Luminous Lyα Emitters with Very Blue UV-continuum Slopes at Redshift 5.7 ≤ z ≤ 6.6

Linhua Jiang,Eiichi Egami,Daniel Schaerer,Fengwu Sun,Kristian Finlator,Seth H Cohen,Rogier A Windhorst

doi:10.3847/1538-4357/ab64ea

Abstract

Abstract We study six luminous Lyα emitters (LAEs) with very blue rest-frame UV continua at 5.7 ≤ z ≤ 6.6. These LAEs have previous Hubble Space Telescope (HST) and Spitzer IRAC observations. Combining our newly acquired HST images, we find that their UV-continuum slopes β are in a range of −3.4 ≤ β ≤ −2.6. Unlike previous, tentative detections of β ≃ −3 in photometrically selected, low-luminosity galaxies, our LAEs are spectroscopically confirmed and luminous (M UV < −20 mag). We model their broadband spectral energy distributions (SEDs), and find that two β ≃ −2.6 ± 0.2 galaxies can be well fitted with young and dust-free stellar populations. However, it becomes increasingly difficult to fit bluer galaxies. We explore further interpretations by including the non-zero LyC escape fraction f esc, very low metallicities, and/or active galactic nucleus contributions. Assuming f esc ≃ 0.2, we achieve the bluest slopes β ≃ −2.7 when nebular emission is considered. This can nearly explain the SEDs of two galaxies with β ≃ −2.8 and −2.9 (σ β = 0.15). Larger f esc values and very low metallicities are not favored by the strong nebular line emission (evidenced by the IRAC flux) or the observed (IRAC 1 − IRAC 2) color. Finally, we find that the β ≃ −2.9 galaxy can potentially be well explained by the combination of a very young population with a high f esc (≥0.5) and an old, dusty population. We are not able to produce two β ≃ −3.4 ± 0.4 galaxies. Future deep spectroscopic observations are needed to fully understand these galaxies.

Highlights

In recent years, a number of high-redshift galaxies have been discovered, thanks to the advances of instrumentation on the Hubble Space Telescope (HST) and large ground-based telescopes
Combining our newly acquired HST images, we find that their UV-continuum slopes β are in a range of −3.4 β −2.6
Tentative detections of β ; −3 in photometrically selected, low-luminosity galaxies, our LAEs are spectroscopically confirmed and luminous (MUV < −20 mag). We model their broadband spectral energy distributions (SEDs), and find that two β ; −2.6 ± 0.2 galaxies can be well fitted with young and dust-free stellar populations

Summary

Introduction

A number of high-redshift galaxies have been discovered, thanks to the advances of instrumentation on the Hubble Space Telescope (HST) and large ground-based telescopes. These galaxies have played an important role in studies of galaxy formation and evolution at early epochs. The narrowband (or Lyα) technique provides a complementary method to find highredshift galaxies, and narrowband observations were mostly done by large-area ground-based observations (e.g., Kashikawa et al 2011; Ota et al 2017; Zheng et al 2017; Ouchi et al 2018; Hu et al 2019). While ground-based observations provide samples of luminous galaxies, the majority of faint galaxies have to come from deep HST observations

Results

Discussion

Conclusion