A Direct Measurement of Major Galaxy Mergers atz3

Abstract

This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z ~ 3. We identify major mergers using the model-independent CAS (concentration, asymmetry, clumpiness) physical morphological system on galaxies detected, and photometrically selected, in the WFPC2 and NICMOS Hubble Deep Field North. We specifically use the asymmetric distributions of rest-frame optical light measured through the asymmetry parameter (A) to determine the fraction of galaxies undergoing major mergers as a function of redshift (z), stellar mass (M*), and absolute magnitude (MB). We find that the fraction of galaxies consistent with undergoing a major merger increases with redshift for all galaxies, but most significantly, at 5?10 ? confidence, for the most luminous and massive systems. The highest merger fractions we find are 40%?50% for galaxies with MB 1010 M? at z > 2.5, e.g., objects identified as Lyman-break galaxies. Using these results, we model the merger fraction evolution in the form fm(A, M*, MB, z) = f0 ? (1 + z). We find mA values ~4?6 for the most luminous and massive galaxies, while lower mass and less luminous galaxies have smaller mA values. We use these merger fractions, combined with merger timescales calculated from N-body simulations, to derive galaxy merger rates to z ~ 3. We also use stellar masses of HDF-N galaxies to determine the mass accretion rate of field galaxies involved in major mergers. We find an average stellar mass accretion rate of G ~ 4 ? 108 M? Gyr-1 per galaxy at z ~ 1 for galaxies with stellar masses M* > 109 M?. This accretion rate changes with redshift as G = 1.6 ? 108(1 + z)0.99?0.32 M? Gyr-1 per galaxy. We also find that the fraction of stellar mass density in galaxies involved in major mergers increases with redshift, with a peak mass fraction ~0.5 for the brightest, MB 1010 M?, systems near z ~ 2.5. By comparing merger fractions predicted in cold dark matter semianalytic models with our results we find a reasonably good agreement for the largest and brightest systems, although we find more low-mass galaxy mergers at lower redshifts than what these models predict.