Chaotic Molecular Gas in Five Dusty Star-forming Galaxies in the Spiderweb Protocluster at $z = 2.16$

TL;DR

This study uses ALMA observations to analyze molecular gas properties in five dusty star-forming galaxies within the Spiderweb protocluster at z=2.16, revealing disturbed kinematics and environmental effects on gas content and star formation.

Contribution

First detailed ALMA analysis of molecular gas in multiple galaxies within a high-redshift protocluster, highlighting disturbed kinematics and environmental influences.

Findings

All galaxies show disturbed kinematics and irregular morphology.

Gas fractions are similar to field galaxies at cosmic noon.

Gas fractions and star formation rates decline with distance from the protocluster core.

Abstract

Measuring the properties of cold molecular gas available for intense star formation in galaxy protoclusters at $z>2$ is a crucial step in understanding large scale structure formation. We present ALMA observations of CO(3$-$2) in five dusty star-forming galaxies within $\sim0.5-4$ cMpc of the core of the Spiderweb protocluster at $z=2.16$ to measure the molecular gas mass and kinematics in the most starbursting members of the protocluster. All five galaxies exhibit evidence for disturbed kinematics including non-Gaussian CO line profiles, irregular spatial morphology, and strong residuals when fitting the galaxies with a classical disk model. This could be indicative of an elevated merger rate in the outskirts of the mature Spiderweb protocluster, as all of the galaxies in our sample have multiple companions detected in H$\alpha$. Both the gas fractions and the gas depletion timescales of the galaxies are similar to field relations at cosmic noon, indicative of the fact that their prodigious star formation rates are compensated by similarly high gas masses. The most massive galaxies, as well as all of the galaxies identified as X-ray AGN in previous works, have gas fractions $<30$%, compared to the sample average of 49%, indicating declining availability of gas for star formation. Finally, we find that the gas fractions and specific star formation rates decline with distance from the Spiderweb Galaxy, supporting the reversal of the SFR density--radius relation in high-redshift protoclusters.