Baryons and Their Halos
  Galaxies are composed of baryonic stars and gas embedded in dark matter
halos. Here I briefly review two aspects of the connection between baryons and
their halos. (1) The observed baryon content of galaxies falls short of the
cosmic baryon fraction by an amount that varies systematically with mass. Where
these missing baryons now reside is unclear. (2) The characteristic
acceleration in disk galaxies correlates strongly with their baryonic mass
surface density. This implies a close coupling between the gravitational
dynamics, which is presumably dominated by dark matter, and the purely baryonic
components of galaxies.
