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

Institute of Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria
e-mail: jan.bischko@uibk.ac.at

Received: 23 September 2013
Accepted: 10 February 2015

Abstract

Aims. We investigate the effect of ram pressure stripping (RPS) on several simulations of merging pairs of gas-rich spiral galaxies. We are concerned with the changes in stripping efficiency and the time evolution of the star formation rate. Our goal is to provide an estimate of the combined effect of merging and RPS compared to the influence of the individual processes.

Methods. We make use of the combined N-body/hydrodynamic code GADGET-2. The code features a threshold-based statistical recipe for star formation, as well as radiative cooling and modeling of galactic winds. 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^-1, 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 10⁷ 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_⊙ yr^-1 (compared to 3 M_⊙ yr^-1 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.