Mergers, starbursts, and quenching in the simba simulation

Abstract

We use the simba cosmological galaxy formation simulation to investigate the relationship between major mergers ($\lesssim$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) $\gt t_{\rm H}^{-1}$ to $\lt 0.2t_{\rm H}^{-1}$, 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 $\lesssim 1{{\ \rm per\ cent}}$ 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 $M_*\gtrsim 10^{10.5} \,\mathrm{ M}_{\odot }$ 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 $z\lesssim 1.5$. simba first quenches galaxies at $z\gtrsim 3$, with a number density in good agreement with observations. The quenching time-scales τq are strongly bimodal, with ‘slow’ quenchings (τq ∼ 0.1tH) dominating overall, but ‘fast’ quenchings (τq ∼ 0.01tH) dominating in M* ∼ 1010–1010.5 M$\odot$ 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.

Interacting galaxies in the IllustrisTNG simulations – II: star formation in the post-merger stage

“…Note that the sSFR enhancements driven by galaxy mergers persist consistently up to z = 1 in our sample. This is consistent with results by Rodríguez Montero et al (2019), who analysed galaxy mergers with µ ≥ 0.25 in the simba simulation ) and found an enhancement of a factor of ∼ 2 − 3 for z ≤ 2. However, the absence of evolution in Q(sSF R) with redshift in our sample is in contrast to recent work by Patton et al (2020), who studied SFR enhancement during the pair phase using the same simulation (2020) includes galaxies to lower masses.…”

“…While both binary merger simulations and cosmological zoom-in simulations provide powerful tools to study galaxy mergers at high resolution (spatial and temporal), they both lack the large and diverse galaxy samples of large-box cosmological simulations. Recent studies have begun to investigate the effect of galaxy mergers in large cosmological boxes (e.g., Blumenthal et al 2019;Patton et al 2020;Rodríguez Montero et al 2019). The work presented here leverages the sizeable sample of galaxy mergers available in large cosmological box hydrodynamical simulations to investigate the impact of mergers on galactic star formation during the post-merger stage (i.e., post-coalescence).…”

Galaxy mergers in eagle do not induce a significant amount of black hole growth yet do increase the rate of luminous AGN

“…However, there are many other properties of galaxies that could also influence the growth behaviour of BHs: for example the available gas content, BH mass, or environmental properties: such as the halo mass (M 200 ), or the N 2 and r 2 parameters (defined for this study as the number of major, M * ,1 /M * ,2 ≥ 1 4 , companions within 2 pMpc, and the distance to the 2 nd closest major companion, respectively, similar to the methods of observational studies, e.g., Ellison et al 2010;Patton et al 2013Patton et al , 2016. We note that we would always argue against matching on the SFR, as the SFR of a galaxy can also be enhanced during the merger process (e.g., Rodríguez Montero et al 2019).…”

The star formation histories of z ∼ 1 post-starburst galaxies

“…We find that the time from the peak of the starburst to the point where the sSFR falls below 0.2/t H , where t H is the age of the Universe at the time of the quenching, is typically 100-200 Myr (Table 3). In order to compare directly to simulations, we also calculated the time taken for the galaxy's sSFR to fall from 1/t H to 0.2/t H , following Rodríguez Montero et al (2019) for the simba cosmological hydrodynamic simulations . They noted strongly bimodal quenching times, split at τ q /t H = 0.03, with the fast quenching events (τ q /t H ∼ 0.01) likely caused by simba's jet-mode black hole feedback.…”

“…We find that the majority of the poststarbursts have quenching times of 30-50 Myr (τ q2 in Table 3), which corresponds to τ q /t H ∼ 0.01 at cosmic times of 4-5 Gyr where most of our quenching events take place, exactly as found for the fast quenching mode in simba. Interestingly, Rodríguez Montero et al (2019) note that neither fast nor slow quenching events are directly linked to galaxy major mergers, where mergers are identified by a sudden increase in the stellar mass of the galaxy by more than 20% (mass ratio > 1 : 4). Interestingly, this is in contrast to Pawlik et al (2019) and Davis et al (2019) who found that local (z ∼ 0) post-starburst galaxies in the eagle cosmological hydrodynamic simulation were predominantly caused by major mergers or multiple minor mergers.…”