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The effects of gas on morphological transformation in mergers: implications for bulge and disc demographics

Hopkins, Philip F. and Somerville, Rachel S. and Cox, Thomas J. and Hernquist, Lars and Jogee, Shardha and Kereš, Dušan and Ma, Chung-Pei and Robertson, Brant and Stewart, Kyle (2009) The effects of gas on morphological transformation in mergers: implications for bulge and disc demographics. Monthly Notices of the Royal Astronomical Society, 397 (2). pp. 802-814. ISSN 0035-8711.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Hopkins, Philip F.0000-0003-3729-1684
Hernquist, Lars0000-0001-6950-1629
Kereš, Dušan0000-0002-1666-7067
Robertson, Brant0000-0002-4271-0364
Additional Information:© 2009 The Authors. Journal compilation © 2009 RAS. Accepted 2009 April 25. Received 2009 April 20; in original form 2009 January 21. We thank Fabio Governato, James Bullock and Chris Purcell for helpful discussions. We also thank the anonymous referee for suggestions that improved this manuscript. This work was supported in part by NSF grants ACI 96-19019, AST 00-71019, AST 02-06299 and AST 03-07690, and NASA ATP grants NAG5-12140, NAG5-13292 and NAG5-13381. Support for PFH was provided by the Miller Institute for Basic Research in Science, University of California Berkeley. Support for TJC was provided by the W. M. Keck Foundation. Support for DK was provided by Harvard University and the Institute for Theory and Computation (ITC). RSS thanks the ITC at Harvard University for supporting visits that contributed to this work.
Funding AgencyGrant Number
NSFACI 96-19019
NSFAST 00-71019
NSFAST 02-06299
NSFAST 03-07690
Miller Institute for Basic Research in ScienceUNSPECIFIED
W. M. Keck FoundationUNSPECIFIED
Harvard UniversityUNSPECIFIED
Subject Keywords:galaxies: active, galaxies: evolution, galaxies: formation, galaxies: spiral, cosmology: theory
Issue or Number:2
Record Number:CaltechAUTHORS:20200529-093432060
Persistent URL:
Official Citation:Philip F. Hopkins, Rachel S. Somerville, Thomas J. Cox, Lars Hernquist, Shardha Jogee, Dusan Kereš, Chung-Pei Ma, Brant Robertson, Kyle Stewart, The effects of gas on morphological transformation in mergers: implications for bulge and disc demographics, Monthly Notices of the Royal Astronomical Society, Volume 397, Issue 2, August 2009, Pages 802–814,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103545
Deposited By: George Porter
Deposited On:01 Jun 2020 14:05
Last Modified:01 Jun 2020 14:05

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