Interacting galaxies in the IllustrisTNG simulations -- V: Comparing the influence of star-forming vs. passive companions

Abstract

We study interacting galaxy pairs in the TNG100-1 and TNG300-1 cosmological simulations using previously generated closest companion samples. We study the specific star formation rates (sSFR) of massive ($10^{10} M_{\odot} < M_* < 10^{12} M_{\odot}$) galaxies at $z \leq 0.2$ as a function of separation from the closest companion galaxy. We split our sample based on whether the companion galaxy is star-forming or passive. We find that galaxies with close star-forming companions have sSFRs that are enhanced (on average) by a factor of $2.9 \pm 0.3$ in TNG100-1 and $2.27 \pm 0.06$ in TNG300-1 compared to controls, with enhancements present out to separations of $\sim 300$ kpc. Galaxies with passive companions in TNG300-1 exhibit mild sSFR suppression ($\sim12$ percent) at 100-300 kpc and small sSFR enhancements at separations below 50 kpc. sSFR suppression is strongest in pairs where the galaxy's stellar mass is more than 2 times that of its passive companion. By generating a stellar mass-matched ("twinned") sample in TNG300-1, we show that differences in sSFR trends between companion types are not a result of intrinsic stellar mass differences in star-forming vs. passive galaxies. We compare with an analogous sample of galaxy pairs from SDSS, finding consistent results between observations and simulations. Overall, we find that star-forming galaxies show enhanced sSFRs regardless of companion type, but that galaxies with close passive companions are more likely to be passive themselves.