Spatially resolved millimeter interferometry of SMM J02399-0136:: A very massive galaxy at z=2.8

We report high-resolution millimeter mapping with the IRAM Plateau de Bure interferometer of rest-frame 335 mum continuum and CO (3-2) line emission from the z = 2.8 submillimeter galaxy SMM J02399-0136. The continuum emission comes from a similar to3 diameter structure whose elongation is approximately east-west and whose centroid is coincident within the astrometric errors with the brightest X-ray and rest-UV peak ( L1). The line data show that this structure is most likely a rapidly rotating disk. Its rotation velocity of greater than or equal to420 km s(-1) implies a total dynamical mass of greater than or equal to3 x 10(11) sin(-2) i h(0.7)(-1) M-. within an intrinsic radius of 8 h(0.7)(-1) kpc, most of which is plausibly in the form of stars and gas. SMM J02399-0136 is thus a very massive system, whose formation at zsimilar to3 is not easy to understand in current cold dark matter hierarchical merger cosmogonies.