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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 Lsolar. 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 Msolar 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 Msolar 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 Msolar 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.

Galaxies: Formation; Galaxies: High-Redshift; Galaxies: ISM; Galaxies: Starburst; Submillimeter

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