  Recent observations and theoretical calculations suggest that some QSO
absorption line systems may be due to gas in small dark matter halos with
circular velocities on the order of 30 km/s. Additional observational evidence
suggests that, in general, many absorption line systems may also be multi-phase
in nature. Thus, computing the absorption lines of mini-halos, in addition to
providing signatures of small halos, is a natural way to explore multi-phase
behavior. The state of gas in mini-halos is strongly affected by the background
UV radiation field. To address this issue a code was developed that includes
many of the chemical and radiative processes found in CLOUDY and also
incorporates spherically symmetric multi-wavelength radiative transfer of an
isotropic field, non-equilibrium chemistry, heating, cooling and
self-consistent quasi hydro-static equilibrium gas dynamics. With this code
detailed simulations were conducted of gas in mini-halos using different types
of background spectra. From these simulations the absorption line signatures of
the gas were computed and compared with a variety of observations: high
redshift metal lines, He lines and low redshift metal line systems. Based on
these results the mini-halo model absorption line signatures appear to be
consistent with many current observations given a sufficiently soft spectrum.
