Free-Free Radio Emission from Little Red Dots as a Probe of Ionized Gas

Takumi S. Tanaka, Kohei Inayoshi, Zu Yan, Tomokazu Kiyota, Yuichi Harikane, John D. Silverman

First listed 2026-09-21 | Last updated 2026-09-17

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.

Short digest

Tanaka et al. propose thermal free-free radio emission as an independent probe of the dense ionized gas thought to surround little red dots. Free-free self-absorption should imprint a spectral turnover whose frequency and luminosity jointly constrain the electron column density and characteristic size of the ionized region, while the low-frequency slope distinguishes a uniform medium from a radially stratified one. Using representative low- and high-redshift LRDs, including J1025+1402 and A2744-45924, they show that 10-hour SKA1-Mid, ngVLA, and ALMA observations can access the relevant parameter space and recover column and size constraints from mock photometry. The approach offers a radio-to-millimeter test of whether the gas geometry invoked to explain LRD optical/infrared spectra is physically viable on unresolved scales.

Key figures to inspect

• Figure 1. This is the core observable-to-physics map: a turnover frequency and luminosity translate into ionized-region radius, electron column density, and density, while the shaded [O III] collisional-de-excitation regime marks an important physical boundary.
• Figure 2. The representative SEDs make the proposal observationally concrete by placing Model II and density-gradient Model V free-free spectra beside A2744-45924 and J1025+1402, dust/blackbody alternatives, and instrument sensitivities.
• Figure 3. This figure identifies which combinations of ionized-gas size and density are detectable in 10-hour SKA1-Mid, ngVLA, and ALMA exposures, directly testing the paper's feasibility claim across low- and high-redshift cases.
• Figure 4. The mock-fitting posteriors show the diagnostic payoff after detection: even with an intrinsic degeneracy among temperature, size, and density-profile parameters, the electron column density and characteristic radius remain recoverable to useful precision.