A Direct Measurement of Galaxy Major and Minor Merger Rates and Stellar Mass Accretion Histories at Z < 3 Using Galaxy Pairs in the REFINE Survey

We measure the role of major and minor mergers in forming the stellar masses of galaxies over redshifts 0 < z < 3 using a combination of ~3.25 deg² of the deepest ground-based near-infrared imaging taken to date (Ultra Deep Survey, Ultra-VISTA, and VIDEO) as part of the collated REFINE survey. We measure the pair fraction and merger fractions for galaxy mergers of different mass ratios, and quantify the merger rate with newly measured timescales derived from the Illustris simulation as a function of redshift and merger mass ratio. For a M _* > 10¹¹ M _⊙ selection, we find that over 0 < z < 3 major mergers with mass ratios greater than 1:4 occur ${0.84}_{-0.2}^{+0.3}$ times on average, while minor mergers down to ratios of 1:10 occur on average ${1.43}_{-0.3}^{+0.5}$ times per galaxy. We also quantify the role of major and minor mergers in galaxy formation, whereby the increase in mass due to major mergers is ${93}_{-31}^{+49} \% $ while minor mergers account for an increase of ${29}_{-12}^{+17} \% $ using a M _* > 10¹¹ M _⊙ selection. We thus find that major mergers add more stellar mass to galaxies than minor mergers over this epoch. Overall, mergers will more than double the mass of massive galaxies over this epoch when selecting by stellar mass. We however find a lower increase in stellar mass when selecting by a constant number density. Finally, we compare our results to simulations, finding that minor mergers are overpredicted in Illustris and in semi-analytical models, suggesting a mismatch between observations and theory in this fundamental aspect of galaxy assembly.