The structures of distant galaxies — IV. A new empirical measurement of the time-scale for galaxy mergers — implications for the merger history
Understanding the role of mergers in galaxy formation is one of the most outstanding problems in extragalactic astronomy. While we now have an idea for how the merger fraction evolves at redshifts z 20 000 galaxies in the Extended Groth Strip Survey and Cosmic Evolution Survey. Using a new methodology described in this Letter, we are able to determine how long a galaxy remains identifiable as a merging system within the CAS system. We find a maximum CAS major merger time-scale of 1.1 ± 0.3 Gyr at z 1010 M⊙ undergo at z < 1.2, with a total number Nm= 0.90+0.44−0.23. We further show that this time-scale is inconsistent with a star formation origin for ultrahigh asymmetries, thereby providing further evidence that structural methods are able to locate mostly merging galaxies.