Absorption Line Signatures of Gas in Mini Dark Matter Halos

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. Kepner, Babul & Spergel (1997) have shown that gas in these “mini-halos” can readily be in a multi-phase state. 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: power-law, power-law + HeII break, Haardt & Madau (1996) 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 mini-halo 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 mini-halo, 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 mini-halo) is necessary to break this degeneracy. Finally, the mini-halo model is a useful tool for analyzing absorption line data in a multi-phase context and should become even more applicable as new space based observations become available. Princeton University Observatory, Peyton Hall, Ivy Lane, Princeton, NJ 08544–1001 (jvkepner/tripp/dns)@astro.princeton.edu Laboratory for Computational Astrophysics, NCSA, University of Illinois at Urbana-Champaign, 405 Mathews Ave., Urbana, IL 61801 Max Planck Institute fur Astrophysik, Karl-Schwarzschild Str. 1, 85768, Garching, Germany [email protected] Current address: MIT Lincoln Laboratory, Lexington, MA