Free-Free Radio Emission from Little Red Dots as a Probe of Ionized Gas
Abstract
Recent studies have hypothesized that little red dots (LRDs) are rapidly accreting black holes surrounded by dense gaseous environments. Both neutral and ionized gas play a key role in explaining many of the puzzling spectral features of LRDs, including the emission line profiles in the optical-infrared bands, although the physical conditions required for these gas components remain poorly constrained. We propose that free-free emission provides a robust, independent probe of ionized gas in LRDs. Free-free self-absorption produces a characteristic spectral turnover whose critical frequency and luminosity depend on the electron column density and the characteristic size of the ionized region, such that radio observations simultaneously constrain both quantities. The spectral slope at frequencies below the turnover further probes the radial density distribution on spatial scales far below those achievable by current and future optical-infrared observatories. We also present predicted free-free spectral energy distributions for two representative LRDs in the local and high-redshift universe and show that future radio-to-millimeter observations with ALMA and ngVLA can probe the range of ionized-gas parameters inferred from current observations. Free-free emission therefore offers a unique way to probe the geometry and physical conditions of the ionized gas surrounding LRDs.
Related
- A radio counterpart of the local little red dot J092359+415736
- Little Red Dot Cosmology: A Matter-Era Baryon Acoustic Oscillations Probe of ΛCDM
- Ultraviolet diversity of Little Red Dots as a probe for direct-collapse black hole ages
- Ultraviolet diversity of Little Red Dots as a probe for direct-collapse black hole ages
- A radio counterpart of the local little-red-dot J092359+415736
BibTeX
@misc{tanaka2026freefree,
title = {{Free-Free Radio Emission from Little Red Dots as a Probe of Ionized Gas}},
author = {Takumi S. Tanaka and Kohei Inayoshi and Zu Yan and Tomokazu Kiyota and Yuichi Harikane and John D. Silverman},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.20913},
eprint = {2609.20913},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.20913},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Free-Free Radio Emission from Little Red Dots as a Probe of Ionized Gas
AU - Takumi S. Tanaka
AU - Kohei Inayoshi
AU - Zu Yan
AU - Tomokazu Kiyota
AU - Yuichi Harikane
AU - John D. Silverman
PY - 2026
DA - 2026/09/17
PB - arXiv
DO - 10.48550/arxiv.2609.20913
UR - https://arxiv.org/abs/2609.20913
AB - Recent studies have hypothesized that little red dots (LRDs) are rapidly accreting black holes surrounded by dense gaseous environments. Both neutral and ionized gas play a key role in explaining many of the puzzling spectral features of LRDs, including the emission line profiles in the optical-infrared bands, although the physical conditions required for these gas components remain poorly constrained. We propose that free-free emission provides a robust, independent probe of ionized gas in LRDs. Free-free self-absorption produces a characteristic spectral turnover whose critical frequency and luminosity depend on the electron column density and the characteristic size of the ionized region, such that radio observations simultaneously constrain both quantities. The spectral slope at frequencies below the turnover further probes the radial density distribution on spatial scales far below those achievable by current and future optical-infrared observatories. We also present predicted free-free spectral energy distributions for two representative LRDs in the local and high-redshift universe and show that future radio-to-millimeter observations with ALMA and ngVLA can probe the range of ionized-gas parameters inferred from current observations. Free-free emission therefore offers a unique way to probe the geometry and physical conditions of the ionized gas surrounding LRDs.
ER -