CHIMPS: Physical properties of molecular clumps across the inner Galaxy

The latest generation of high-angular-resolution unbiased Galactic plane surveys in molecular-gas tracers are enabling the interiors of molecular clouds to be studied across a range of environments. The CO Heterodyne Inner Milky Way Plane Survey (CHIMPS) simultaneously mapped a sector of the inner Galactic plane, within 27.8\(^\text{o}\) \(\lesssim\) \(\ell\) \(\lesssim\) 46.2\(^\text{o}\) and |\(\textit{b}\)| ≤ 0.5\(^{\rm o}\), in \({^1}{^3}\)CO (3–2) and C\({^1}{^8}\)O (3–2) at an angular resolution of 15 arcsec. The combination of the CHIMPS data with \({^1}{^2}\)CO (3–2) data from the CO High Resolution Survey (COHRS) has enabled us to perform a voxel-by-voxel local-thermodynamic-equilibrium (LTE) analysis, determining the excitation temperature, optical depth, and column density of \({^1}{^3}\)CO at each \(\ell\), \(\textit{b}\), \(\textit{v}\) position. Distances to discrete sources identified by FellWalker in the \({^1}{^3}\)CO (3–2) emission maps were determined, allowing the calculation of numerous physical properties of the sources, and we present the first source catalogues in this paper. We find that, in terms of size and density, the CHIMPS sources represent an intermediate population between large-scale molecular clouds identified by CO and dense clumps seen in thermal dust continuum emission, and therefore represent the bulk transition from the diffuse to the dense phase of molecular gas. We do not find any significant systematic variations in the masses, column densities, virial parameters, mean excitation temperature, or the turbulent pressure over the range of Galactocentric distance probed, but we do find a shallow increase in the mean volume density with increasing Galactocentric distance. We find that inter-arm clumps have significantly narrower linewidths, and lower virial parameters and excitation temperatures than clumps located in spiral arms. When considering the most reliable distance-limited subsamples, the largest variations occur on the clump-to-clump scale, echoing similar recent studies that suggest that the star-forming process is largely insensitive to the Galactic-scale environment, at least within the inner disc.