Effect of galaxy mergers on star formation rates

TL;DR

This study uses large galaxy samples and neural networks to analyze how galaxy mergers influence star formation rates, finding minimal overall impact but a higher merger fraction among starburst galaxies.

Contribution

It introduces a neural network-based method to identify galaxy mergers across multiple surveys and statistically assesses their effect on star formation rates.

Findings

Merging galaxies' SFRs are not significantly different from non-merging galaxies.

The typical SFR change due to mergers is less than 0.1 dex.

Higher SFR above the main sequence correlates with increased merger fraction.

Abstract

Galaxy mergers and interactions are an integral part of our basic understanding of how galaxies grow and evolve over time. However, the effect that galaxy mergers have on star formation rates (SFR) is contested, with observations of galaxy mergers showing reduced, enhanced and highly enhanced star formation. We aim to determine the effect of galaxy mergers on the SFR of galaxies using statistically large samples of galaxies, totalling over 200\,000, over a large redshift range, 0.0 to 4.0. We train and use convolutional neural networks to create binary merger identifications (merger or non-merger) in the SDSS, KiDS and CANDELS imaging surveys. We then compare the galaxy main sequence subtracted SFR of the merging and non-merging galaxies to determine what effect, if any, a galaxy merger has on SFR. We find that the SFR of merging galaxies are not significantly different from the SFR of non-merging systems. The changes in the average SFR seen in the star forming population when a galaxy is merging are small, of the order of a factor of 1.2. However, the higher the SFR above the galaxy main sequence, the higher the fraction of galaxy mergers. Galaxy mergers have little effect on the SFR of the majority of merging galaxies compared to the non-merging galaxies. The typical change in SFR is less than 0.1~dex in either direction. Larger changes in SFR can be seen but are less common. The increase in merger fraction as the distance above the galaxy main sequence increases demonstrates that galaxy mergers can induce starbursts.