Misc,
Comparing CII, HI, and CO dynamics of nearby galaxies
(2016)cite arxiv:1604.08777Comment: Accepted for publication in the Astronomical Journal.
Abstract
The HI 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 $\mu$m line of atomic carbon CII 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 HI, CO and CII. HI and CO are well-characterised observationally, however,
for CII only very few measurements exist. Here we compare observations of CO,
HI, and CII emission of a sample of nearby galaxies, drawn from the HERACLES,
THINGS and KINGFISH surveys. We find that within R_25, the average CII
exponential radial profile is slightly shallower than that of the CO, but much
steeper than the HI distribution. This is also reflected in the integrated
spectrum ("global profile"), where the CII spectrum looks more like that of
the CO than that of the HI. 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
CII data. Using high-spectral-resolution SOFIA CII data of a number of star
forming regions in two nearby galaxies, we find that their CII linewidths
agree better with those of the CO than the HI. 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
CII data, is that of the well-characterised CO distribution. This length
scale is similar to that of the optical disk.
