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

TL;DR

This study targets the gas content of three Little Red Dots (LRDs) at z=3.10, 4.46, and 7.04 with ALMA, observing CO(7-6) and [CI](2-1) to constrain molecular and neutral gas reservoirs. CO(7-6) remains undetected, yielding molecular gas mass upper limits of ≲1–5×10^10 M⊙. A tentative 4.9σ detection of [CI](2-1) is reported for A2744-45924 at z=4.46, with a narrow line that implies M_dyn ≲ 10^10 M⊙, significantly below expectations from the local $M_{ m BH}$–$M_{ m dyn}$ relation and well under the stellar mass from SED fitting, suggesting a non-stellar origin for the Balmer break. These results imply LRDs host gas properties that differ markedly from typical high-z quasars, which are generally gas-rich and actively star-forming.

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 ($4.9σ$) 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.