A Vision for Little Red Dots in the Low-Redshift Universe with ALMA2040
Abstract
The discovery of Little Red Dots (LRDs) at high redshifts with JWST has revealed a previously unknown phase of black-hole growth. However, their black-hole (BH) masses, BH-to-stellar mass ratios, cold-gas reservoirs, and depletion times remain uncertain. The recent discovery of low-redshift LRDs at z≃0.1 − 0.9 creates a decisive opportunity to access their low-J CO lines for measuring molecular gas reservoirs, and the nearest systems can in principle be spatially resolved. We use J1025+1402 ("The Egg") at z=0.1007, the best-studied local LRD, as a benchmark to explore the scientific opportunities with ALMA2040. Assuming a molecular gas mass of 109 M⊙, the CO(2-1) detectability strongly depends on the assumed CO-to-H2 conversion (αCO). With a large αCO ≃ 315 − 725 M⊙ (K km s−1 pc2)−1 at a low gas-phase metallicity, the CO line would remain very challenging for the current ALMA to detect, while ALMA2040 will likely deliver a robust CO detection and thus a gas-reservoir constraint for The Egg. Furthermore, with the best angular resolution at its CO(2-1) frequency (14.6 mas, or 27 pc), ALMA2040 may spatially resolve the CO emission from The Egg. Assuming an optimistic, SMC-like αCO with a 50-hour deep integration, we produce mock position-velocity diagrams that may distinguish a 107 M⊙ black hole from the no-BH model through its steeper central velocity structure. We conclude that ALMA2040 has the potential to directly measure the gas masses, depletion times, and excitation of local LRDs, while the CO-bright subset may ultimately yield dynamical constraints on the mass of their central black holes.
Related
- ALMA2040: The High-Redshift Circumgalactic Medium. Imaging the fueling and feedback of galaxies in the early universe
- A Cosmological Framework for Stellar Collisions at High Redshift in Proto-globular Clusters, Nuclear Star Clusters, and Little Red Dots
- (LRDs)2: The Low-ReDshift Little Red Dots Survey. II. DESI DR1 Sample
- Can Little Red Dots Contribute To The Early Universe Cosmic Dust Budget?
- A critical evaluation of the physical nature of the little red dots
BibTeX
@misc{sun2026vision,
title = {{A Vision for Little Red Dots in the Low-Redshift Universe with ALMA2040}},
author = {Fengwu Sun and Xiaojing Lin and Miguel Pereira-Santaella and Dieu D. Nguyen},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.20697},
eprint = {2610.20697},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.20697},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Vision for Little Red Dots in the Low-Redshift Universe with ALMA2040
AU - Fengwu Sun
AU - Xiaojing Lin
AU - Miguel Pereira-Santaella
AU - Dieu D. Nguyen
PY - 2026
DA - 2026/10/14
PB - arXiv
DO - 10.48550/arxiv.2610.20697
UR - https://arxiv.org/abs/2610.20697
AB - The discovery of Little Red Dots (LRDs) at high redshifts with JWST has revealed a previously unknown phase of black-hole growth. However, their black-hole (BH) masses, BH-to-stellar mass ratios, cold-gas reservoirs, and depletion times remain uncertain. The recent discovery of low-redshift LRDs at z≃0.1 − 0.9 creates a decisive opportunity to access their low-J CO lines for measuring molecular gas reservoirs, and the nearest systems can in principle be spatially resolved. We use J1025+1402 ("The Egg") at z=0.1007, the best-studied local LRD, as a benchmark to explore the scientific opportunities with ALMA2040. Assuming a molecular gas mass of 10^9 M_⊙, the CO(2-1) detectability strongly depends on the assumed CO-to-H_2 conversion (α_CO). With a large α_CO ≃ 315 − 725 M_⊙ (K km s^−1 pc^2)^−1 at a low gas-phase metallicity, the CO line would remain very challenging for the current ALMA to detect, while ALMA2040 will likely deliver a robust CO detection and thus a gas-reservoir constraint for The Egg. Furthermore, with the best angular resolution at its CO(2-1) frequency (14.6 mas, or 27 pc), ALMA2040 may spatially resolve the CO emission from The Egg. Assuming an optimistic, SMC-like α_CO with a 50-hour deep integration, we produce mock position-velocity diagrams that may distinguish a 10^7 M_⊙ black hole from the no-BH model through its steeper central velocity structure. We conclude that ALMA2040 has the potential to directly measure the gas masses, depletion times, and excitation of local LRDs, while the CO-bright subset may ultimately yield dynamical constraints on the mass of their central black holes.
ER -