RESOLVING THE DISCREPANCY OF GALAXY MERGER FRACTION MEASUREMENTS AT z ∼ 0–3

Abstract

ABSTRACT We measure the merger fraction of massive galaxies using the UltraVISTA/COSMOS catalog, complemented with the deeper, higher resolution 3DHST+CANDELS catalog, presenting the largest mass-complete photometric merger sample up to . We find that the variation in the mass ratio probe can explain the discrepant redshift evolution of the merger fraction in the literature: selecting mergers using the H 160-band flux ratio leads to an increasing merger fraction with redshift, while selecting mergers using the stellar mass ratio reveals a merger fraction with little redshift dependence at . Defining major and minor mergers as having stellar mass ratios of 1:1–4:1 and 4:1–10:1, respectively, the results imply ∼1 major merger and ∼0.7 minor merger on average for a massive (log ) galaxy during . There may be an additional major (minor) merger if we use the H-band flux ratio selection. The observed amount of major merging alone is sufficient to explain the observed number density evolution for the very massive (log ) galaxies. The observed number of major and minor mergers can increase the size of a massive quiescent galaxy by a factor of two at most. This amount of merging is enough to bring the compact quiescent galaxies formed at to lie at below the mean of the stellar mass–size relation as measured in some works (e.g., Newman et al.), but additional mechanisms are needed to fully explain the evolution, and to be consistent with works suggesting stronger evolution.