Orbiting Circum-galactic Gas as a Signature of Cosmological Accretion
  We use cosmological SPH simulations to study the kinematic signatures of cool
gas accretion onto a pair of well-resolved galaxy halos. Cold-flow streams and
gas-rich mergers produce a circum-galactic component of cool gas that generally
orbits with high angular momentum about the galaxy halo before falling in to
build the disk. This signature of cosmological accretion should be observable
using background-object absorption line studies as features that are offset
from the galaxy's systemic velocity by ~100 km/s. Accreted gas typically
co-rotates with the central disk in the form of a warped, extended cold flow
disk, such that the observed velocity offset is in the same direction as galaxy
rotation, appearing in sight lines that avoid the galactic poles. This
prediction provides a means to observationally distinguish accreted gas from
outflow gas: the accreted gas will show large one-sided velocity offsets in
absorption line studies while radial/bi-conical outflows will not (except
possibly in special polar projections). This rotation signature has already
been seen in studies of intermediate redshift galaxy-absorber pairs; we suggest
that these observations may be among the first to provide indirect
observational evidence for cold accretion onto galactic halos. Cold mode halo
gas typically has ~3-5 times more specific angular momentum than the dark
matter. The associated cold mode disk configurations are likely related to
extended HI/XUV disks seen around galaxies in the local universe. The fraction
of galaxies with extended cold flow disks and associated offset absorption-line
gas should decrease around bright galaxies at low redshift, as cold mode
accretion dies out.
