Gas-rich major mergers in high-redshift proto-clusters are important events , perhaps leading to the creation of the slowly rotating remnants seen in the cores of clusters in the present day .
Here , we present a deep Jansky Very Large Array observation of CO J = 1–0 emission line in a proto-cluster at z = 2.5 , USS1558-003 .
The target field is an extremely dense region , where 20 H \alpha emitters ( HAEs ) are clustering .
We have successfully detected the CO emission line from three HAEs and discovered a close pair of red and blue CO-emitting HAEs .
Given their close proximity ( \sim 30 kpc ) , small velocity offset ( \sim 300 km s ^ { -1 } ) , and similar stellar masses , they could be in the early phase of a gas-rich major merger .
For the red HAE , we derive a total infrared luminosity of L _ { \mathrm { IR } } = 5.1 \times 10 ^ { 12 } ~ { } L _ { \odot } using MIPS 24 \mu m and radio continuum images .
The L _ { \mathrm { IR } } / L ^ { \prime } _ { \mathrm { CO } } ratio is significantly enhanced compared to local spirals and high-redshift disks with a similar CO luminosity , which is indicative of a starburst mode .
We find the gas depletion timescale is shorter than that of normal star-forming galaxies regardless of adopted CO-H _ { 2 } conversion factors .
The identification of such a rare event suggests that gas-rich major mergers frequently take place in proto-clusters at z > 2 and may involve the formation processes of slow rotators seen in local massive clusters .