CO Line Width Differences in Early Universe Molecular Emission-Line Galaxies: Submillimeter Galaxies versus QSO Hosts

Abstract

We investigate the CO line widths in early universe molecular emission-line galaxies. There is a clear difference in the CO line widths in QSO host galaxies compared to submillimeter galaxies, as first pointed out by Greve et al., with the QSO hosts' line widths typically a factor of 2.3 narrower in velocity. Powerful radio galaxies fall in between. At the same time, there is no major difference between the molecular gas mass distributions of the different galaxy types. We consider possible explanations for the narrower line widths in the QSO hosts and favor an explanation that the QSO hosts are systematically closer to the sky plane than the submillimeter galaxies, QSO = 18°, consistent with the lack of strong obscuration toward the active galactic nucleus. However, we cannot rule out other possible factors, such as different stages in a merger sequence or galaxy mass or size. Only high-resolution imaging of a large sample of sources can address this interesting dichotomy.