Associated HI absorption and AGN feedback

TL;DR

This paper reviews recent findings on atomic neutral hydrogen (HI) absorption in galaxy centers, emphasizing AGN-driven outflows, their structures, and implications for galaxy evolution, highlighting advances enabled by new radio surveys.

Contribution

It provides a concise overview of recent discoveries in HI absorption related to AGN feedback, especially focusing on outflows and their role in galaxy evolution, supported by new observational capabilities.

Findings

Detection of fast, massive HI outflows in galaxy centers

Clumpy HI structures on parsec scales confirmed by observations

Numerical simulations agree with observed AGN-driven HI outflows

Abstract

The presence, distribution and kinematics of atomic neutral hydrogen in the central regions of galaxies can be traced by the HI 21~cm line observed in absorption. Depending only on the strength of the radio continuum, the associated absorption can trace the gas down to pc scale, which is ideal for exploring the HI in the nuclear regions of radio AGN. This paper gives a brief overview of the main recent findings with particular focus on the AGN-driven HI outflows. Absorption has made possible the discovery of fast and massive HI outflows and their clumpy structure on pc scales. The similarities with the predictions of numerical simulations confirm the impact of young radio jets in the feedback cycle and galaxy evolution. The field of HI absorption is rapidly expanding thanks to new ``blind" surveys and the increased spectral capabilities of the radio telescopes. This opens many possibilities for future discoveries and complements the studies of HI emission.