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

Hollis B. Akins; Caitlin M. Casey; John Chisholm; Danielle A. Berg; Olivia Cooper; Maximilien Franco; Seiji Fujimoto; Erini Lambrides; Arianna S. Long; Jed McKinney

arXiv · 2503.00998

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

Hollis B. Akins·Caitlin M. Casey·John Chisholm·Danielle A. Berg·Olivia Cooper·Maximilien Franco·Seiji Fujimoto·Erini Lambrides·Arianna S. Long·Jed McKinney

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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.

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Hollis B. Akins, Caitlin M. Casey, John Chisholm, Danielle A. Berg, Olivia Cooper, Maximilien Franco, Seiji Fujimoto, Erini Lambrides, Arianna S. Long, Jed McKinney. 2025-03-02. Tentative detection of neutral gas in a Little Red Dot at $z=4.46$. https://arxiv.org/abs/2503.00998

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