A direct measurement of major galaxy mergers at z≲3

This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z similar to 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 (M-B). 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 sigma confidence, for the most luminous and massive systems. The highest merger fractions we find are 40% - 50% for galaxies with M-B < -21, or M* > 10(10) 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 f(m)(A, M*, M-B, z) = f(0) x (1 + z)(mA). We find m(A) values similar to4-6 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 similar to 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 (M) over dot(G) similar to 4 x 10(8) M-. Gyr(-1) per galaxy at z similar to 1 for galaxies with stellar masses M* > 10(9) M-.. This accretion rate changes with redshift as (M) over dot G = 1.6 x 10(8) (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 similar to 0.5 for the brightest, M-B < - 21, and most massive, M-* > 10(10) M-., systems near z similar to 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.