Ugmrt Detects 82 Kpc Hydrogen Outflow in Distant Radio Galaxy

Scientists have detected a substantial outflow of neutral hydrogen gas extending far beyond the host galaxy of the radio galaxy 0731+438, offering a new insight into how active galactic nuclei influence their environments. Renzhi Su from Shanghai Astronomical Observatory, Chinese Academy of Sciences, Stephen J. Curran from Victoria University of Wellington, and James R. Allison from First Light Fusion Ltd., alongside colleagues including Marcin Glowacki et al., used upgraded Giant Metrewave Radio Telescope observations to identify this blueshifted gas 47 kiloparsecs from the galaxy’s core. This discovery is significant because it reveals jet-gas interactions occurring on a cosmic scale at a redshift of 2.429 , a period known as cosmic noon , and demonstrates a powerful mechanism for feedback, where the energy released by the galaxy’s central engine expels gas, potentially suppressing star formation. The observed outflow’s high mass and energy outflow rates, exceeding previously known examples, expand our understanding of how galaxies and their supermassive black holes co-evolve throughout the universe.

The radio galaxy 429, which exhibits lobes separated by 82 kpc, shows a blueshifted, faint, and broad H I 21 cm absorption line against its southern radio lobe. This feature provides clear evidence for a neutral hydrogen outflow driven by jet–gas interactions occurring well beyond the boundaries of the host galaxy. The outflow is detected at a projected distance of 47 kpc from the radio core and has a velocity full width at half maximum (FWHM) of approximately 600 km s⁻¹, offering important insights into gas expulsion processes in distant active galaxies.

The study estimates a mass outflow rate of about 0.4 Tₛ Ω M⊙ yr⁻¹, which could rise to 4.0 Tₛ Ω M⊙ yr⁻¹. This corresponds to an energy outflow rate in the range of 2.4 Tₛ Ω × 10⁴⁰ to 1.5 Tₛ Ω × 10⁴¹ erg s⁻¹, where Tₛ is the spin temperature and Ω denotes the solid angle subtended by the outflow. These measurements highlight the significant role of radio jets in driving large-scale neutral gas outflows in active galaxies.

This detection is particularly significant given the rarity of observed jet-driven neutral hydrogen outflows, especially at such high redshifts and large radii. Previous optical observations had already identified an extended emission line region aligned with the radio axis, ionized by the central Active Galactic Nucleus (AGN), and a warm ionized outflow with a mass outflow rate of approximately 50 M⊙yr−1 and an energy outflow rate of approximately 1.7×1043 erg s−1. The. This work expands the known parameter space of neutral hydrogen outflows, demonstrating a high redshift, large outflow radii, substantial mass outflow rate, and significant energy outflow rate, characteristics not previously observed in such detail.
Furthermore, the researchers found tentative correlations between the neutral hydrogen mass outflow rate (or energy outflow rate) and the rest-frame 1.4GHz radio power of jets for the known broad neutral hydrogen outflows. This finding suggests a potential link between jet activity and the magnitude of the resulting outflow, offering a new avenue for understanding the interplay between AGN, jets, and their host galaxies0.429, seeking to confirm the presence of neutral hydrogen outflow driven by jet-gas interaction. The study utilized a 25MHz bandwidth centered on 414.3MHz, divided into 2048 channels, achieving a velocity coverage of -9000 to 9000km s−1 with a velocity resolution of approximately 8.8km s−1. Crucially, the team identified clear absorption features within the calibrated visibilities spanning 413.610, 414.831MHz, corresponding to a redshift range of z=2.424, 2.434, indicating the presence of Hi absorption. To generate a continuum image, the study excluded line channels and binned visibilities to a coarse resolution of 244kHz, applying two iterations of phase-only self-calibration and one iteration of amplitude-and-phase self-calibration to enhance image quality. The spectral cube was then constructed by applying self-calibration solutions to the original high-resolution visibilities, followed by second-order polynomial fitting in the frequency ranges of 412.390, 413.610MHz and 414.831, 416.052MHz to subtract the continuum emission.

Further spectral baseline inspection and flagging ensured accurate continuum subtraction before generating the final cube. This innovative approach enabled the detection of a blueshifted, faint, and broad H{\sc i} 21cm absorption line against the southern radio lobe, located 47 kpc from the radio core, revealing a neutral hydrogen outflow. The detected outflow exhibits a mass outflow rate of, potentially increasing to, and an associated energy outflow rate of –, where the spin temperature and solid angle of the outflow are key parameters0.429. This detection, situated 47 kpc from the radio core, indicates a neutral hydrogen outflow resulting from jet-gas interaction extending beyond the host galaxy itself. Experiments revealed the outflow possesses a velocity full width at half maximum (FWHM) and a mass outflow rate ranging from 10⁴⁰ to 1.5x 10⁴¹ erg s⁻¹, dependent on the spin temperature (Ts) and solid angle (Ω) of the outflow. The team measured an energy outflow rate of 2.4TsΩ x 10⁴⁰, 1.5TsΩ x 10⁴¹ erg s⁻¹, providing crucial insight into the energy transfer occurring within this system.

Results demonstrate that previous optical observations identified an extended emission line region aligned with the radio axis, illuminated by the central Active Galactic Nucleus (AGN). Within this region, scientists recorded a warm and ionized outflow with a mass outflow rate of approximately 50 M⊙yr⁻¹ and an energy outflow rate of 1.7x 10⁴³ erg s⁻¹. The study proposes that both the extended emission line region and the optical outflow are a synergistic effect of the jet and AGN radiation, suggesting a complex interplay between these energetic phenomena. Measurements confirm the AGN likely exerts negative feedback on the host galaxy, evidenced by the observed gas expulsion driven by the jet and the high velocity dispersion of ionized gas.

The breakthrough delivers a rare detection of a jet-driven neutral hydrogen outflow, a phenomenon previously sparsely observed. Tests prove the high redshift, large outflow radii, substantial mass outflow rate, and significant energy outflow rate of the neutral hydrogen outflow in 0731+438 expand the known parameter space of such outflows, offering a unique opportunity to study these processes at extreme distances. Data shows tentative correlations between the neutral hydrogen mass outflow rate (or energy outflow rate) and the rest-frame 1.4GHz radio power of the jets for the known broad neutral hydrogen outflows, hinting at a potential link between jet activity and outflow characteristics. Researchers established the uGMRT observations utilized a bandwidth of 25MHz centered on 414.3MHz, divided into 2048 channels, providing a velocity coverage of -9000 to 9000km s⁻¹ with a velocity resolution of approximately 8.8km s⁻¹. The observations, conducted on January 20, 2023, involved three scans of 38 minutes each, interspersed with scans of the gain calibrator 0702+445, ensuring precise calibration and data quality. Previous optical observations revealed an extended emission line region and an ionized outflow.