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Growth of galaxies in SPH simulations

Dusan Keres, University of Massachusetts Amherst


We explore the growth of galaxies formed in self-consistent Smoothed Particle Hydrodynamic (SPH) cosmological simulations. In the first Chapter, we examine the temperature history of gas accreted by forming galaxies in SPH simulations. Some of this gas follows the track expected in the conventional picture of galaxy formation, shock heating to roughly the virial temperature of the galaxy potential well (T ∼ 106 K for a Milky Way type galaxy) before cooling, condensing, and forming stars. However, a large fraction of the accreted gas radiates its acquired gravitational energy at much lower temperatures, typically T < 10 5 K, and the histogram of maximum gas temperatures is clearly bimodal. The "cold mode" of gas accretion dominates for low mass galaxies (baryonic mass Mgal [special characters omitted] 10 10.3[special characters omitted] or halo mass Mhalo [special characters omitted] 1011.4[special characters omitted]), while the conventional "hot mode" dominates the growth of high mass systems. Cold accretion is often directed along filaments, allowing galaxies to efficiently draw gas from large distances, while hot accretion is quasi-spherical. The galaxy and halo mass dependence leads to a redshift and environmental dependence of the cold and hot mode accretion rates, with cold mode dominating at high redshifts and in low density regions today, and hot mode dominating in group and cluster environments at low redshifts. The cosmic star formation rate tracks the overall history of gas accretion, and its decline at low redshifts follows the combined decline of the cold and hot mode accretion rates. The drop in cold accretion is driven by the decreasing infall rate of material into halos, while for hot accretion this slower mass growth is further modified by the longer cooling times within larger halos. If we allowed hot accretion to be suppressed by conduction or AGN feedback, then the simulation predictions would change in interesting ways, perhaps resolving conflicts with the colors of ellipticals and the cutoff of the galaxy luminosity function. The transition at Mhalo ∼ 1011.4[special characters omitted] between cold and hot mode domination is similar to that found by Birnboim & Dekel (2003) using 1-d simulations and analytic arguments. The corresponding baryonic mass is tantalizingly close to the scale at which Kauffmann et al. (2003a) find a marked shift in galaxy properties, and we speculate on possible connections between these theoretical and observational transitions. (Abstract shortened by UMI.)

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Recommended Citation

Keres, Dusan, "Growth of galaxies in SPH simulations" (2007). Doctoral Dissertations Available from Proquest. AAI3275739.