Quantified HI Morphology III: Merger Visibility Times from HI in Galaxy Simulations

TL;DR

This study assesses how long galaxy mergers are visible in HI using simulations and observational criteria, providing key timescales to improve merger rate estimates from upcoming HI surveys.

Contribution

It evaluates and compares different HI morphological criteria for detecting galaxy mergers and determines their effective visibility timescales.

Findings

The Concentration and M20 criterion detects mergers for about 0.4 Gyr.

The Gini parameter (GM) criterion detects mergers for about 0.69 Gyr.

Some criteria also select isolated disks, especially in face-on, gas-rich galaxies.

Abstract

Major mergers of disk galaxies are thought to be a substantial driver in galaxy evolution. To trace the fraction and the rate galaxies are in mergers over cosmic times, several observational techniques, including morphological selection criteria, have been developed over the last decade. We apply this morphological selection of mergers to 21 cm radio emission line (HI) column density images of spiral galaxies in nearby surveys. In this paper, we investigate how long a 1:1 merger is visible in HI from N-body simulations. We evaluate the merger visibility times for selection criteria based on four parameters: Concentration, Asymmetry, M20, and the Gini parameter of second order moment of the flux distribution (GM). Of three selection criteria used in the literature, one based on Concentration and M20 works well for the HI perspective with a merger time scale of 0.4 Gyr. Of the three selection criteria defined in our previous paper, the GM performs well and cleanly selects mergers for 0.69 Gyr. The other two criteria (A-M20 and C-M20), select isolated disks as well, but perform best for face-on, gas-rich disks (T(merger) ~ 1 Gyr). The different visibility scales can be combined with the selected fractions of galaxies in any large HI survey to obtain merger rates in the nearby Universe. All-sky surveys such as WALLABY with ASKAP and the Medium Deep Survey with the APETIF instrument on Westerbork are set to revolutionize our perspective on neutral hydrogen and will provide an accurate measure of the merger fraction and rate of the present epoch.