The Effect of Gas Fraction on the Morphology and Time-scales of Disc Galaxy Mergers

TL;DR

This study investigates how the gas content in galaxy mergers influences their observable features and the implications for estimating galaxy merger rates across cosmic time.

Contribution

It provides a detailed analysis of how gas fraction affects morphological indicators and merger time-scales in simulated disc galaxy mergers, highlighting potential biases in merger rate estimates.

Findings

High gas-fraction mergers show longer asymmetry time-scales.

G-M20 based merger identification is weakly affected by gas fraction.

Redshift evolution in asymmetry may reflect gas properties, not merger rate.

Abstract

Gas-rich galaxy mergers are more easily identified by their disturbed morphologies than mergers with less gas. Because the typical gas fraction of galaxy mergers is expected to increase with redshift, the under-counting of low gas-fraction mergers may bias morphological estimates of the evolution of galaxy merger rate. To understand the magnitude of this bias, we explore the effect of gas fraction on the morphologies of a series of simulated disc galaxy mergers. With the resulting g-band images, we determine how the time-scale for identifying major and minor galaxy mergers via close projected pairs and quantitative morphology (the Gini coefficient G, the second-order moment of the brightest 20% of the light M20, and asymmetry A) depends on baryonic gas fraction f(gas). Strong asymmetries last significantly longer in high gas-fraction mergers of all mass ratios, with time-scales ranging from >= 300 Myr for f(gas) ~ 20% to >= 1 Gyr for f(gas) ~ 50%. Therefore the strong evolution with redshift observed in the fraction of asymmetric galaxies may reflect evolution in the gas properties of galaxies rather than the global galaxy merger rate. On the other hand, the time-scale for identifying a galaxy merger via G-M20 is weakly dependent on gas-fraction (~ 200-400 Myr), consistent with the weak evolution observed for G-M20 mergers.