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

One of the most prominent features of galaxy clusters is the presence of a dominant population of massive ellipticals in their cores. While stellar archaeology suggests that these massive beasts assembled most of their stars in the early Universe via intense starbursts, as what has been observed at high redshift, detailed physical mechanisms and the role of dense environment in triggering the starburst activities remain unknown. Here we report 0.3-arcsec Atacama Large Millimeter/submillimeter Array (ALMA) observations of the molecular gas towards a forming galaxy cluster core with intense starburst galaxies at z=2.51. In contrast to starburst galaxies in the field often associated with mergers or gas disks with high velocity dispersion, our observations show that the two starbursts in the cluster exhibit exclusively cold disks with extremely low velocity dispersion with ∼ 20 − 30 km s−1 that is 3 times lower than their field counterparts. The high gas fraction and suppressed velocity dispersion suggest gravitationally unstable gas disks, which enable highly efficient star formation. Together with the elevated rotational velocities of the two starbursts, we argue that suppressed velocity dispersion induced likely from the accretion of co-rotating cold gas serves as an important avenue in triggering starbursts in massive halos at high redshift