The Rest-Frame Far-Ultraviolet Morphologies of Star-Forming Galaxies at z ~ 1.5 and 4

We apply a new approach to quantifying galaxy morphology and identifying galaxy mergers to the rest-frame far-ultraviolet images of 82 z ~ 4 Lyman break galaxies (LBGs) and 55 1.2 < z < 1.8 emission-line galaxies in the GOODS and Ultra Deep Field survey. We compare the distributions of the Gini coefficient (G), the second-order moment of the brightest 20% of galaxy light (M₂₀), and concentration (C) for high- and low-redshift galaxies with average signal-to-noise ratios per pixel >2.5 and Petrosian radii >03. Ten of the 82 LBGs have M₂₀ ≥ -1.1 and possess bright double or multiple nuclei, implying a major-merger fraction of star-forming galaxies ~10%-25% at M_FUV < -20, depending on our incompleteness corrections. Galaxies with bulge-like morphologies (G ≥ 0.55, M₂₀ < -1.6) make up ~30% of the z ~ 4 LBG sample, while the remaining ~50% have G- and M₂₀-values higher than expected for smooth bulges and disks and may be star-forming disks, minor mergers, or postmergers. The star-forming z ~ 1.5 galaxy sample has a morphological distribution that is similar to the UDF z ~ 4 LBGs, with an identical fraction of major-merger candidates but fewer spheroids. The observed morphological distributions are roughly consistent with current hierarchical model predictions for the major-merger rates and minor-merger-induced starbursts at z ~ 1.5 and ~4. We also examine the rest-frame FUV - NUV and FUV - B colors as a function of morphology and find no strong correlations at either epoch.