Interacting galaxies in the IllustrisTNG simulations -- II: Star
  formation in the post-merger stage
  Galaxy mergers are a major evolutionary transformation whose effects are
borne out by a plethora of observations and numerical simulations. However,
most previous simulations have used idealised, isolated, binary mergers and
there has not been significant progress on studying statistical samples of
galaxy mergers in large cosmological simulations. We present a sample of 27,691
post-merger (PM) galaxies ($0\le z \le 1$) identified from IllustrisTNG: a
cosmological, large box, magneto-hydrodynamical simulation suite. The PM sample
spans a wide range of merger and galaxy properties ($M_\star$, $\mu$,
$f_\mathrm{gas}$). We demonstrate that star forming (SF) PMs exhibit enhanced
star formation rates (SFRs) on average by a factor of $\sim 2$, while the
passive PMs show no statistical enhancement. We find that the SFR enhancements:
(1) show no dependence on redshift, (2) anti-correlate with the PM's stellar
mass, and (3) correlate with the gas fraction of the PM's progenitors. However,
SF PMs show stronger enhancements which may indicate other processes being at
play (e.g., gas phase, feedback efficiency). Although the SFR enhancement
correlates mildly with the merger mass ratio, the more abundant minor mergers
($0.1 \le \mu < 0.3 $) still contribute $\sim 50\%$ of the total SFR
enhancement. By tracing the PM sample forward in time, we find that galaxy
mergers can drive significant SFR enhancements which decay over $\sim 0.5$ Gyr
independent of the merger mass ratio, although the decay timescale is dependent
on the simulation resolution. The strongest merger-driven starburst galaxies
evolve to be passive/quenched on faster timescales than their controls.
