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

Abstract

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 ~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 M _dyn sin² i = 5.6 × 10¹¹ M _⊙, corresponding to a gas mass fraction of sime21%. Assuming a typical M _BH/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.