How many stars form in galaxy mergers?

Abstract

We forward model the difference in stellar age between post-coalescence mergers and a control sample with the same stellar mass, environmental density, and redshift. In particular, we use a pure sample of 445 post-coalescence mergers from the recent visually confirmed post-coalescence merger sample identified by Bickley et al. and find that post-coalescence mergers are on average younger than control galaxies for 10 < 11. The difference in age from matched controls is up to 1.5 Gyr, highest for lower stellar mass galaxies. We forward model this difference using parametric star formation histories, accounting for the pre-coalescence inspiral phase of enhanced star formation using close pair data, and a final additive burst of star formation at coalescence. We find a best-fitting stellar mass burst fraction of fburst = ΔM∗/M∗, merger = 0.18 ± 0.02 for 10 < 11 galaxies, with no evidence of a trend in stellar mass. The modelled burst fraction is robust to choice of parametric star formation history, as well as differences in burst duration. The result appears consistent with some prior observationally derived values, but is significantly higher than that found in hydrodynamical simulations. Using published luminous infrared galaxy star formation rates, we find a burst duration increasing with stellar mass, from 120 to 250 Myr. A comparison to published cold gas measurements indicates there is enough molecular gas available in very close pairs to fuel the burst. Additionally, given our stellar mass burst estimate, the predicted cold gas fraction remaining after the burst is consistent with observed post-coalescence mergers.