Comparing [C II] , HI, and CO Dynamics of Nearby Galaxies

The HI and CO components of the interstellar medium (ISM) are often used to derive the dynamical mass of nearby galaxies. Both components become too faint to be used as a tracer in observations of high-redshift galaxies. In those cases, the 158 μm line of atomic carbon ([C II]) may be the only way to derive this. As the distribution and kinematics of the ISM tracer affects the determination of the dynamical mass, it is important to quantify the relative distributions of HI, CO, and [C II]. We compare observations of CO, HI, and [C II] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS, and KINGFISH surveys. In addition, we use high-spectral-resolution SOFIA [C II] data of a number of star forming regions in two nearby galaxies, to compare their [C II] linewidths with those of CO and HI. Our results suggests that the relevant length-scale to use in determining a dynamical mass based on [C II] data, is that of the well-characterized CO distribution. This length scale is similar to that of the optical disk.