Physical Properties of Spectroscopically-Confirmed Galaxies at z >= 6. I. Basic Characteristics of the Rest-Frame UV Continuum and Lyman-alpha emission

TL;DR

This study analyzes the UV continuum and Lyman-alpha emission properties of 67 spectroscopically-confirmed galaxies at z >= 6, revealing insights into their stellar populations, star formation, and implications for cosmic reionization.

Contribution

It provides the first comprehensive analysis of UV slopes, Lyman-alpha emission, and their relation to galaxy properties at z >= 6, highlighting the diversity and characteristics of early galaxies.

Findings

Galaxies exhibit steep UV slopes, averaging around -2.3.

Presence of very young, low-metallicity stellar populations indicated by slopes near -3.

Faint galaxies with weak Lyman-alpha emission likely dominate ionizing flux at z >= 6.

Abstract

We present deep HST near-IR and Spitzer mid-IR observations of a large sample of spectroscopically-confirmed galaxies at z >= 6. The 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. The near-IR images were mostly obtained with WFC3 in the F125W and F160W bands, and the mid-IR images were obtained with IRAC in the 3.6um and 4.5um bands. Our galaxies also have deep optical imaging data from Subaru Suprime-Cam. We utilize the multi-band data and secure redshifts to derive their rest-frame UV properties. These galaxies have steep UV continuum slopes roughly between beta ~ -1.5 and -3.5, with an average value of beta ~ -2.3, slightly steeper than the slopes of LBGs in previous studies. The slope shows little dependence on UV continuum luminosity except for a few of the brightest galaxies. We find a statistically significant excess of galaxies with slopes around beta ~ -3, suggesting the existence of very young stellar populations with extremely low metallicity and dust content. Our galaxies have moderately strong rest-frame Lyman-alpha equivalent width (EW) in a range of ~10 to ~200 \AA. The star-formation rates are also moderate, from a few to a few tens solar masses per year. The LAEs and LBGs in this sample share many common properties, implying that LAEs represent a subset of LBGs with strong Lyman-alpha emission. Finally, the comparison of the UV luminosity functions between LAEs and LBGs suggests that there exists a substantial population of faint galaxies with weak Lyman-alpha emission (EW < 20 \AA) that could be the dominant contribution to the total ionizing flux at z >= 6.