2023ApJ...944...93E

We report the detection of stimulated hydrogen radio recombination line (RRL) emission from ionized gas in a z = 0.89 galaxy using 580-1670 MHz observations from the MeerKAT Absorption Line Survey. The RRL emission originates in a galaxy that intercepts and strongly lenses the radio blazar PKS 1830-211 (z = 2.5). This is the second detection of RRLs outside of the local Universe and the first clearly associated with hydrogen. We detect effective H144α (and H163α) transitions at observed frequencies of 1156 (798) MHz by stacking 17 (27) RRLs with 21σ (14σ) significance. The RRL emission contains two main velocity components and is coincident in velocity with H I 21 cm and OH 18 cm absorption. We use the RRL spectral line energy distribution and a Bayesian analysis to constrain the density (n_e) and the volume-averaged path length (ℓ) of the ionized gas. We determine $\mathrm{log}({n}_{e})={2.0}_{-0.7}^{+1.0}$ cm^–3 and $\mathrm{log}({\ell })=-{0.7}_{-1.1}^{+1.1}$ pc toward the northeast (NE) lensed image, likely tracing the diffuse thermal phase of the ionized ISM in a thin disk. Toward the southwest (SW) lensed image, we determine $\mathrm{log}({n}_{e})={3.2}_{-1.0}^{+0.4}$ cm^–3 and $\mathrm{log}({\ell })=-{2.7}_{-0.2}^{+1.8}$ pc, tracing gas that is more reminiscent of H scii regions. We estimate a star formation (surface density) rate of Σ_SFR ∼ 0.6 M_⊙ yr^–1 kpc^–2 or SFR ∼ 50 M_⊙ yr^–1, consistent with a star-forming main-sequence galaxy of M_⋆ ∼ 10¹¹M_⊙. The discovery presented here opens up the possibility of studying ionized gas at high redshifts using RRL observations from current and future (e.g., SKA and ngVLA) radio facilities.