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

Abstract

We report the detection of CO molecular line emission in the z=4.5 millimeter-detected galaxy COSMOS_J100054+023436 (hereafter: J100+0234) using the IRAM Plateau de Bure interferometer (PdBI) and NRAO's Very Large Array (VLA). The CO(4-3) line as observed with PdBI has a full line width of ~1000 km/s, an integrated line flux of 0.66 Jy km/s, and a CO luminosity of 3.2e10 L_sun. Comparison to the 3.3sigma 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 2.6e10 M_sun assuming an ULIRG-like conversion factor. From the spatial offset of the red- and blue-shifted line peaks and the line width a dynamical mass of 1.1e11 M_sun 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 Ly$α$ emission. J1000+0234 exhibits very typical properties for lower redshift (z~2) sub-millimeter 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_sun/yr and molecular gas content this object could be the precursor of a 'red-and-dead' elliptical observed at a redshift of z=2.