Resolving the Discrepancy of Galaxy Merger Fraction Measurements at z ∼ 0-3

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 z∼ 3. 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 z=1{--}3. 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 ({M}_\star /{M}_⊙ )≥slant 10.8) galaxy during z=0.1{--}2.5. There may be an additional ∼ 0.5(0.4) 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 ({M}_\star /{M}_⊙ )≥slant 11.1) 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 z\gt 2 to lie at 1.5σ 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.