Physical Properties of Spectroscopically-Confirmed Galaxies at z >= 6. II. Morphology of the Rest-Frame UV Continuum and Lyman-alpha Emission

TL;DR

This study analyzes the morphology and structural properties of galaxies at z >= 6, revealing their sizes, merger fractions, and the relationship between luminosity and size, while also examining Lyman-alpha halos and emission alignment.

Contribution

It provides the first detailed morphological analysis of spectroscopically-confirmed z >= 6 galaxies using high-resolution imaging and nonparametric methods, including the size-luminosity relation and halo searches.

Findings

Median galaxy size ~0.9 kpc at z >= 6.

Merger/interacting fraction reaches 40-50% at bright magnitudes.

No evidence of extended Lyman-alpha halos was found.

Abstract

We present a detailed structural and morphological study of a large sample of spectroscopically-confirmed galaxies at z >= 6, using deep HST near-IR broad-band images and Subaru optical narrow-band images. The galaxy sample consists of 51 Lyman-alpha emitters (LAEs) at z ~ 5.7, 6.5, and 7.0, and 16 Lyman-break galaxies (LBGs) at 5.9 < z < 6.5. These galaxies exhibit a wide range of rest-frame UV continuum morphology in the HST images, from compact features to multiple component systems. The fraction of merging/interacting galaxies reaches 40% ~ 50% at the brightest end of M_1500 <= -20.5 mag. The intrinsic half-light radii r_{hl,in}, after correction for PSF broadening, are roughly between r_{hl,in} ~ 0.05" (0.3 kpc) and 0.3" (1.7 kpc) at M_1500 <= -19.5 mag. The median r_{hl,in} value is 0.16" (~0.9 kpc). This is consistent with the sizes of bright LAEs and LBGs at z >= 6 in previous studies. In addition, more luminous galaxies tend to have larger sizes, exhibiting a weak size-luminosity relation r_{hl,in} \propto L^{0.14} at M_1500 <= -19.5 mag. The slope of 0.14 is significantly flatter than those in fainter LBG samples. We discuss the morphology of z >= 6 galaxies with nonparametric methods, including the CAS system and the Gini and M_20 parameters, and demonstrate their validity through simulations. We search for extended Lyman-alpha emission halos around LAEs at z ~ 5.7 and 6.5, by stacking a number of narrow-band images. We do not find evidence of extended halos predicted by cosmological simulations. Such Lyman-alpha halos, if they exist, could be weaker than predicted. Finally, we investigate any positional misalignment between UV continuum and Lyman-alpha emission in LAEs. While the two positions are generally consistent, several merging galaxies show significant positional differences. This is likely caused by a disturbed ISM distribution due to merging activity.