Evidence for extended gaseous reservoirs around AGN at cosmic noon from ALMA CO(3-2) observations

TL;DR

This study uses ALMA CO(3-2) observations to detect extended molecular gas reservoirs around z~2 AGN host galaxies, suggesting past AGN-driven outflows influenced their evolution.

Contribution

It provides the first evidence of extended CO reservoirs around AGN at cosmic noon, linking molecular gas distribution to AGN feedback history.

Findings

Extended CO emission with radius ~13 kpc detected

No significant extended stellar or dust emission observed

Diffuse molecular gas likely results from past AGN-driven outflows

Abstract

Gaseous outflows are key phenomena in the evolution of galaxies, as they affect star formation (either positively or negatively), eject gas from the core or disk, and directly cause mixing of pristine and processed material. Active outflows may be detected through searches for broad spectral line emission or high-velocity gas, but it is also possible to determine the presence of past outflows by searching for extended reservoirs of chemically enriched molecular gas in the circumgalactic medium (CGM) around galaxies. In this work, we examine the CO(3-2) emission of a set of seven z~2.0-2.5 AGN host galaxies, as observed with ALMA. Through a three-dimensional stacking analysis we find evidence for extended CO emission of radius r~13kpc. We extend this analysis to the HST/ACS i-band images of the sample galaxies, finding a complex small-scale (r<10kpc) morphology but no robust evidence for extended emission. In addition, the dust emission (traced by rest-frame FIR emission) shows no evidence for significant spatial extension. This indicates that the diffuse CO emission revealed by ALMA is morphologically distinct from the stellar component, and thus traces an extended reservoir of enriched gas. The presence of a diffuse, enriched molecular reservoir around this sample of AGN host galaxies at cosmic noon hints at a history of AGN-driven outflows that likely had strong effects on the star formation history of these objects.