Starbursts with suppressed velocity dispersion revealed in a forming cluster at z=2.51

TL;DR

This study reveals that starburst galaxies within a forming cluster at z=2.51 have dynamically cold, rotation-dominated gas disks with low velocity dispersion, facilitating efficient star formation, contrasting with field starbursts often linked to turbulence or mergers.

Contribution

First spatially resolved ALMA observations of starburst galaxies in a forming cluster at high redshift showing suppressed velocity dispersion and rotation-dominated gas disks.

Findings

Starburst galaxies in the cluster have low velocity dispersion (~20-30 km/s).

Gas disks are rotation-dominated and gravitationally unstable.

Suppressed velocity dispersion may be induced by cold gas accretion.

Abstract

One of the most prominent features of galaxy clusters is the presence of a dominant population of massive ellipticals in their cores. Stellar archaeology suggests that these gigantic beasts assembled most of their stars in the early Universe via starbursts. However, the role of dense environments and their detailed physical mechanisms in triggering starburst activities remain unknown. Here we report spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of the CO $J= 3-2$ emission line, with a resolution of about 2.5 kiloparsecs, toward a forming galaxy cluster core with starburst galaxies at $z=2.51$. In contrast to starburst galaxies in the field often associated with galaxy mergers or highly turbulent gaseous disks, our observations show that the two starbursts in the cluster exhibit dynamically cold (rotation-dominated) gas-rich disks. Their gas disks have extremely low velocity dispersion ($\sigma_{\mathrm{0}} \sim 20-30$ km s$^{-1}$), which is three times lower than their field counterparts at similar redshifts. The high gas fraction and suppressed velocity dispersion yield gravitationally unstable gas disks, which enables highly efficient star formation. The suppressed velocity dispersion, likely induced by the accretion of corotating and coplanar cold gas, might serve as an essential avenue to trigger starbursts in massive halos at high redshifts.