Mergers, Starbursts, and Quenching in the Simba Simulation

We use the simfiAeosmological galaxy formation simulation to investigate the relationship between major mergers (less than or similar to 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 < 0.2t(H)(-1), where t(H) is the Hubble time. Al z approximate to 0-3, mergers show similar to 2-3 x higher SFR than a mass matched sample of star -forming galaxies, but globally represent less than or similar to 1 per cent of the cosmic SF budget. At low masses, the increase in SFR in mergers is mostly attributed to an increase in the H-2 content, but for M-* greater than or similar to 10(10.5) M-circle dot 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, alpha(1 + z)(3.5), but the quenching rate evolves much more slowly, alpha(1+z)(0.9); there are insufficient mergers to explain the quenching rate at z less than or similar to 1.5. SIMBA first quenches galaxies at z greater than or similar to 3, with a number density in good agreement with observations. The quenching time-scales iota(q) are strongly bimodal, with 'slow' quenchings iota(q) similar to 0.1t(Pi)) dominating overall, but 'fast' quenchings (iota(q) similar to 0.1t(Pi))) dominating in M-* similar to 10(10)-10(10.5) M-circle dot galaxies, likely induced by S1MBA-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.