A Bare Molecular Cloud at z~0.45
  Several neutral species (MgI, SiI, CaI, FeI) have been detected in a weak
MgII absorption line system (W_r(2796)~0.15 Angstroms) at z~0.45 along the
sightline toward HE0001-2340. These observations require extreme physical
conditions, as noted in D'Odorico (2007). We place further constraints on the
properties of this system by running a wide grid of photoionization models,
determining that the absorbing cloud that produces the neutral absorption is
extremely dense (~100-1000/cm^3), cold (<100 K), and has significant molecular
content (~72-94%). Structures of this size and temperature have been detected
in Milky Way CO surveys, and have been predicted in hydrodynamic simulations of
turbulent gas. In order to explain the observed line profiles in all neutral
and singly ionized chemical transitions, the lines must suffer from unresolved
saturation and/or the absorber must partially cover the broad emission line
region of the background quasar. In addition to this highly unusual cloud,
three other ordinary weak MgII clouds (within densities of ~0.005/cm^3 and
temperatures of ~10000K) lie within 500 km/s along the same sightline. We
suggest that the "bare molecular cloud", which appears to reside outside of a
galaxy disk, may have had in situ star formation and may evolve into an
ordinary weak MgII absorbing cloud.
