Mergers, starbursts, and quenching in the Simba simulation

Francisco Rodríguez Montero, Romeel Davé, Vivienne Wild, Daniel Anglés-Alcázar, Desika Narayanan

Research output: Contribution to journalArticlepeer-review

72 Citations (Scopus)


We use the Simbacosmological galaxy formation simulation to investigate the
relationship between major mergers (≤ 4:1), starbursts, and galaxy quenching. Mergers are identified via sudden jumps in stellar mass M∗ well above that expected from in situ star formation, while quenching is defined as going from specific star formation rate sSFR>t−1H to sSFR<0.2t−1H, where tH is the Hubble time. At z≈0−3, mergers show ∼×2−3 higher SFR than a mass-matched sample of star-forming galaxies, but globally represent ≤1% of the cosmic SF budget. At low masses, the increase in SFR in mergers is mostly attributed to an increase in the H2 content, but for M∗≥1010.5M⊙ mergers also show an elevated star formation efficiency suggesting denser gas within merging galaxies. The merger rate for star-forming galaxies shows a rapid increase with redshift ∝(1+z)3.5, but the quenching rate evolves much more slowly, ∝(1+z)0.9; there are insufficient mergers to explain the quenching rate at z≤1.5. Simba first quenches galaxies at z≥3, with a number density in good agreement with observations. The quenching timescales τq are strongly bimodal, with `slow' quenchings (τq∼0.1tH) dominating overall, but `fast' quenchings (τq∼0.01tH) dominating in M∗∼1010−1010.5M⊙ galaxies, likely induced by Simba's jet-mode black hole feedback. The delay time distribution between mergers and quenching events suggests no physical connection to either fast or slow quenching. Hence, Simba predicts that major mergers induce starbursts, but are unrelated to quenching in either fast or slow mode.
Original languageEnglish
Pages (from-to)2139–2154
Number of pages16
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date17 Sept 2019
Publication statusPublished - Dec 2019


  • Galaxies: evolution
  • Galaxies: formation


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