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

TL;DR

This study uses VLBA observations to reveal the structure, proper motion, and cold gas environment of the AGN in galaxy group NGC 5044, shedding light on AGN fueling and jet evolution at parsec scales.

Contribution

It provides the first high-resolution measurements of jet proper motion, collimation transition, and cold gas coexistence near the AGN in NGC 5044, advancing understanding of AGN feeding mechanisms.

Findings

Jet components expand at 0.10c, with a dynamical age of ~180 years.

Detected cold atomic gas within 10-20 pc of the AGN, coexisting with molecular gas.

Jet collimation transitions from parabolic to conical at a few ×10^4 Schwarzschild radii.

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.