Giant galaxy growing from recycled gas: ALMA maps the circumgalactic molecular medium of the Spiderweb in [CI]

TL;DR

This study uses ALMA and ATCA observations to map the molecular gas in the circumgalactic medium of the Spiderweb Galaxy at z=2.2, revealing a metal-rich, recycled gas reservoir fueling galaxy growth.

Contribution

First detailed mapping of the molecular CGM in the Spiderweb Galaxy showing recycled gas as a growth mechanism, contrasting direct cosmic web accretion.

Findings

Molecular gas extends over ~50 kpc in the CGM.

The CGM's molecular properties resemble those of star-forming galaxies.

The galaxy's core shows signs of non-star-formation-related heating.

Abstract

The circumgalactic medium (CGM) of the massive Spiderweb Galaxy, a conglomerate of merging proto-cluster galaxies at z=2.2, forms an enriched interface where feedback and recycling act on accreted gas. This is shown by observations of [CI], CO(1-0) and CO(4-3) performed with the Atacama Large Millimeter Array (ALMA) and Australia Telescope Compact Array (ATCA). [CI] and CO(4-3) are detected across ~50 kpc, following the distribution of previously detected low-surface-brightness CO(1-0) across the CGM. This confirms our previous results on the presence of a cold molecular halo. The central radio galaxy MRC1138-262 shows a very high global $L'_{\rm CO(4-3)}$/$L'_{\rm CO(1-0)}$ ~ 1, suggesting that mechanisms other than FUV-heating by star formation prevail at the heart of the Spiderweb Galaxy. Contrary, the CGM has $L'_{\rm CO(4-3)}$/$L'_{\rm CO(1-0)}$ and $L'_{\rm [CI]}$/$L'_{\rm CO(1-0)}$ similar to the ISM of five galaxies in the wider proto-cluster, and its carbon abundance, $X_{\rm [CI]}$/$X_{\rm H2}$, resembles that of the Milky Way and starforming galaxies. The molecular CGM is thus metal-rich and not diffuse, confirming a link between the cold gas and in-situ star formation. Thus, the Spiderweb Galaxy grows not directly through accretion of gas from the cosmic web, but from recycled gas in the GCM.