the mass of its dark matter halo. Although the existence of dark matter halos is well-accepted, relatively few direct constraints exist for the sizes and shapes of the halos that surround the galaxies we observe in our Universe. The reason for this is simple: at present we cannot directly detect the dark matter halos at any wavelength of light. (In the future, however, it may be possible to detect the gamma rays that are produced when CDM particles in the dark matter halos annihilate.) If we wish to place strong constraints on the nature of the dark matter halos, we need a way to “see” that which is invisible. In the past, a number of different techniques have been used to study the dark matter halos of galaxies, and in all cases the techniques use some type of luminous material as a “tracer” of the gravitational potentials of the halos. Here I will focus on recent work that has been carried out with two of these techniques: satellite galaxies that orbit large “primary” galaxies, and weak gravitational lensing. Observationally, the most poorly constrained property of the dark matter halos is their overall shape distribution. CDM predicts that the halos are not spherical. Rather, they