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