Three Lyman-alpha emitting filaments converging to a massive galaxy group at z=2.91: discussing the case for cold gas infall

TL;DR

This study reports the discovery of a giant Lyman-alpha nebula with converging cold gas filaments around a massive galaxy group at z=2.91, supporting the cold gas infall scenario in galaxy formation.

Contribution

It provides observational evidence of cold gas filaments feeding a massive, star-forming galaxy group at high redshift, testing models of galaxy accretion.

Findings

Detection of three cold gas filaments converging into a galaxy group

Kinematic evidence consistent with gas accretion rather than outflows

Supports gravity-driven cold gas inflow as a key galaxy feeding mechanism

Abstract

We have discovered a 300kpc-wide giant Lya nebula centered on the massive galaxy group RO-1001 at z=2.91 in the COSMOS field. Keck Cosmic Web Imager observations reveal three cold gas filaments converging into the center of the potential well of its ~4x10^13Msun dark matter halo, hosting 1200Msun/yr of star formation as probed by ALMA and NOEMA observations. The nebula morphological and kinematics properties and the prevalence of blueshifted components in the Lya spectra are consistent with a scenario of gas accretion. The upper limits on AGN activity and overall energetics favor gravity as the primary Lya powering source and infall as the main source of gas flows to the system. Although interpretational difficulties remain, with outflows and likely also photoionization with ensuing recombination still playing a role, this finding provides arguably an ideal environment to quantitatively test models of cold gas accretion and galaxy feeding inside an actively star-forming massive halo at high redshift.