2014A&A...562A..35N

New high-resolution maps with the IRAM interferometer of the redshifted C⁺ 158µm line and the 0.98 mm dust continuum of HDF 850.1 at z=5.185 show the source to have a blueshifted northern component and a redshifted southern component, with a projected separation of 0.3'', or 2kpc. We interpret these components as primordial galaxies that are merging to form a larger galaxy. We think it is the resulting merger-driven starburst that makes HDF 850.1 an ultraluminous infrared galaxy, with L_IR∼10¹³L_☉. The observed line and continuum brightness temperatures and the constant line-to-continuum ratio across the source imply (1) high C⁺ line optical depth, (2) a C⁺ excitation temperature of the same order as the dust temperature, and (3) dust continuum emission that is nearly optically thick at 158µm. These conclusions for HDF 850.1 probably also apply to other high-redshift submillimeter galaxies and quasar hosts in which the C⁺ 158µm line has been detected, as indicated by their roughly constant C⁺-to-158µm continuum ratios, in sharp contrast to the large dispersion in their C⁺-to-FIR luminosity ratios. In brightness temperature units, the C⁺ line luminosity is about the same as the predicted CO(1-0) luminosity, implying that the C⁺ line can also be used to estimate the molecular gas mass, with the same assumptions as for CO.