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No assembly required: mergers are mostly irrelevant for the growth of low-mass dwarf galaxies

Fitts, Alex and Boylan-Kolchin, Michael and Bullock, James S. and Weisz, Daniel R. and El-Badry, Kareem and Wheeler, Coral and Faucher-Giguère, Claude-André and Quataert, Eliot and Hopkins, Philip F. and Kereš, Dušan and Wetzel, Andrew and Hayward, Christopher C. (2018) No assembly required: mergers are mostly irrelevant for the growth of low-mass dwarf galaxies. Monthly Notices of the Royal Astronomical Society, 479 (1). pp. 319-331. ISSN 0035-8711.

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We investigate the merger histories of isolated dwarf galaxies based on a suite of 15 high-resolution cosmological zoom-in simulations, all with masses of M_(halo) ≈ 10^(10) M⊙ (and M⋆∼10^5−10^7M⊙) at z = 0, from the Feedback in Realistic Environments project. The stellar populations of these dwarf galaxies at z = 0 are formed essentially entirely ‘in situ’: over 90 percent of the stellar mass is formed in the main progenitor in all but two cases, and all 15 of the galaxies have >70 percent of their stellar mass formed in situ. Virtually all galaxy mergers occur prior to z ∼ 3, meaning that accreted stellar populations are ancient. On average, our simulated dwarfs undergo five galaxy mergers in their lifetimes, with typical pre-merger galaxy mass ratios that are less than 1:10. This merger frequency is generally comparable to what has been found in dissipationless simulations when coupled with abundance matching. Two of the simulated dwarfs have a luminous satellite companion at z= 0. These ultra-faint dwarfs lie at or below current detectability thresholds but are intriguing targets for next-generation facilities. The small contribution of accreted stars makes it extremely difficult to discern the effects of mergers in the vast majority of dwarfs either photometrically or using resolved-star colour–magnitude diagrams (CMDs). The important implication for near-field cosmology is that star formation histories (SFHs) of comparably massive galaxies derived from resolved CMDs should trace the build-up of stellar mass in one main system across cosmic time as opposed to reflecting the contributions of many individual SFHs of merged dwarfs.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Boylan-Kolchin, Michael0000-0002-9604-343X
Weisz, Daniel R.0000-0002-6442-6030
El-Badry, Kareem0000-0002-6871-1752
Faucher-Giguère, Claude-André0000-0002-4900-6628
Quataert, Eliot0000-0001-9185-5044
Hopkins, Philip F.0000-0003-3729-1684
Kereš, Dušan0000-0002-1666-7067
Wetzel, Andrew0000-0003-0603-8942
Hayward, Christopher C.0000-0003-4073-3236
Additional Information:© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2018 May 25. Received 2018 April 13; in original form 2017 December 28. Published: 06 June 2018. MBK and AF acknowledge support from the National Science Foundation (grant AST-1517226). MBK was also partially supported by NASA through HST grants AR-12836, AR-13888, AR-13896, AR-14282, AR-14554, GO-12914, and GO-14191 awarded by the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555. JSB was supported by NSF AST-1518291, HST-AR-14282, and HST-AR-13888. Support for PFH was provided by an Alfred P. Sloan Research Fellowship, NASA ATP Grant NNX14AH35G, and NSF Collaborative Research Grant #1411920 and CAREER grant #1455342. CAFG was supported by NSF through grants AST-1412836, AST-1517491, AST-1715216, and CAREER award AST-1652522, and by NASA through grant NNX15AB22G. DRW is supported by a fellowship from the Alfred P. Sloan Foundation. EQ was supported by NASA ATP grant 12-APT12-0183, a Simons Investigator award from the Simons Foundation, and the David and Lucile Packard Foundation. DK was supported by NSF grant AST-1715101 and the Cottrell Scholar Award from the Research Corporation for Science Advancement. AW was supported by a Caltech-Carnegie Fellowship, in part through the Moore Center for Theoretical Cosmology and Physics at Caltech, and by NASA through grant HST-GO-14734 from STScI. This work used computational resources of the University of Texas at Austin and the Texas Advanced Computing Center (TACC;, the NASA Advanced Supercomputing (NAS) Division and the NASA Center for Climate Simulation (NCCS) through allocations SMD-15-5902, SMD-15-5904, SMD-16-7043, and SMD-16-6991, and the Extreme Science and Engineering Discovery Environment (XSEDE, via allocations TG-AST110035, TG-AST130039, and TG-AST140080), which is supported by National Science Foundation grant number OCI-1053575.
Group:TAPIR, Moore Center for Theoretical Cosmology and Physics
Funding AgencyGrant Number
NASA Hubble FellowshipAR-12836
NASA Hubble FellowshipAR-13888
NASA Hubble FellowshipAR-13896
NASA Hubble FellowshipAR-14282
NASA Hubble FellowshipAR-14554
NASA Hubble FellowshipGO-12914
NASA Hubble FellowshipGO-14191
Alfred P. Sloan FoundationUNSPECIFIED
Simons FoundationUNSPECIFIED
David and Lucile Packard FoundationUNSPECIFIED
Research CorporationUNSPECIFIED
Caltech-Carnegie FellowshipUNSPECIFIED
Moore Center for Theoretical Cosmology and Physics, CaltechUNSPECIFIED
Subject Keywords:galaxies: dwarf – galaxies: evolution – galaxies: formation – galaxies: star formation – galaxies: structure – dark matter
Record Number:CaltechAUTHORS:20180829-152027447
Persistent URL:
Official Citation:Alex Fitts, Michael Boylan-Kolchin, James S Bullock, Daniel R Weisz, Kareem El-Badry, Coral Wheeler, Claude-André Faucher-Giguère, Eliot Quataert, Philip F Hopkins, Dušan Kereš, Andrew Wetzel, Christopher C Hayward; No assembly required: mergers are mostly irrelevant for the growth of low-mass dwarf galaxies, Monthly Notices of the Royal Astronomical Society, Volume 479, Issue 1, 1 September 2018, Pages 319–331,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:89288
Deposited By: Tony Diaz
Deposited On:29 Aug 2018 22:36
Last Modified:29 Aug 2018 22:36

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