AGN Fueling and Radio Jet Evolution in the Galaxy Group NGC 5044 revealed by VLBA HI Absorption and Proper-Motion Radio Observations

Abstract

The role of cooling gas in triggering active galactic nucleus (AGN) feedback in the centers of galaxy groups and clusters remains a key open question. NGC 5044, the X-ray brightest galaxy group, hosts the largest known reservoir of molecular gas among cool-core groups and exhibits multiple AGN outbursts, making it an ideal system to study AGN feeding. We present new multi-frequency Very Long Baseline Array (VLBA) observations of NGC 5044 at 1.4 GHz, 4.9 GHz, and 8.4 GHz, combining continuum imaging with HI spectroscopy. At 1.4 GHz, we recovered the previously known symmetric northeast-southwest jets extending for 5.5 pc each, along with evidence for previously undetected, more extended faint emission aligned with the older, kpc-scale outbursts. Comparison of 4.9 GHz and 8.4 GHz data from 2020 and 2024 reveals clear outward proper motion of jet components, yielding an average expansion speed of $(0.10\pm0.02)\,c$ and implying a dynamical age of $\sim$180 yr for the ejection of the parsec-scale jet components. The jet width profile suggests a transition from parabolic to conical collimation at a few $\times 10^{4}$ Schwarzschild radii. We detect a narrow, redshifted HI absorption line at $+264\,\mathrm{km\,s^{-1}}$ against the VLBA core, tracing a compact, cold atomic cloud within $\sim$10-20 pc of the AGN. The close velocity correspondence with previously detected CO and HI absorption features in ALMA and MeerKAT data, respectively, demonstrates that cold atomic and molecular gas coexists in infalling clouds at parsec scales. Overall, these results provide an unprecedented high angular resolution view of AGN cycling, jet growth, and feeding in a galaxy group environment.

Figures (9)

• Figure 1: Left panels: VLBA L band continuum images of the radio core of NGC 5044 obtained with Briggs weighting with robust parameter R=-1 (top), R=+2 (bottom). White contours start at 3$\sigma_{rms}$ and increase by a factor of 2. In the top panel, black contours trace the 4.9 GHz emission seen in VLBA images from 2020 schellenberger2021. Main background panel:Chandra X-ray residual image of NGC 5044 from ubertosi2024a, showing the hot IGrM. We overlay in black the dashed ellipses corresponding to the AGN-driven bubbles in the gas (see david2017schellenberger2021ubertosi2024arajpurohit2025). We also overlay in white the contours from the uGMRT Band 3 image at $10"$ resolution of NGC 5044 presented in rajpurohit2025. The contours are drawn at 3, 6, 12...$\times\sigma_{rms}$, where $\sigma_{rms}=21\mu$Jy beam$^{-1}$ is the rms noise of the radio image. The central red circle shows the position of the AGN.
• Figure 2: Total 1.4 GHz flux density as a function of Briggs weighting (and optional tapering or (u,v)-selection) used during imaging, plotted as a function of the largest linear size of the 5$\sigma_{rms}$ contour in the final image. The flux density saturates at $\sim$23 mJy for R $\geq+$0.5, suggesting that the largest linear size of the jets in NGC 5044 is $\sim$11 pc.
• Figure 3: Atomic HI absorption against the radio core in NGC 5044. Left panel: moment 0 map obtained by integrating the signal between $v = 255$ km s$^{-1}$ and $v = 275$ km s$^{-1}$ in the R$=+0.5$, $\delta v = 1.66$ km s$^{-1}$ cube, showing a $\sim5\sigma$ (white contour) absorption feature coincident with the radio core of NGC 5044. Black contours trace the continuum emission from the image shown in Figure \ref{['fig:continuum']} (top left), while the blue ellipse shows the beam-sized region used to extract the spectra. Right: HI spectra against the radio core of NGC 5044 extracted from three data cubes, see Table \ref{['tab:datacube']} for imaging details. The narrow HI absorption lines, identified as significant and discussed in the text, are labeled. We note that the three spectra were extracted from the same data but imaged using different channel resolution or robustness.
• Figure 4: HI absorption against the radio core of NGC 5044. Central panel: black contours trace the continuum emission from the image shown in Figure \ref{['fig:continuum']} (top left), while the ellipses show the beam-sized region used to extract the spectra. The beam is shown in the bottom-left corner. Left, Top, Right panels: HI spectra against the northeast jet, the core, and the southwest jet of NGC 5044, respectively, extracted from the data cube obtained by setting R$=+0.5$ during imaging and with a channel width $\delta v = 1.66$ km s$^{-1}$ (see Table \ref{['tab:datacube']}). The narrow HI absorption lines, identified as significant and discussed in the text, are labeled.
• Figure 5: Atomic and molecular gas absorption features against the radio core in NGC 5044. We show the spectra of HI from VLBA data (blue; from Figure \ref{['fig:HIcore']}, right panel, top spectrum) and MeerKAT data (orange and scaled by a factor of $\times$5; rajpurohit2025), and of CO(2-1) from ALMA data (gray; schellenberger2020). The spectra are zoomed on the velocity range of the redshifted atomic and molecular clouds seen in absorption.