The effect of galaxy mass ratio on merger-driven starbursts

Abstract

We employ numerical simulations of galaxy mergers to explore the effect of galaxy mass ratio on merger-driven starbursts. Our numerical simulations include radiative cooling of gas, star formation, and stellar feedback to follow the interaction and merger of four disc galaxies. The galaxy models span a factor of 23 in total mass and are designed to be representative of typical galaxies in the local universe. We find that the merger-driven star formation is a strong function of merger mass ratio, with very little, if any, induced star formation for large mass ratio mergers. We define a burst efficiency that is useful to characterize the merger-driven star formation and test that it is insensitive to uncertainties in the feedback parametrization. In accord with previous work we find that the burst efficiency depends on the structure of the primary galaxy. In particular, the presence of a massive stellar bulge stabilizes the disc and suppresses merger-driven star formation for large mass ratio mergers. Direct, coplanar merging orbits produce the largest tidal disturbance and yield the most intense burst of star formation. Contrary to naive expectations, a more compact distribution of gas or an increased gas fraction both decrease the burst efficiency. Owing to the efficient feedback model and the newer version of smoothed particle hydrodynamics employed here, the burst efficiencies of the mergers presented here are smaller than in previous studies.

Star formation in mergers with cosmologically motivated initial conditions

“…The relation between the starburst efficiency and the merger ratio for simulations with BHs is shown in Fig. 14, while Table 5 is the analogue of Table 3 for these simulations. It is clear from the figure that the inclusion of both the hot halo component and BHs strongly reduces the efficiency of massive major mergers in triggering additional SF with respect to previous studies (e.g Cox et al 2008). Both the slope γ and the normalisation e1:1 of the power law are lower than found above, showing that the correlation between starburst effiency and merger ratio is much weaker.…”

“…In addition, the blobs formed in SPH might spuriously contribute to the heating of the The blue points show all major merger events in simulations with a BH and a hot gaseous halo around the galaxy, while the green squares show the simulations with black holes and without a hot halo. The black line is the starburst efficiency of Cox et al (2008) and the green line represents the least squares fit to our results. Figure 15.…”

“…Cox et al 2008); the SFR drops rapidly when the gas is depleted, and a starburst occurs during the final merging process. The starburst increases the SFR by an order of magnitude and there is almost no SF after the merger, because all the gas has been consumed during the starburst.…”

“…For µ < 5 there is a large scatter in the values of e burst , while for µ > 5 the starburst efficiency seems to converge to values around 0.15 (though the scatter is difficult to assess due to small number statistics). Following Cox et al (2008), we performed a power law fit to the burst efficiency as a function of mass ratio:…”

Dusty star-forming galaxies at high redshift

“…First, the majority of stars formed in galaxy mergers are done during early quiescent phases, rather than bursts (e.g. Cox et al, 2008). Hence, modifying the IMF in bursts alone does not severely impact the present-epoch stellar mass function.…”