2016AJ....152...51D

The H i and CO components of the interstellar medium (ISM) are usually used to derive the dynamical mass {M}_{dyn} 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 {M}_{dyn} . As the distribution and kinematics of the ISM tracer affects the determination of {M}_{dyn} , it is important to quantify the relative distributions of H i, CO, and [C ii]. H i and CO are well-characterized observationally, however, for [C ii] only very few measurements exist. Here we compare observations of CO, H i, and [C ii] emission of a sample of nearby galaxies, drawn from the HERACLES, THINGS, and KINGFISH surveys. We find that within R₂₅, the average [C ii] exponential radial profile is slightly shallower than that of the CO, but much steeper than the H i distribution. This is also reflected in the integrated spectrum ("global profile"), where the [C ii] spectrum looks more like that of the CO than that of the H i. For one galaxy, a spectrally resolved comparison of integrated spectra was possible; other comparisons were limited by the intrinsic line-widths of the galaxies and the coarse velocity resolution of the [C ii] data. Using high-spectral-resolution SOFIA [C ii] data of a number of star forming regions in two nearby galaxies, we find that their [C ii] linewidths agree better with those of the CO than the H i. As the radial extent of a given ISM tracer is a key input in deriving {M}_{dyn} from spatially unresolved data, we conclude that the relevant length-scale to use in determining {M}_{dyn} based on [C ii] data, is that of the well-characterized CO distribution. This length scale is similar to that of the optical disk.