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

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 $\lesssim 1$-$5\times 10^{10}$ 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 $M_{\mathrm{BH}}$-$M_{\mathrm{dyn}}$ 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.

arXiv comment10 pages, 3 figures, submitted to ApJL