  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 (~30 kpc), small velocity offset (~300 km/s),
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(IR)=5.1e12 Lsun using MIPS 24 um and radio continuum images. The L(IR)/L'(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-H2 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.
