Discovery of Hydrogen Radio Recombination Lines at z = 0.89 toward PKS 1830-211
Journal article, 2023

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 log ( n e ) = 2.0 − 0.7 + 1.0 cm−3 and log ( ℓ ) = − 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 log ( n e ) = 3.2 − 1.0 + 0.4 cm−3 and log ( ℓ ) = − 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 ΣSFR ∼ 0.6 M ⊙ yr−1 kpc−2 or SFR ∼ 50 M ⊙ yr−1, consistent with a star-forming main-sequence galaxy of M ⋆ ∼ 1011 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.


K.L. Emig

National Radio Astronomy Observatory

Neeraj Gupta

Inter-University Centre for Astronomy and Astrophysics

P. Salas

Green Bank Observatory

Sebastien Muller

Chalmers, Space, Earth and Environment, Onsala Space Observatory

S. Balashev

Russian Academy of Sciences

National Research University Higher School of Economics

Françoise Combes

Paris Observatory

E. Momjian

National Radio Astronomy Observatory Socorro

Y. Song

National Radio Astronomy Observatory

University of Virginia

P. Jagannathan

National Radio Astronomy Observatory Socorro

Partha P. Deka

Inter-University Centre for Astronomy and Astrophysics

G. I.G. Józsa

Rhodes University

Max Planck Society

H. R. Klockner

Max Planck Society

Abhisek Mohapatra

Inter-University Centre for Astronomy and Astrophysics

P. Noterdaeme

Institut d 'Astrophysique de Paris

University of Chile (UCH)

Patrick Petitjean

Institut d 'Astrophysique de Paris

R. Srianand

Inter-University Centre for Astronomy and Astrophysics

Jonah D. Wagenveld

Max Planck Society

Astrophysical Journal

0004-637X (ISSN) 1538-4357 (eISSN)

Vol. 944 1 93

Subject Categories

Astronomy, Astrophysics and Cosmology

Atom and Molecular Physics and Optics

Fusion, Plasma and Space Physics



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