# Confirming Herschel candidate protoclusters from ALMA/VLA CO observations

## TL;DR

This study confirms that multiple dusty star-forming galaxies detected by ALMA are physically associated within the same protocluster structures at high redshift, revealing new gas-rich protocluster cores and insights into their evolutionary stages.

## Contribution

First confirmation that multiple DSFGs are part of the same protocluster structures using CO line observations, discovering two new gas-rich protocluster cores at high redshift.

## Key findings

- Two new protocluster cores identified at z=2.171 and z=2.602.
- Confirmed physical association of multiple DSFGs within the same structures.
- Sources have high molecular gas fractions and low stellar masses, possibly indicating early galaxy formation stages.

## Abstract

ALMA 870$\mu$m continuum imaging has uncovered a population of blends of multiple dusty star-forming galaxies (DSFGs) in sources originally detected with the Herschel Space Observatory. However, their pairwise separations are much smaller that what is found by ALMA follow-up of other single-dish surveys or expected from theoretical simulations. Using ALMA and VLA, we have targeted three of these systems to confirm whether the multiple 870$\mu$m continuum sources lie at the same redshift, successfully detecting $^{12}$CO($J = 3$-2) and $^{12}$CO($J = 1$-0) lines and being able to confirm that in the three cases all the multiple DSFGs are likely physically associated within the same structure. Therefore, we report the discovery of two new gas-rich dusty protocluster cores (HELAISS02, $z = 2.171 \pm 0.004$; HXMM20, $z = 2.602 \pm 0.002$). The third target is located in the well known COSMOS overdensity at $z = 2.51$ (named CL J1001+0220 in the literature), for which we do not find any new secure CO(1-0) detection, although some of its members show only tentative detections and require further confirmation. From the gas, dust, and stellar properties of the two new protocluster cores, we find very large molecular gas fractions yet low stellar masses, pushing the sources above the main sequence, while not enhancing their star formation efficiency. We suggest that the sources might be newly formed galaxies migrating to the main sequence. The properties of the three systems compared to each other and to field galaxies may suggest a different evolutionary stage between systems.