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

Abstract

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 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.