Science Highlight - Spatially Measured CO(2-1)/CO(1-0) Ratios in the Galaxy M83

Figure 1.  (a) A map of R_21/10 with CO(1-0) contours. (b) Far-UV map with R_21/10 contours. (c) 70μm-to-350μm emission map with R_21/10 contours.

While the rotational transition of CO(1-0) is commonly used to derive the bulk molecular gas properties (e.g., the total star-forming molecular gas mass) of galaxies, observations of higher CO transitions are gaining popularity due to their brightness and thus the lower investment in telescope time relative to CO(1-0). In a recent paper, Dr. Koda and his collaborators make use of the Total Power Array of ALMA to map out the distribution of CO(1-0) and CO(2-1) in the spiral galaxy M83. They measure the CO(2-1)/CO(1-0) ratio, R_21/10, of M83, and compare it with beam-matched intensity maps of GALEX far-UV and Herschel infrared emission, as well as the ratio of 70μm-to-350μm emission (a proxy for dust temperature). The authors find that R_21/10 > 0.7 in regions with high molecular gas surface density, and that R_21/10 in excess of 0.55 correlates well with the far-UV intensity (r = 0.82) and the dust temperature (r = 0.83).  They speculate that the correlations may be due to a thermal coupling between the UV-heated dust and the gas. No clear correlation was found for lower values of R_21/10.