Gas flows in galaxy mergers: supersonic turbulence in bridges, accretion from the circumgalactic medium, and metallicity dilution

TL;DR

This study uses cosmological MHD simulations to analyze gas dynamics in galaxy mergers, revealing supersonic turbulence in bridges, significant CGM accretion fueling star formation, and rapid metallicity dilution in merging galaxies.

Contribution

It provides new insights into the role of supersonic turbulence, CGM accretion, and rapid metallicity changes during galaxy mergers using detailed simulations.

Findings

Bridges exhibit supersonic turbulence with Mach numbers 1.6-3.3.

CGM accretion contributes up to 51% of star formation during mergers.

Metallicity-diluted gas is rapidly accreted, affecting metallicity relations.

Abstract

In major galaxy mergers, the orbits of stars are violently perturbed, and gas is torqued to the centre, diluting the gas metallicity and igniting a starburst. In this paper, we study the gas dynamics in and around merging galaxies using a series of cosmological magneto-hydrodynamical (MHD) zoom-in simulations. We find that the gas bridge connecting the merging galaxies pre-coalescence is dominated by turbulent pressure, with turbulent Mach numbers peaking at values of 1.6-3.3. This implies that bridges are dominated by supersonic turbulence, and are thus ideal candidates for studying the impact of extreme environments on star formation. We also find that gas accreted from the circumgalactic medium (CGM) during the merger significantly contributes (27-51 per cent) to the star formation rate (SFR) at the time of coalescence and drives the subsequent reignition of star formation in the merger remnant. Indeed, 19-53 per cent of the SFR at z=0 originates from gas belonging to the CGM prior the merger. Finally, we investigate the origin of the metallicity-diluted gas at the centre of merging galaxies. We show that this gas is rapidly accreted onto the galactic centre with a time-scale much shorter than that of normal star-forming galaxies. This explains why coalescing galaxies are not well-captured by the fundamental metallicity relation.