High-redshift major mergers weakly enhance star formation

Abstract

Galaxy mergers are believed to trigger strong starbursts. This is well assessed by observations in the local Universe. However the efficiency of this mechanism has poorly been tested so far for high redshift, actively star forming, galaxies. We present a suite of pc-resolution hydrodynamical numerical simulations to compare the star formation process along a merging sequence of high and low z galaxies, by varying the gas mass fraction between the two models. We show that, for the same orbit, high-redshift gasrich mergers are less efficient than low-redshift ones at producing starbursts: the star formation rate excess induced by the merger and its duration are both around 10 times lower than in the low gas fraction case. The mechanisms that account for the star formation triggering at low redshift -the increased compressive turbulence, gas fragmentation, and central gas inflows -are only mildly, if not at all, enhanced for high gas fraction galaxy encounters. Furthermore, we show that the strong stellar feedback from the initially high star formation rate in high redshift galaxies does not prevent an increase of the star formation during the merger. Our results are consistent with the observed increase of the number of major mergers with increasing redshift being faster than the respective increase in the number of starburst galaxies.

Interacting galaxies in the IllustrisTNG simulations - I: Triggered star formation in a cosmological context

“…This trend is consistent with predictions from simulations that SFR enhancements are smaller at higher redshift (Perret et al 2014;Kaviraj et al 2015;Fensch et al 2017;Martin et al 2017). There is also some observational support for this trend (e.g., Kaviraj et al 2013;Lofthouse et al 2017).…”

Interacting galaxies on FIRE-2: the connection between enhanced star formation and interstellar gas content

“…Various authors have also find that the strength of the SFR enhancement depends on the progenitor galaxies' gas fractions. For example, Fensch et al (2017) argue that mergers are less efficient at high redshift, owing to galaxies typically having higher gas fractions than at low redshift.…”

A model for core formation in dark matter haloes and ultra-diffuse galaxies by outflow episodes

“…Amongst the processes potentially induced by mergers and fly-bys, dynamical friction can heat the background particles (e.g. El-Zant et al 2001Tonini et al 2006;Goerdt et al 2010) and gravitational torques can drive large amounts of gas towards the center in a non-spherical way (Barnes & Hernquist 1991;Mihos & Hernquist 1996;Fensch et al 2017). As presented in section 3.3, we account for the difference between the total enclosed mass M tot and the collisionless component mass M through a power-law radial dependence according to Eq.…”