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

Abstract

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 R ₂₅, 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.