AAS 207th Meeting, 8-12 January 2006
Session 170 Cosmology, Early Universe, Cosmic Distance Scale
Poster, Thursday, 9:20am-4:00pm, January 12, 2006, Exhibit Hall

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[170.21] The Lack of Influence of Metallicity on Cooling and Collapse of Ionized Gas in Small Protogalactic Halos

A.-K. Jappsen (Astrophysikalisches Institut Potsdam, Germany), S. C. O. Glover (American Museum of Natural History, New York, USA), R. S. Klessen (Astrophysikalisches Institut Potsdam, Germany), M.-M. Mac Low (American Museum of Natural History, New York, USA; Columbia University, New York, USA)

We study the influence of low levels of metal enrichment on the cooling and collapse of ionized gas in small protogalactic halos. We use three-dimensional, smoothed particle hydrodynamics simulations, run with the publicly available parallel code GADGET (Springel et al. 2001). We implement a sink particle algorithm. This allows us to safely represent gas that has collapsed beyond the resolution limit without causing numerical errors within the resolved regions of the simulation. We also include the necessary framework for following the non-equilibrium chemistry of H2 in the protogalactic gas, and a treatment of radiative heating and cooling.

Our initial conditions represent protogalaxies forming within a fossil H~{\sc ii} region---a previously ionized H~{\sc ii} region that 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, H2 is the dominant and most effective coolant, even in the presence of small amounts of metals. It is the amount of H2 which forms that controls whether or not the gas can collapse and form stars. For metallicities {\rm Z} \leq 10-3\, {\rm Z}\odot, we find that metal line cooling alters the density and temperature evolution of the gas by less than 1% compared to the metal-free case at densities below 1\,\mathrm{cm-3} and temperatures above 2000\,\mathrm{K}. However, at higher densities and lower temperatures, metal line cooling does become rather more important, and will affect the ability of the gas to fragment. We also show that an external ultraviolet background delays or suppresses the cooling and collapse of the gas regardless of whether or not it is metal-enriched.

RSK and A-KJ acknowledge support from the Emmy Noether Program of the Deutsche Forschungsgemeinschaft (grant no. KL1358/1). M-MML acknowledges support from NSF grants AST99-85392 and AST03-07854, and NASA grant NAG5-10103. SCOG acknowledges support from NSF grant AST03-07793, and NASA grant NAG5-13028. The simulations were performed on the cluster "sanssouci" at Astrophysikalisches Institut Potsdam.

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