Hydrodynamics of high-redshift galaxy collisions: From gas-rich disks to
  dispersion-dominated mergers and compact spheroids
  Disk galaxies at high redshift (z~2) are characterized by high fractions of
cold gas, strong turbulence, and giant star-forming clumps. Major mergers of
disk galaxies at high redshift should then generally involve such turbulent
clumpy disks. Merger simulations, however, model the ISM as a stable,
homogeneous, and thermally pressurized medium. We present the first merger
simulations with high fractions of cold, turbulent, and clumpy gas. We discuss
the major new features of these models compared to models where the gas is
artificially stabilized and warmed. Gas turbulence, which is already strong in
high-redshift disks, is further enhanced in mergers. Some phases are
dispersion-dominated, with most of the gas kinetic energy in the form of
velocity dispersion and very chaotic velocity fields, unlike merger models
using a thermally stabilized gas. These mergers can reach very high star
formation rates, and have multi-component gas spectra consistent with
SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas
are also characterized by highly dissipative gas collapse to the center of
mass, with the stellar component following in a global contraction. The final
galaxies are early-type with relatively small radii and high Sersic indices,
like high-redshift compact spheroids. The mass fraction in a disk component
that survives or re-forms after a merger is severely reduced compared to models
with stabilized gas, and the formation of a massive disk component would
require significant accretion of external baryons afterwards. Mergers thus
appear to destroy extended disks even when the gas fraction is high, and this
lends further support to smooth infall as the main formation mechanism for
massive disk galaxies.
