Edinburgh Research Explorer

Galaxy merger statistics and inferred bulge-to-disk ratios in cosmological SPH simulations

Research output: Contribution to journalArticle

  • Ariyeh H. Maller
  • Neal Katz
  • Dušan Kereš
  • Romeel Davé
  • David H. Weinberg

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Original languageEnglish
Pages (from-to)763-772
Number of pages10
JournalAstrophysical Journal
Issue number2 I
Publication statusPublished - 20 Aug 2006


We construct merger trees for galaxies identified in a cosmological hydrodynamic simulation and use them to characterize predicted merger rates as a function of redshift, galaxy mass, and merger mass ratio. Atz = 0.3, we find a mean rate of 0.054 mergers per galaxy per Gyr above a 1:2 mass ratio threshold for massive galaxies (baryonic mass above 6.4 × 1010 M ), but only 0.018 Gyr-1 for lower mass galaxies. The mass ratio distribution is prop;Rmerg-1.2 for the massive galaxy sample, so high-mass mergers dominate the total merger growth rate. The predicted rates increase rapidly with increasing redshift, and they agree reasonably well with observational estimates. A substantial fraction of galaxies do not experience any resolved mergers during the course of the simulation, and even for the high-mass sample, only 50% of galaxies experience a greater than 1:4 merger since z = 1. Typical galaxies thus have fairly quiescent merger histories. We assign bulge-to-disk ratios to simulated galaxies by assuming that mergers above a mass ratio threshold Rmajor convert stellar disks into spheroids. With Rmajor values of 1:4, we obtain a fairly good match to the observed dependence of the early-type fraction on galaxy mass. However, the predicted fraction of truly bulge-dominated systems (f bulge > 0.8) is small, and producing a substantial population of bulge-dominated galaxies may require a mechanism that shuts off gas accretion at late times and/or additional processes (besides major mergers) for producing bulges.

    Research areas

  • Galaxies: elliptical and lenticular, cD, Galaxies: formation, Galaxies: spiral, Methods: n-body simulations

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