SPURS: Evidence for Clumpy Neutral Envelopes and Ionized IGM Surrounding Little Red Dots in Abell 2744 from Ultra-Deep Rest-UV Spectroscopy

TL;DR

This study uses ultra-deep JWST/NIRSpec spectra of two Little Red Dots in Abell 2744 to investigate their neutral gas and ionized intergalactic medium, revealing clumpy neutral gas and dense environments influencing Ly$ extalpha$ emission.

Contribution

First detailed spectral analysis of LRDs at high redshift showing evidence for clumpy neutral gas and environmental effects on Ly$ extalpha$ emission.

Findings

Abell2744-QSO1 has broad Ly$ extalpha$ and narrow metal lines, with a less redshifted profile than dense gas models predict.

UNCOVER-2476 exhibits narrow [NeIV] emission, indicating hard radiation or shocks.

LRDs are associated with dense environments that may ionize surrounding IGM, affecting Ly$ extalpha$ visibility.

Abstract

Rest-frame ultraviolet (UV) spectra of Little Red Dots (LRDs) often show Ly$\alpha$ emission. Along with broad Balmer emission, LRDs are expected to produce broad Ly$\alpha$ emission. However, the large column density of neutral gas invoked to explain the Balmer break should significantly redshift and further broaden the Ly$\alpha$ line, making it challenging to detect without sensitive, moderate-resolution spectra. We present ultra-deep (29 hours) G140M JWST/NIRSpec observations covering the rest-UV of two LRDs in Abell2744 from the SPURS Cycle 4 Large Program. One of our targets is Abell2744-QSO1, a gravitationally-lensed LRD at $z=7.04$ with faint UV emission (M$_{\rm UV}=-16.9$), and the other source (UNCOVER-2476) is newly-confirmed at $z=4.02$ with a very bright UV continuum (M$_{\rm UV}=-19.6$). We find that Abell2744-QSO1 has a broad Ly$\alpha$ profile, along with narrow CIV, FeII$\lambda1786$, and OI$\lambda1302$ emission. The Ly$\alpha$ profile suggests an origin similar to the broad H$\alpha$, but the line is considerably less redshifted than expected from existing dense gas models. We show that the line profile can be explained if the dense neutral gas is clumpy, allowing Ly$\alpha$ to escape by scattering off of the clump surfaces. We find that UNCOVER-2476 has narrow [NeIV] emission, indicating either a hard radiation field or shocks. We confirm two close neighbors with Ly$\alpha$ emission around Abell2744-QSO1, indicating it traces a dense environment that may have ionized its surrounding IGM. We suggest that LRDs may preferentially trace bubbles carved by their dense environments, contributing to the prevalence of Ly$\alpha$ in the population.