Extracting Galaxy Merger Timescales II: A new fitting formula

TL;DR

This paper introduces a new fitting formula for galaxy merger timescales that improves upon previous models by accounting for tidal stripping and orbital properties, enhancing predictions in cosmological simulations.

Contribution

The paper develops a novel model for galaxy merger timescales based on insights from N-body simulations, addressing limitations of prior models and enabling more accurate predictions.

Findings

Published models underpredict merger times inside the host virial radius.

Models considering orbital angular momentum can under or overpredict depending on system binding.

The new model provides a more accurate fit for merger times at any orbital phase.

Abstract

Predicting the merger timescale ($\tau_{\rm merge}$) of merging dark matter halos, based on their orbital parameters and the structural properties of their hosts, is a fundamental problem in gravitational dynamics that has important consequences for our understanding of cosmological structure formation and galaxy formation. Previous models predicting $\tau_{\rm merge}$ have shown varying degrees of success when compared to the results of cosmological $N$-body simulations. We build on this previous work and propose a new model for $\tau_{\rm merge}$ that draws on insights derived from these simulations. We find that published predictions can provide reasonable estimates for $\tau_{\rm merge}$ based on orbital properties at infall, but tend to underpredict $\tau_{\rm merge}$ inside the host virial radius ($R_{200}$) because tidal stripping is neglected, and overpredict it outside $R_{200}$ because the host mass is underestimated. Furthermore, we find that models that account for orbital angular momentum via the circular radius $R_{\rm circ}$ underpredict (overpredict) $\tau_{\rm merge}$ for bound (unbound) systems. By fitting for the dependence of $\tau_{\rm merge}$ on various orbital and host halo properties,we derive an improved model for $\tau_{\rm merge}$ that can be applied to a merging halo at any point in its orbit. Finally, we discuss briefly the implications of our new model for $\tau_{\rm merge}$ for semi-analytical galaxy formation modelling.