Tentative detection of neutral gas in a Little Red Dot at $z=4.46$
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2025
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| author | Akins, Hollis B. Casey, Caitlin M. Chisholm, John Berg, Danielle A. Cooper, Olivia Franco, Maximilien Fujimoto, Seiji Lambrides, Erini Long, Arianna S. McKinney, Jed |
| author_facet | Akins, Hollis B. Casey, Caitlin M. Chisholm, John Berg, Danielle A. Cooper, Olivia Franco, Maximilien Fujimoto, Seiji Lambrides, Erini Long, Arianna S. McKinney, Jed |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2503_00998 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Tentative detection of neutral gas in a Little Red Dot at $z=4.46$ Akins, Hollis B. Casey, Caitlin M. Chisholm, John Berg, Danielle A. Cooper, Olivia Franco, Maximilien Fujimoto, Seiji Lambrides, Erini Long, Arianna S. McKinney, Jed Astrophysics of Galaxies 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. |
| title | Tentative detection of neutral gas in a Little Red Dot at $z=4.46$ |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2503.00998 |