A jet-gas interaction beyond the host galaxy: Detection of a neutral hydrogen outflow at cosmic noon

¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences 80 Nandan Road Shanghai 200030, China
² School of Chemical and Physical Sciences, Victoria University of Wellington PO Box 600 Wellington 6140, New Zealand
³ First Light Fusion Ltd., Unit 9/10 Oxford Pioneer Park Mead Road Yarnton Kidlington OX5 1QU, UK
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh EH9 3HJ, UK
⁵ Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town Cape Town, South Africa
⁶ International Centre for Radio Astronomy Research (ICRAR), Curtin University Bentley WA, Australia
⁷ ATNF, CSIRO Space and Astronomy PO Box 76 Epping NSW 1710, Australia
⁸ State Key Laboratory of Radio Astronomy and Technology, A20 Datun Road Chaoyang District Beijing 100101, PR China
⁹ College of Physics, Guizhou University 550025 Guiyang, PR China
¹⁰ National Astronomical Observatories, Chinese Academy of Sciences 20A Datun Road Beijing 100101, China

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Received: 6 November 2025
Accepted: 18 January 2026

Abstract

We present upgraded Giant Metrewave Radio Telescope (uGMRT) observations of 0731+438, an FR II radio galaxy at a redshift of 2.429 with two lobes separated by 82 kpc. A blueshifted, faint, and broad H I 21 cm absorption line with a velocity full width at half maximum of ∼ 600 km s⁻¹ is detected against the southern radio lobe that is 47 kpc from radio core, indicating a neutral hydrogen outflow associated with jet-gas interaction beyond the host galaxy. The outflow has a mass outflow rate of ∼0.4T_sΩ M_⊙ yr⁻¹, which could increase to ∼ 4.0T_sΩ M_⊙ yr⁻¹, corresponding to an energy outflow rate of 2.4T_sΩ × 10⁴⁰–1.5T_s Ω × 10⁴¹ erg s⁻¹, where T_s is the spin temperature and Ω is the solid angle of the outflow. Previous optical observations identified an extended emission line region aligned with the radio axis, ionized by the central active galactic nucleus (AGN). Within this region, a warm and ionized outflow with a mass outflow rate of ∼50 M_⊙ yr⁻¹ and an energy outflow rate of ∼ 1.7 × 10⁴³ erg s⁻¹ was detected. We propose that both the extended emission line region and the optical outflow are results of a synergistic effect between the jet and AGN radiation. The AGN likely exerts negative feedback on the host galaxy, as is evidenced by the gas expulsion by the jet and the high-velocity dispersion of ionized gas observed optically. So far, detections of jet-driven neutral hydrogen outflows remain rare. The high redshift, large outflow radii, substantial mass outflow rate, and energy outflow rate of the neutral hydrogen outflow in 0731+438 expand the known parameter space of such outflows. Finally, we find tentative correlations between the neutral hydrogen mass outflow rate (or energy outflow rate) and the rest-frame 1.4 GHz radio power of jets for the known broad neutral hydrogen outflows.