Absorption-line signatures of gas in dark matter minihalos

Recent observations and theoretical calculations suggest that some QSO abso rption-line systems may be due to gas in small dark matter halos with circu lar velocities on the order of 30 km s(-1). Kepner, Babul & Spergel have sh own that gas in these "minihalos" can readily be in a multiphase state. Add itional observational evidence suggests that, in general, many absorption-l ine systems may also be multiphase in nature. Thus, computing the absorptio n lines of minihalos, in addition to providing signatures of small halos, i s a natural way to explore multiphase behavior. The state of gas in minihal os is strongly affected by the background UV radiation field. To address th is issue, a code was developed that includes many of the chemical and radia tive processes found in CLOUDY and also incorporates spherically symmetric multiwavelength radiative transfer of an isotropic held, nonequilibrium che mistry, heating, cooling and self-consistent quasi-hydrostatic equilibrium gasdynamics. With this code detailed simulations were conducted of gas in m inihalos using different types of background spectra: power-law, power-law + He II break, Haardt & Madau, and O-star. From these simulations, the abso rption-line signatures of the gas were computed and compared with a variety of observations: high-redshift metal lines, He lines, and low-redshift met al-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 mod el is a useful tool for analyzing absorption-line data in a multiphase cont ext and should become even more applicable as new space-based observations become available.