Molecular Line Ratio Diagnostics Along the Radial Cut and Dusty UV-bright Clumps in a Spiral Galaxy NGC 0628

Abstract: Molecular emission lines are essential tools to shed lights on many questions regarding star formation in galaxies. Multiple molecular lines are particularly useful to probe different phases of star-forming molecular clouds. In this study, we investigate the physical properties of giant molecular clouds (GMCs) using multiple lines of CO, i.e. CO(1-0, 2-1, 3-2) and $^{13}$CO(1-0), obtained at selected 20 positions in the disc of NGC 0628. Eleven positions were selected over the radial cut, including the centre, and remaining nine positions were selected across the southern and northern arms of the galaxy. 13 out of 20 positions are brighter at 24micron and ultraviolet (UV) emission and hosting significantly more HII regions compared to the rest of the positions indicating opposite characteristics. Our line ratio analysis shows that the gas gets warmer and thinner as a function of radius from the galaxy centre up to 1.7 kpc, and then the ratios start to fluctuate. Our empirical and model results suggest that the UV-bright positions have colder and thinner CO gas with higher hydrogen and CO column densities. However, the UV-dim positions have relatively warmer CO gas with lower densities bathed in GMCs surrounded by less number of HII regions. Analysis of multi-wavelength infrared and UV data indicates that the UV-bright positions have higher star formation efficiency than that of the UV-dim positions.