The Origin of Extended Disk Galaxies at z=2
  Galaxy formation models typically assume that the size and rotation speed of
galaxy disks are largely dictated by the mass, concentration, and spin of their
surrounding dark matter haloes. Equally important, however, are the fraction of
baryons in the halo that collect into the central galaxy, as well as the net
angular momentum that they are able to retain during its assembly process. We
explore the latter using a set of four large cosmological N-body/gasdynamical
simulations drawn from the OWLS (OverWhelmingly Large Simulations) project.
These runs differ only in their implementation of feedback from supernovae. We
find that, when expressed as fractions of their virial values, galaxy mass and
net angular momentum are tightly correlated. Galaxy mass fractions,
m_d=M_gal/M_vir, depend strongly on feedback, but only weakly on halo mass or
spin over the halo mass range explored here (M_vir>1e11 h^{-1}M_sun). The
angular momentum of a galaxy, j_d=J_gal/J_vir, correlates with m_d in a manner
that is insensitive to feedback and that deviates strongly from the simple j_d
= m_d assumption often adopted in semi-analytic models of galaxy formation. The
m_d-j_d correlation implies that, in a given halo, galaxy disk size is maximal
when the central galaxy makes up a substantial fraction (~20%-30%) of all
baryons within the virial radius. At z=2, such systems may host gaseous disks
with radial scale lengths as large as those reported for star-forming disks by
the SINS survey, even in moderately massive haloes of average spin. Extended
disks at z=2 may thus signal the presence of systems where galaxy formation has
been particularly efficient, rather than the existence of haloes with unusually
high spin parameter.
