Internal Alignments of Red Versus Blue Discs in Dark Matter Halos
  Large surveys have shown that red galaxies are preferentially aligned with
their halos while blue galaxies have a more isotropic distribution. Since halos
generally align with their filaments this introduces a bias in the measurement
of the cosmic shear from weak lensing. It is therefore vitally important to
understand why this difference arises. We explore the stability of different
disc orientations within triaxial halos. We show that, in the absence of gas,
the disc orientation is most stable when its spin is along the minor axis of
the halo. Instead when gas cools onto a disc it is able to form in almost
arbitrary orientation, including off the main planes of the halo (but avoiding
an orientation perpendicular to the halo's intermediate axis). Substructure
helps gasless galaxies reach alignment with the halo faster, but have less
effect on galaxies when gas is cooling onto the disc. Our results provide a
novel and natural interpretation for why red, gas poor galaxies are
preferentially aligned with their halo, while blue, star-forming, galaxies have
nearly random orientations, without requiring a connection between galaxies'
current star formation rate and their merger history.
