Smm J04135+10277: a Candidate Early–stage “wet–dry” Merger of Two Massive Galaxies at Z=2.8

We report interferometric imaging of CO(J=3→2) emission toward the z=2.846 submillimeterselected galaxy SMMJ04135+10277, using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). SMMJ04135+10277 was previously thought to be a gas-rich, submillimeterselected quasar, with the highest molecular gas mass among high-z quasars reported in the literature. Our maps at ∼6× 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 sin i=5.6×10M⊙, 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. Subject headings: galaxies: active — galaxies: starburst — galaxies: formation — galaxies: highredshift — cosmology: observations — radio lines: galaxies