J 02399 - 0136 : a Very Massive Galaxy at z = 2 . 8 1

We report high-resolution millimeter mapping with the IRAM Plateau de Bure interferometer of rest-frame 335μm continuum and CO(3–2) line emission from the z = 2.8 submillimeter galaxy SMMJ02399-0136. The continuum emission comes from a ∼ 3 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 ≥ 420 km s implies a total dynamical mass of ≥ 3× 10 sin i h 0.7M⊙ within an intrinsic radius of 8 h 0.7 kpc, most of which is plausibly in the form of stars and gas. SMMJ023990136 is thus a very massive system, whose formation at z ∼ 3 is not easy to understand in current CDM hierarchical merger cosmogonies. Subject headings: galaxies: formation, kinematics and dynamics, active, starburst; cosmology: observations