Highly-ionized gas in lensed z = 6.027 Little Red Dot seen through [OIII] 88$\mu$m with ALMA

TL;DR

This study uses deep ALMA observations to analyze the highly-ionized gas in a faint, gravitationally lensed galaxy at z=6.027, revealing its low metallicity, dust-poor nature, and potential role in cosmic reionization.

Contribution

First detection of [OIII] 88μm line in a faint, lensed galaxy at z=6.027, providing insights into early galaxy properties and reionization contributions.

Findings

High [OIII]/[CII] luminosity ratio (~14) indicating low metallicity.

No dust continuum detected, suggesting dust-poor galaxy.

Strong ultraviolet radiation field likely contributes to intergalactic medium ionization.

Abstract

Determining the physical properties of galaxies during the first billion years after the big bang is key to understanding both early galaxy evolution and how galaxies contributed to the epoch of reionization. We present deep ALMA observations of the redshifted [OIII] 88um line for the gravitationally lensed ($\mu = 11.4\pm1.9$) galaxy A383-5.1 (z=6.027) that has previously been detected in [CII] 158um. Recent James Webb Space Telescope (JWST) imaging identified this sub-L* galaxy as a ''Little Red Dot'' (LRD). With a line luminosity of $L_{\rm [OIII]} = (1.29\pm0.24)\times10^8$ L$_\odot$ (corrected for lensing magnification) A383-5.1 is one of the faintest galaxies with combined [CII] and [OIII] detections. The ALMA data reveal no dust continuum emission, consistent with previous observations. The high line luminosity ratio of [OIII]/[CII] $\sim 14\pm5$ is consistent with A383-5.1 being low-metallicity and dust-poor. The non-detection of dust continuum in bands 6 and 8 is consistent with the high [OIII]/[CII] ratio and suggests a presence of a strong ultraviolet radiation field, which would be less affect by dust attenuation, implying that galaxies of this type could contribute significantly to the ionization of the intergalactic medium. The presence of strong ionizing field could provide an important piece of information for understanding the nature of LRDs and their role in cosmic reionization.