The UV Properties of Star Forming Galaxies I: HST WFC3 Observations of Very-high Redshift Galaxies

TL;DR

This study uses HST WFC3 near-infrared imaging to analyze the UV continuum colors of high-redshift galaxies between z=4.7 and 7.7, revealing insights into their dust content, metallicity, and star formation properties.

Contribution

First detailed measurement of UV continuum slopes of galaxies at z=4.7-7.7 using carefully selected dropout samples to minimize bias.

Findings

High-redshift galaxies have UV colors consistent with dust-free, low-metallicity star formation.

Lower-redshift and more luminous galaxies tend to be redder in UV color.

Color variations are influenced by dust, metallicity, star formation history, and initial mass function.

Abstract

The acquisition of deep Near-IR imaging with Wide Field Camera 3 on the Hubble Space Telescope has provided the opportunity to study the very-high redshift Universe. For galaxies up to $z\approx 7.7$ sufficient wavelength coverage exists to probe the rest-frame ultraviolet (UV) continuum without contamination from either Lyman-$\alpha$ emission or the Lyman-$\alpha$ break. In this work we use Near-IR imaging to measure the rest-frame UV continuum colours of galaxies at $4.7<z<7.7$. We are have carefully defined a colour-colour selection to minimise any inherent bias in the measured UV continuum slope for the drop-out samples. For the highest-redshift sample ($6.7<z<7.7$), selected as $z_{f850lp}$-band dropouts, we find mean UV continuum colours approximately equal to zero (AB), consistent with a dust-free, solar metallicity, star forming population (or a moderately dusty population of low metallicity). At lower-redshift we find that the mean UV continuum colours of galaxies (over the same luminosity range) are redder, and that galaxies with higher luminosities are also slightly redder on average. One interpretation of this is that lower-redshift and more luminous galaxies are dustier, however this interpretation is complicated by the effects of the star formation history and metallicity and potentially the initial mass function on the UV continuum colours.