Absorption-Line Signatures of Gas in Dark Matter Minihalos

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Abstract
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-1. Kepner, Babul & Spergel have shown that gas in these "minihalos" can readily be in a multiphase state. Additional observational evidence suggests that, in general, many absorption-line systems may also be multiphase in nature. Thus, computing the absorption lines of minihalos, in addition to providing signatures of small halos, is a natural way to explore multiphase behavior. The state of gas in minihalos 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 multiwavelength radiative transfer of an isotropic field, nonequilibrium chemistry, heating, cooling and self-consistent quasi-hydrostatic equilibrium gasdynamics. With this code detailed simulations were conducted of gas in minihalos using different types of background spectra: power-law, power-law + He II break, Haardt & Madau, and O-star. 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 minihalo model absorption-line signatures appear to be consistent with many current observations, given a sufficiently soft spectrum. Thus, in any given instance it is difficult to either rule in or rule out a minihalo, and in most cases additional data (e.g., optical counterparts or the lack thereof) or contextual information (e.g., evidence of significant star formation, which would disrupt gas in a minihalo) are necessary to break this degeneracy. Finally, the minihalo model is a useful tool for analyzing absorption-line data in a multiphase context and should become even more applicable as new space-based observations become available.
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