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 >0″.3. 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 MFUV < -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.