Cold Mode Accretion in Galaxy Formation
  A generic expectation for gas accreted by high mass haloes is that it is
shock heated to the virial temperature of the halo. In low mass haloes, or at
high redshift, however, the gas cooling rate is sufficiently rapid that an
accretion shock is unlikely to form. Instead, gas can accrete directly into the
centre of the halo in a `cold mode' of accretion. Although semi-analytic models
have always made a clear distinction between hydrostatic and rapid cooling they
have not made a distinction between whether or not an accretion shock forms.
Starting from the well-established Galform code, we investigate the effect of
explicitly accounting for cold mode accretion using the shock stability model
of Birnboim & Dekel. When we modify the code so that there is no effective
feedback from galaxy formation, we find that cold mode accretion is the
dominant channel for feeding gas into the galaxies at high redshifts. However,
this does not translate into a significant difference in the star formation
history of the universe compared to the previous code. When effective feedback
is included in the model, we find that the the cold mode is much less apparent
because of the presence of gas ejected from the galaxy. Thus the inclusion of
the additional cold mode physics makes little difference to basic results from
earlier semi-analytic models which used a simpler treatment of gas accretion.
For more sophisticated predictions of its consequences, we require a better
understanding of how the cold mode delivers angular momentum to galaxies and
how it interacts with outflows.
