An infrared study of local galaxy mergers

TL;DR

This study analyzes approximately 3000 local galaxy mergers using infrared data, revealing that minor mergers can significantly boost star formation and that the transition from LIRG to ULIRG occurs rapidly over about 160 million years.

Contribution

It provides a comprehensive infrared analysis of local galaxy mergers, highlighting the role of minor mergers in star formation and the quick transition from LIRG to ULIRG phases.

Findings

Minor mergers can drive high star formation rates.

Most FIR-bright mergers are spiral-spiral systems.

LIRG to ULIRG transition occurs in about 160 Myr.

Abstract

We combine a large, homogeneous sample of $\sim$3000 local mergers with the Imperial IRAS Faint Source Redshift Catalogue (IIFSCz), to perform a blind far-infrared (FIR) study of the local merger population. The IRAS-detected mergers are mostly ($98\%$) spiral-spiral systems, residing in low density environments, a median FIR luminosity of $10^{11} L_\odot$ (which translates to a median star formation rate of around 15$M_\odot yr^{-1}$). The FIR luminosity -- and therefore the star formation rate -- shows little correlation with group richness and scales with the total stellar mass of the system, with little or no dependence on the merger mass ratio. In particular, minor mergers (mass ratios $<1:3$) are capable of driving strong star formation (between 10 and $173 M_\odot yr^{-1}$) and producing systems that are classified as LIRGs, luminous infrared galaxies ($65\%$ of our LIRGs are minor mergers), with some minor-merging systems being close to the ultra luminous infrared galaxy (ULIRG) limit. Optical emission line ratios indicate that the AGN fraction increases with increasing FIR luminosity, with all ULIRG mergers having some form of AGN activity. Finally, we estimate that the LIRG-to-ULIRG transition along a merger sequence typically takes place over a relatively short timescale of $\sim$160 Myr.