Shocks and angular momentum flips: a different path to feeding the nuclear regions of merging galaxies

TL;DR

This study uses high-resolution hydrodynamical simulations to reveal a new gas feeding mechanism in galaxy mergers involving shocks and angular momentum flips, which may explain observed kinematic features.

Contribution

It uncovers a novel gas inflow process driven by ram-pressure shocks and demonstrates how these shocks can cause large-scale flips in galactic gas rotation, affecting galaxy evolution models.

Findings

Ram-pressure shocks enhance gas inflows to galactic centers.

Galactic gas discs can abruptly flip their rotation direction.

Long-lived decoupled gas-stellar discs may explain some observed kinematic features.

Abstract

We study the dynamics of galaxy mergers, with emphasis on the gas feeding of nuclear regions, using a suite of hydrodynamical simulations of galaxy encounters. The high spatial and temporal resolution of the simulations allows us to not only recover the standard picture of tidal-torque induced inflows, but also to detail another, important feeding path produced by ram pressure. The induced shocks effectively decouple the dynamics of the gas from that of the stars, greatly enhancing the loss of gas angular momentum and leading to increased central inflows. The ram-pressure shocks also cause, in many cases, the entire galactic gas disc of the smaller galaxy to abruptly change its direction of rotation, causing a complete "flip" and, several $10^8$ yr later, a subsequent "counter-flip". This phenomenon results in the existence of long-lived decoupled gas-stellar and stellar-stellar discs, which could hint at a new explanation for the origin of some of the observed kinematically decoupled cores/counter-rotating discs. Lastly, we speculate, in the case of non-coplanar mergers, on the possible existence of a new class of remnant systems similar to some of the observed X-shaped radio galaxies.