How Do Disks Survive Mergers?

TL;DR

This paper presents a physical model predicting how galactic disks survive or are destroyed during mergers, validated by hydrodynamic simulations, highlighting the role of gas content and orbital parameters.

Contribution

The study introduces a predictive model for disk survival in galaxy mergers based on gravitational physics, applicable across various simulation conditions and informing semi-analytic models.

Findings

Gas primarily loses angular momentum to stars, leading to central starbursts.

Disk re-formation depends on initial gas content and merger parameters.

Gas-rich mergers can result in systems with small bulges.

Abstract

We develop a physical model for how galactic disks survive and/or are destroyed in interactions. Based on dynamical arguments, we show gas primarily loses angular momentum to internal torques in a merger. Gas within some characteristic radius (a function of the orbital parameters, mass ratio, and gas fraction of the merging galaxies), will quickly lose angular momentum to the stars sharing the perturbed disk, fall to the center and be consumed in a starburst. A similar analysis predicts where violent relaxation of the stellar disks is efficient. Our model allows us to predict the stellar and gas content that will survive to re-form a disk in the remnant, versus being violently relaxed or contributing to a starburst. We test this in hydrodynamic simulations and find good agreement as a function of mass ratio, orbital parameters, and gas fraction, in simulations spanning a wide range in these properties and others, including different prescriptions for gas physics and feedback. In an immediate sense, the amount of disk that re-forms can be understood in terms of well-understood gravitational physics, independent of details of ISM gas physics or feedback. This allows us to explicitly quantify the requirements for such feedback to (indirectly) enable disk survival, by changing the pre-merger gas content and distribution. The efficiency of disk destruction is a strong function of gas content: we show how and why sufficiently gas-rich major mergers can, under general conditions, yield systems with small bulges (B/T<0.2). We provide prescriptions for inclusion of our results in semi-analytic models.