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

First listed 2025-03-02 | Last updated 2025-03-02

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.

Short digest

ALMA Band 3/4/5 spectroscopy targets CO(7–6) and [CI](2–1) in three Little Red Dots at z=3.10, 4.46, and 7.04, finding no CO and no far-IR continuum, implying conservative molecular gas limits ≲1–5×10^10 Msun. A2744-45924 (z=4.4655) shows a tentative 4.9σ [CI](2–1) line with a narrow FWHM ~80 km s−1. The [CI]-inferred dynamical mass is ≲10^10 Msun for conservative size assumptions, well below the local MBH–Mdyn relation and an order of magnitude under SED-based stellar mass. If confirmed, this points to LRD hosts unlike massive, gas-rich quasar hosts and lends weight to a non-stellar origin for the Balmer break.

Key figures to inspect

• Figure 1 (CO and [CI] spectra): Check the CO(7–6) non-detections and the single [CI](2–1) feature in A2744-45924; compare line centroids to the NIRSpec/WFSS redshifts and assess per-channel noise using the grey-error tracks, including A/B image extracts for A2744-QSO1.
• Figure 2 ([CI] moment-0 + spectrum): Inspect [CI] contours over the NIRCam RGB to see alignment/offset with the LRD; use the Gaussian fit and corner plot to verify the ~80 km s−1 width, integrated flux, and the 4.9σ significance given the Band 4 beam (~1.6×1.3″).
• Figure 3 (MBH vs Mdyn): Locate A2744-45924 relative to local MBH–Mdyn and high-z quasar [CII]-based dynamical masses; quantify how far it lies below the local relation and compare to other JWST BLAGNs with dynamical estimates.