Interacting galaxies in the IllustrisTNG simulations – VIII. Pericentric star formation rate enhancements are driven both by increased fuelling and efficiency

ABSTRACT Using the TNG100-1 cosmological simulations, we explore how galaxy properties, such as specific star formation rate ($\rm sSFR=SFR/\mathit{ M}_*$), gas fraction ($f_{\rm gas} \, = \, M_{\rm H}/M_{*}$), and star formation efficiency ($\rm SFE_{H} = SFR/\mathit{ M}_{H}$), change over the course of galaxy–galaxy interactions. We identify 18 534 distinct encounters from the reconstructed orbits of a sample of massive galaxies ($M_{*} > 10^{10} \,\, {\rm M}_{\odot }$) with companions within a stellar mass ratio of 0.1–10. Using these encounters, we study the variation of galaxy properties over time as they approach and move away from pericentric encounters over a redshift range of $0 \le z \lt 1$. Following the closest pericentric encounters ($\le 50$ kpc) of a host galaxy with its companion, we find that sSFR is enhanced by a factor of $1.6 \pm 0.1$ on average within the central stellar half-mass radius (R1/2) compared to pre-encounter values. Our results show a time delay between pericentre and maximum sSFR enhancement of $\sim$0.1 Gyr with a mean galaxy separation of 75 kpc. We similarly find that $f_{\rm gas}$ is enhanced by a factor of $1.2 \pm 0.1$, and $\rm SFE_{H}$ is enhanced by a factor of $1.4 \pm 0.1$ following the pericentre of an encounter within the same time-scale. Additionally, we find evidence of inflowing gas towards the centre, measured by comparing the $f_{\rm gas}$ and metallicity within the central R1/2 to the galactic outskirts. We find that approximately 70 per cent of the peak sSFR enhancement can be attributed to the increase in $\rm SFE_{H}$, with the increase in $f_{\rm gas}$ contributing the remaining 30 per cent.