Bright Galaxies and their Dark Matter Halos
For almost 40 years the astronomical community has agreed that large, bright galaxies are embedded within massive halos of dark matter. Although the existence of dark matter halos is well-accepted, the nature of the halos, and their relationship to the bright galaxies that they surround, is not well-constrained observationally. From a theoretical standpoint, the most successful model of structure formation in the universe is the Cold Dark Matter (CDM) model in which the dark matter accounts for about 85% of the total mass in the universe.
CDM makes specific predictions about the nature of dark matter galaxy halos, including the fact that the halos are triaxial and, therefore, should appear "flattened" (i.e., elliptical) in the plane of the sky. Here I will compare the locations of satellite galaxies in the Sloan Digital Sky Survey to the locations of satellite galaxies in the CDM Millennium Run computer simulation to show:
-  the dark matter halos of bright galaxies are, indeed, flattened and
-  in projection on the sky, the dark matter mass surrounding elliptical galaxies is in fairly good alignment with the luminous galaxy, while the dark matter mass surrounding disk galaxies is poorly-aligned with the luminous galaxy.
These results have important implications for the comparison theoretical predictions for intrinsic alignments of galaxies with observations, as well as the use of weak gravitational lensing to directly measure of the shapes of dark matter halos.