The effects of gas on morphological transformation in mergers: implications for bulge and disc demographics |
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| Authors: | Philip F Hopkins Rachel S Somerville Thomas J Cox † Lars Hernquist Shardha Jogee Dusan Kere&#; Chung-Pei Ma Brant Robertson ‡ Kyle Stewart |
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| Institution: | Department of Astronomy, University of California Berkeley, Berkeley, CA 94720, USA;Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA;Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA;Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA;Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA;Kavli Institute for Cosmological Physics, The Department of Astronomy and Astrophysics, University of Chicago, 933 East 56th Street, Chicago, IL 60637, USA;Enrico Fermi Institute, 5640 South Ellis Avenue, Chicago, IL 60637, USA;Center for Cosmology, Department of Physics and Astronomy, The University of California at Irvine, Irvine, CA 92697, USA |
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| Abstract: | Transformation of discs into spheroids via mergers is a well-accepted element of galaxy formation models. However, recent simulations have shown that the bulge formation is suppressed in increasingly gas-rich mergers. We investigate the global implications of these results in a cosmological framework, using independent approaches: empirical halo-occupation models (where galaxies are populated in haloes according to observations) and semi-analytic models. In both, ignoring the effects of gas in mergers leads to the overproduction of spheroids: low- and intermediate-mass galaxies are predicted to be bulge-dominated ( B / T ～ 0.5 at <1010 M⊙ , with almost no 'bulgeless' systems), even if they have avoided major mergers. Including the different physical behaviour of gas in mergers immediately leads to a dramatic change: bulge formation is suppressed in low-mass galaxies, observed to be gas-rich (giving B / T ～ 0.1 at <1010 M⊙ , with a number of bulgeless galaxies in good agreement with observations). Simulations and analytic models which neglect the similarity-breaking behaviour of gas have difficulty reproducing the strong observed morphology–mass relation. However, the observed dependence of gas fractions on mass, combined with suppression of bulge formation in gas-rich mergers, naturally leads to the observed trends. Discrepancies between observations and models that ignore the role of gas increase with redshift; in models that treat gas properly, galaxies are predicted to be less bulge-dominated at high redshifts, in agreement with the observations. We discuss implications for the global bulge mass density and future observational tests. |
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| Keywords: | galaxies: active galaxies: evolution galaxies: formation galaxies: spiral cosmology: theory |
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