Extracting Galaxy Merger Timescales II: A new fitting formula

Abstract

Predicting the merger timescale (${\tau }_{\mathrm{merge}}$) 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 ${\tau }_{\mathrm{merge}}$ 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 ${\tau }_{\mathrm{merge}}$ that draws on insights derived from these simulations. We find that published predictions can provide reasonable estimates for ${\tau }_{\mathrm{merge}}$ based on orbital properties at infall, but tend to underpredict ${\tau }_{\mathrm{merge}}$ 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 $R_{\mathrm{circ}}$ underpredict (overpredict) ${\tau }_{\mathrm{merge}}$ for bound (unbound) systems. By fitting for the dependence of ${\tau }_{\mathrm{merge}}$ on various orbital and host halo properties,we derive an improved model for ${\tau }_{\mathrm{merge}}$ 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 ${\tau }_{\mathrm{merge}}$ for semi-analytical galaxy formation modelling.

arXiv comment12 pages, 11 figures, accepted for publication in MNRAS

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Cited by in corpus (2)

• On the accuracy of dark matter halo merger trees and the consequences for semi-analytic models of galaxy formation
• Describing the Nonuniversal Galaxy Merger Timescales in IllustrisTNG: Effects of Host Halo Mass, Baryons, and Sample Selection