Stellar archaeology shows that massive elliptical galaxies formed
rapidly about ten billion years ago with star-formation rates of above
several hundred solar masses per year. Their progenitors are probably
the submillimetre bright galaxies at redshifts z greater than 2.
Although the mean molecular gas mass (5 × 1010 solar
masses) of the submillimetre bright galaxies can explain the formation
of typical elliptical galaxies, it is inadequate to form elliptical
galaxies that already have stellar masses above 2 ×
1011 solar masses at z ~ 2. Here we report multi-wavelength
high-resolution observations of a rare merger of two massive
submillimetre bright galaxies at z = 2.3. The system is seen to be
forming stars at a rate of 2,000 solar masses per year. The
star-formation efficiency is an order of magnitude greater than that of
normal galaxies, so the gas reservoir will be exhausted and star
formation will be quenched in only around 200 million years. At a
projected separation of 19 kiloparsecs, the two massive starbursts are
about to merge and form a passive elliptical galaxy with a stellar mass
of about 4 × 1011 solar masses. We conclude that
gas-rich major galaxy mergers with intense star formation can form the
most massive elliptical galaxies by z ~ 1.5.