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Title:
The effects of gas on morphological transformation in mergers: implications for bulge and disc demographics
Authors:
Hopkins, Philip F.; Somerville, Rachel S.; Cox, Thomas J.; Hernquist, Lars; Jogee, Shardha; Kereš, Dusan; Ma, Chung-Pei; Robertson, Brant; Stewart, Kyle
Affiliation:
AA(Department of Astronomy, University of California Berkeley, Berkeley, CA 94720, USA), AB(Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA; Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA), AC(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA), AD(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA), AE(Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA), AF(Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA), AG(Department of Astronomy, University of California Berkeley, Berkeley, CA 94720, USA), AH(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), AI(Center for Cosmology, Department of Physics and Astronomy, The University of California at Irvine, Irvine, CA 92697, USA)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 397, Issue 2, pp. 802-814. (MNRAS Homepage)
Publication Date:
08/2009
Origin:
MNRAS
MNRAS Keywords:
galaxies: active , galaxies: evolution , galaxies: formation , galaxies: spiral , cosmology: theory
DOI:
10.1111/j.1365-2966.2009.14983.x
Bibliographic Code:
2009MNRAS.397..802H

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 <1010Msolar, 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 <1010Msolar, 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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