Papers › Mergers, Starbursts, and Quenching in the Simba Simulation
Mergers, Starbursts, and Quenching in the Simba Simulation
Francisco Rodríguez Montero, Romeel Davé, Vivienne Wild, Daniel Anglés-Alcázar, Desika Narayanan
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We use the Simba cosmological 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_H⁻¹ to sSFR<0.2t_H⁻¹, where t_H 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_*≥10^(10.5) M_⊙ 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 τ_q are strongly bimodal, with `slow' quenchings (τ_q ∼0.1t_H) dominating overall, but `fast' quenchings (τ_q∼0.01 t_H) dominating in M_*∼10¹⁰-10^(10.5)M_⊙ 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.
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