Gas Accretion and Angular Momentum
  In this chapter, we review the role of gas accretion to the acquisition of
angular momentum, both in galaxies and in their gaseous halos. We begin by
discussing angular momentum in dark matter halos, with a brief review of tidal
torque theory and the importance of mergers, followed by a discussion of the
canonical picture of galaxy formation within this framework, where halo gas is
presumed to shock-heat to the virial temperature of the halo, following the
same spin distribution as the dark matter halo before cooling to the center of
the halo to form a galaxy there. In the context of recent observational
evidence demonstrating the presence of high angular momentum gas in galaxy
halos, we review recent cosmological hydrodynamic simulations that have begun
to emphasize the role of "cold flow" accretion---anisotropic gas accretion
along cosmic filaments that does not shock-heat before sinking to the central
galaxy. We discuss the implications of these simulations, reviewing a number of
recent developments in the literature, and suggest a revision to the canonical
model as it relates to the expected angular momentum content of gaseous halos
around galaxies.
