An infrared study of local galaxy mergers

¹ Blackett Laboratory, Imperial College London, London SW7 2AZ, UK
e-mail: alfredo.carpineti07@imperial.ac.uk
² University of Hertfordshire, Hatfield, Hertfordshire, AL10 9AB, UK
³ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁴ Adler Planetarium, 1300 S. Lakeshore Drive, Chicago, IL 60605, USA
⁵ Institute for Astronomy, Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 16, 8093 Zurich, Switzerland

Received: 4 November 2014
Accepted: 16 March 2015

Abstract

We combine a large, homogeneous sample of ~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 10¹¹L_⊙ (which translates to a median star formation rate of around 15 M_⊙ 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_⊙ yr^-1) and producing systems that are classified as luminous infrared galaxies (LIRGS; 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 ~160 Myr.