SMM J04135+10277: A Candidate Early-Stage "Wet-Dry" Merger of Two Massive Galaxies at z=2.8

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We report interferometric imaging of CO(J=3-2) emission toward the z=2.846 submillimeter-selected galaxy SMM J04135+10277, using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). SMM J04135+10277 was previously thought to be a gas-rich, submillimeter-selected quasar, with the highest molecular gas mass among high-z quasars reported in the literature. Our maps at 6x improved linear resolution relative to earlier observations spatially resolve the emission on 1.7" scales, corresponding to a (lensing-corrected) source radius of 5.2 kpc. They also reveal that the molecular gas reservoir, and thus, likely the submillimeter emission, is not associated with the host galaxy of the quasar, but with an optically faint gas-rich galaxy at 5.2", or 41.5 kpc projected distance from the active galactic nucleus (AGN). The obscured gas-rich galaxy has a dynamical mass of Mdyn sin2(i)=5.6x10¹1 Mₛun, corresponding to a gas mass fraction of 21%. Assuming a typical MBH/M* ratio for z>2 quasars, the two galaxies in this system have an approximate mass ratio of 1.9. Our findings suggest that this quasar-starburst galaxy pair could represent an early stage of a rare major, gas-rich/gas-poor ("wet-dry") merger of two massive galaxies at z=2.8, rather than a single, gas-rich AGN host galaxy. Such systems could play an important role in the early buildup of present-day massive galaxies through a submillimeter-luminous starburst phase, and may remain hidden in larger numbers among rest-frame far-infrared-selected quasar samples at low and high redshift.