Kiloparsec-Scale Simulations of Star Formation in Disk Galaxies. I. The
  unmagnetized and zero-feedback limit
  We present hydrodynamic simulations of the evolution of self-gravitating
dense gas on scales of 1 kiloparsec down to < parsec in a galactic disk,
designed to study dense clump formation from giant molecular clouds (GMCs).
These structures are expected to be the precursors to star clusters and this
process may be the rate limiting step controling star formation rates in
galactic systems as described by the Kennicutt-Schmidt relation. We follow the
thermal evolution of the gas down to ~5K using extinction-dependent heating and
cooling functions. We do not yet include magnetic fields or localized stellar
feedback, so the evolution of the GMCs and clumps is determined solely by
self-gravity balanced by thermal and turbulent pressure support and the large
scale galactic shear. While cloud structures and densities change significantly
during the simulation, GMC virial parameters remain mostly above unity for time
scales exceeding the free-fall time of GMCs indicating that energy from
galactic shear and large-scale cloud motions continuously cascades down to and
within the GMCs. We implement star formation at a slow, inefficient rate of 2%
per local free-fall time, but even this yields global star formation rates that
are about two orders of magnitude larger than the observed Kennicutt-Schmidt
relation due to over-production of dense gas clumps. We expect a combination of
magnetic support and localized stellar feedback is required to inhibit dense
clump formation to ~1% of the rate that results from the nonmagnetic,
zero-feedback limit.
