  We study the influence of low levels of metal enrichment on the cooling and
collapse of ionized gas in small protogalactic halos using three-dimensional,
smoothed particle hydrodynamics simulations. Our initial conditions represent
protogalaxies forming within a fossil HII region -- a previously ionized HII
region which has not yet had time to cool and recombine. Prior to cosmological
reionization, such regions should be relatively common, since the
characteristic lifetimes of the likely ionizing sources are significantly
shorter than a Hubble time. We show that in these regions, H_2 is the dominant
and most effective coolant, and that it is the amount of H_2 formed that
determines whether or not the gas can collapse and form stars.
  At the low metallicities (Z < 10^{-3} Z_sun) thought to be associated with
the transition from population III to early population II star formation, metal
line cooling has an almost negligible effect on the evolution of low density
gas, altering the density and temperature evolution of the gas by less than 1%
compared to the metal-free case at densities below 1 cm^{-3} and temperatures
above 2000 K. Although there is evidence that metal line cooling becomes more
effective at higher density, we find no significant differences in behaviour
from the metal-free case at any density below our sink particle creation
threshold at n = 500 cm^{-3}. Increasing the metallicity also increases the
importance of metal line cooling, but it does not significantly affect the
dynamical evolution of the low density gas until Z = 0.1 Z_sun. This result
holds regardless of whether or not an ultraviolet background is present.
