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

TL;DR

This study reports upgraded uGMRT detections of a blueshifted HI 21 cm absorption feature in the z=2.429 FR II galaxy 0731+438, probing jet–gas interactions well beyond the host galaxy. The neutral gas exhibits multiple absorption components toward the southern lobe, with HI column densities dependent on the spin temperature, yielding mass outflow rates from roughly 0.4 to 4.0 $T_s$ Ω $M_☉$ yr^{-1} and kinetic powers up to ~1.5 × 10^{41} erg s^{-1}. Multi-wavelength data reveal concurrent warm ionized outflows aligned with the radio jet, suggesting a synergistic jet–AGN radiation feedback that can drive negative regulation of star formation and CGM accretion. With its high redshift, extended outflow radii, and sizable outflow energetics, this work expands the parameter space for jet-driven HI outflows and motivates extended-source HI surveys at high redshift, as well as follow-up IFU observations of the ionized gas. Tentative correlations between outflow properties and jet power are explored, though a larger, unbiased sample is required for robust conclusions.

Abstract

We present upgraded Giant Metrewave Radio Telescope (uGMRT) observations of 0731+438, an \mbox{FR II} radio galaxy at a redshift of 2.429 with two lobes separated by 82 kpc. A blueshifted, faint and broad \mbox{H{\sc i}} 21 cm absorption line with velocity full width at half maximum (FWHM) $\sim 600\,\rm km\,s^{-1}$ 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 $\sim\,0.4T_{\rm s}Ω\rm\, M_\odot\,{\rm yr}^{-1}$, which could increase to $\sim\,4.0T_{\rm s}Ω\rm\,M_\odot\,{\rm yr}^{-1}$, corresponding to an energy outflow rate of $2.4T_{\rm s}Ω\times10^{40}$ -- $1.5T_{\rm s}Ω\times10^{41}\,\rm erg\,s^{-1}$, where $T_{\rm s}$ 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 $\sim\,50\rm\, M_\odot\,{\rm yr}^{-1}$ and an energy outflow rate of $\sim1.7\times10^{43}\,\rm erg\,s^{-1}$ was detected. We propose that both the extended emission line region and the optical outflow are results of synergistic effect between jet and AGN radiation. The AGN likely exerts negative feedback on the host galaxy, as 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.

Figures (4)

• Figure 1: The continuum images and H i absorption moment 0 map. The black contours represent the image with natural weighting, which start at $3\sigma$ noise and increase by a factor of 2 while the green contours show the image with uniform weighting, which start at $3\sigma$ noise and increase by a factor of 2. The color image is H i absorption moment 0 map with natural weighting. Only pixels with absorption stronger than 1.5$\sigma$ are shown, where $\sigma$ is the noise of the map. The right color bar shows the depth of the absorption. The map contains three unresolved regions, compared to the beam size. H i absorption lines were extracted from the peak absorption pixels of the three regions, which are presented in Figure \ref{['fig:hi_spectra']}.
• Figure 2: The H i absorption lines extracted from the peak absorption pixels of the three regions in Figure \ref{['fig:gas_distri']}. The blue lines are observed spectra; the green lines are observed spectra resampled to a velocity resolution of 50 $\rm km\,s^{-1}$ with using SpecRes carnall2017; the red lines are fitted Gaussian components while the gray lines are residuals.
• Figure 3: The distributions of H i column density, mass outflow rate and energy outflow rate as a function of $\Omega$ and $T_{\rm spin}/f$ for the Line 1 and Line 2, where $\Omega$ is the solid angle of the outflow, $T_{\rm spin}/f$ is spin temperature divided by covering factor. Note that the column density limits are constrained by $T_{\rm spin}/f>T_{\rm cmb}$ at z=2.4 and $\rm N_{HI}<3\times10^{22}$schaye2001curran2024, where $T_{\rm cmb}$ is the temperature of cosmic microwave background.
• Figure 4: The correlations between the neutral hydrogen outflow properties, mass outflow rate and energy outflow rate, and the rest-frame 1.4 GHz radio power of jets. The red markers represent 0731+438. The blue lines are linear fits.