Discovery of Hydrogen Radio Recombination Lines at z = 0.89 toward PKS 1830-211

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 H144 α (and H163 α ) transitions at observed frequencies of 1156 (798) MHz by stacking 17 (27) RRLs with 21 σ (14 σ ) 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 _⋆ ∼ 10 ^11 M _⊙ . 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.