Molecular Gas in a Submillimeter Galaxy at z = 4.5: Evidence for a Major Merger at 1 Billion Years after the Big Bang^*

We report the detection of CO molecular line emission in the z = 4.5 millimeter-detected galaxy COSMOS J100054+023436 (hereafter J1000+0234) using the IRAM Plateau de Bure interferometer (PdBI) and NRAO's Very Large Array (VLA). The ¹²CO(4-3) line as observed with PdBI has a full line width of ~1000 km s⁻¹, an integrated line flux of 0.66 Jy km s⁻¹, and a CO luminosity of 3.2 × 10¹⁰ L_☉. Comparison to the 3.3 σ detection of the CO(2-1) line emission with the VLA suggests that the molecular gas is likely thermalized to the J = 4–3 transition level. The corresponding molecular gas mass is 2.6 × 10¹⁰ M_☉ assuming an ULIRG-like conversion factor. From the spatial offset of the red- and blueshifted line peaks and the line width a dynamical mass of 1.1 × 10¹¹ M_☉ is estimated assuming a merging scenario. The molecular gas distribution coincides with the rest-frame optical and radio position of the object while being offset by 0.5'' from the previously detected Lyα emission. J1000+0234 exhibits very typical properties for lower redshift (z ∼ 2) submillimeter galaxies (SMGs) and thus is very likely one of the long sought after high-redshift (z > 4) objects of this population. The large CO(4-3) line width taken together with its highly disturbed rest-frame UV geometry suggest an ongoing major merger about a billion years after the big bang. Given its large star formation rate (SFR) of >1000 M_☉ yr⁻¹ and molecular gas content this object could be the precursor of a "red and dead" elliptical observed at a redshift of z = 2.