12CO/13CO ratio in 126 nearby galaxy centers

CO emission covers only a quarter of the HI galaxy disks. Using radiative transfer models (RADEX), we derived model gas parameters. The assumed carbon elemental abundances and carbon gas depletion onto dust are the main causes of uncertainty.

The new CO data and published [CI] and [CII] data imply that CO, C, and C^+^ each represent about one-third of the gas-phase carbon in the molecular interstellar medium. The mean beam-averaged molecular hydrogen column density is N(H_2_)=(1.5+/-0.2)10^21^cm^-2^. Galaxy center CO-to- H2 conversion factors are typically ten times lower than the 'standard' Milky Way X disk value, with a mean X(CO)=(1.9+/-0.2)10^19^cm^-2^/(K.km/s) and a dispersion 1.7.

The corresponding [CI]-H_2_ factor is five times higher than X(CO), with X[CI]=(9+/-2)10^19^cm^-2^/(K.km/s). No unique conversion factor can be determined for [CII]. The low molecular gas content of galaxy centers relative to their CO intensities is explained in roughly equal parts by high central gas-phase carbon abundances, elevated gas temperatures, and large gas velocity dispersions relative to the corresponding values in galaxy disks.