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

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, Dusan Keres, Andrew Wetzel, Chris Hayward

Research output: Contribution to journalArticlepeer-review


We investigate the merger histories of isolated dwarf galaxies based on a suite of 15 highresolution cosmological zoom-in simulations, all with masses of Mhalo ≈ 1010M⊙ (and M ∼ 105-107M⊙) at z = 0, from the Feedback in Realistic Environments (FIRE) project. The stellar populations of these dwarf galaxies at z = 0 are formed essentially entirely "in situ": Over 90% of the stellar mass is formed in the main progenitor in all but two cases, and all 15 of the galaxies have >70% 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 5 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 make it extremely difficult to discern the effects of mergers in the vast majority of dwarfs either photometrically or using resolvedstar color-magnitude diagrams (CMDs). The important implication for near-field cosmology is that star formation histories 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 star formation histories of merged dwarfs.

Original languageEnglish (US)
JournalUnknown Journal
StatePublished - Jan 18 2018


  • dark matter
  • Galaxies: Dwarf
  • Galaxies: Evolution
  • Galaxies: Formation
  • Galaxies: Star formation
  • Galaxies: Structure

ASJC Scopus subject areas

  • General

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