2016ApJ...818..144R

Emission from carbon monoxide (CO) is ubiquitously used as a tracer of dense star-forming molecular clouds. There is, however, growing evidence that a significant fraction of CO emission originates from diffuse molecular gas. Quantifying the contribution of diffuse CO-emitting gas is vital for understanding the relation between molecular gas and star formation. We examine the Galactic distribution of two CO-emitting gas components, a high column density component detected in ¹³CO and ¹²CO, and a low column density component detected in ¹²CO, but not in ¹³CO. The ''diffuse'' and ''dense'' components are identified using a combination of smoothing, masking, and erosion/dilation procedures, making use of three large-scale ¹²CO and ¹³CO surveys of the inner and outer Milky Way. The diffuse component, which globally represents 25% (1.5x10⁸M_☉) of the total molecular gas mass (6.5 x 10⁸ M_☉), is more extended perpendicular to the Galactic plane. The fraction of diffuse gas increases from ∼10%-20% at a galactocentric radius of 3-4 kpc to 50% at 15 kpc, and increases with decreasing surface density. In the inner Galaxy, a yet denser component traced by CS emission represents 14% of the total molecular gas mass traced by ¹²CO emission. Only 14% of the molecular gas mass traced by ¹²CO emission is identified as part of molecular clouds in ¹³CO surveys by cloud identification algorithms. This study indicates that CO emission not only traces star-forming clouds, but also a significant diffuse molecular ISM component.