Role of Galaxy Mergers in Cosmic Star Formation History

Abstract

We present a morphology study of intermediate-redshift (0.2<z<1.2) luminous infrared galaxies (LIRGs) and general field galaxies in the GOODS fields using a revised asymmetry measurement method optimized for deep fields. By taking careful account of the importance of the underlying sky-background structures, our new method does not suffer from systematic bias and offers small uncertainties. By redshifting local LIRGs and low-redshift GOODS galaxies to different higher redshifts, we have found that the redshift dependence of the galaxy asymmetry due to surface-brightness dimming is a function of the asymmetry itself, with larger corrections for more asymmetric objects. By applying redshift-, IR-luminosity- and optical-brightness-dependent asymmetry corrections, we have found that intermediate-redshift LIRGs generally show highly asymmetric morphologies, with implied merger fractions ~50% up to z=1.2, although they are slightly more symmetric than local LIRGs. For general field galaxies, we find an almost constant relatively high merger fraction (20-30%). The B-band LFs of galaxy mergers are derived at different redshifts up to z=1.2 and confirm the weak evolution of the merger fraction after breaking the luminosity-density degeneracy. The IR luminosity functions (LFs) of galaxy mergers are also derived, indicating a larger merger fraction at higher IR luminosity. The integral of the merger IR LFs indicates a dramatic evolution of the merger-induced IR energy density [(1+z)^(5-6)}], and that galaxy mergers start to dominate the cosmic IR energy density at z>~1.

Accepted for publication in ApJ, 25 pages, 23 figures (2 colors). The high-resolution pdf is at http://cztsy.as.arizona.edu/~yong/Research/SHI_MERGER.pdf