Tentative detection of neutral gas in a Little Red Dot at $z=4.46$

TL;DR

This study reports a tentative detection of neutral gas in a high-redshift galaxy, revealing a compact, possibly non-stellar origin of its Balmer break, contrasting with typical gas-rich quasars.

Contribution

First ALMA observations of Little Red Dots at high redshift, providing constraints on their molecular and neutral gas content and dynamical mass.

Findings

No CO detection, limiting molecular gas mass to $ extless{} 1$-$5 imes10^{10}$ M$_igodot$

Tentative [CI](2-1) detection indicating low dynamical mass

Dynamical mass lower than stellar mass from SED fitting

Abstract

JWST has revealed a population of broad-line active galactic nuclei at $z>4$ with remarkably red colors, so-called "Little Red Dots." Ubiquitous Balmer breaks suggest that they harbor old stellar populations in massive, compact host galaxies. We present ALMA observations of three LRDs at $z=3.10$, $4.46$, and $7.04$, targeting molecular and neutral gas via CO(7-6) and [CI](2-1), respectively. We do not detect CO in any target, placing conservative limits on the host molecular gas mass $\lesssim 1$-$5\times10^{10}$ M$_\odot$. We report the tentative ($4.9\sigma$) detection of the [CI](2-1) line in A2744-45924 ($z=4.46$), one of the brightest known LRDs. The [CI] line is narrow (FWHM $\sim 80$ km s$^{-1}$), implying a dynamical mass $\lesssim 10^{10}$ M$_\odot$, adopting conservative limits for the galaxy size. The dynamical mass limit is significantly lower than expected from the local $M_{\rm BH}$-$M_{\rm dyn}$ relation, and is an order of magnitude below the stellar mass derived from SED fitting, potentially supporting a non-stellar origin of the Balmer break. These results, while tentative, paint a picture of LRDs that is markedly different than typical high-$z$ quasars, which live in massive, gas-rich, and actively star-forming host galaxies.