The collaborative effect of ram pressure and merging on star formation and stripping fraction

Abstract

Aims: We investigate the effect of ram pressure stripping (RPS) on simulations of merging pairs of gas-rich spiral galaxies. Our goal is to provide an estimate of the combined effect of merging and RPS on stripping efficiency and star formation rate. Methods: We make use of the combined N-body/hydrodynamic code GADGET-2. In our simulations, we vary mass ratios between 1:4 and 1:8 in a binary merger. We sample different geometric configurations of the merging systems (edge-on and face-on mergers, different impact parameters). Furthermore, we vary the properties of the intracluster medium (ICM) in rough steps: The speed of the merging system relative to the ICM between 500 and 1000 km/s, the ICM density between $10^{-29}$ and $10^{-27}$ g/cm$^3$, and the ICM direction relative to the mergers' orbital plane. Ram pressure is kept constant within a simulation time period, as is the ICM temperature of $10^7$ K. Each simulation in the ICM is compared to simulations of the merger in vacuum and the non-merging galaxies with acting ram pressure. Results: Averaged over the simulation time (1 Gyr) the merging pairs show a negligible 5% enhancement in SFR, when compared to single galaxies under the same environmental conditions. The SFRs peak at the time of the galaxies first fly-through. There, our simulations show SFRs of up to 20 M$_{\odot}$/yr (compared to 3 M$_{\odot}$/yr of the non-merging galaxies in vacuum). In the most extreme case, this constitutes a short-term ($<50$ Myr) SFR increase of 50% over the non-merging galaxies experiencing ram pressure. The wake of merging galaxies in the ICM typically has a third to half the star mass seen in the non-merging galaxies and 5% to 10 % less gas mass. The joint effect of RPS and merging, according to our simulations, is not significantly different from pure ram pressure effects.