CARMA Survey toward Infrared-bright Nearby Galaxies (STING). IV. Spatially Resolved 13 CO in Spiral Galaxies

Abstract

We present a mapping survey of 12 nearby galaxies from the CARMA STING sample. The line intensity ratio is derived to study the variations in molecular gas properties. For 11 galaxies where it can be measured with high significance, the spatially resolved on (sub)kiloparsec scales varies by up to a factor of 3–5 within a galaxy. Lower values are usually found in regions with weaker . We attribute this apparent trend to a bias against measuring large values when is weak. Limiting our analysis to the -bright regions that are less biased, we do not find that on (sub)kiloparsec scales correlate with galactocentric distance, velocity dispersion, or the star formation rate. The lack of correlation between star formation rate and  indicates that the CO optical depth is not sensitive to stellar energy input, or that any such sensitivity is easily masked by other factors. Extending the analysis to all regions with  emission by spectral stacking, we find that 5 out of 11 galaxies show higher stacked  for galactocentric radii of and yr −1 kpc −2 , which could result from a greater contribution from diffuse gas. Moreover, significant galaxy-to-galaxy variations are found in , but the global does not strongly depend on dust temperature, inclination, or metallicity of the galaxy.