Central molecular zones in galaxies: 12CO-to-13CO ratios, carbon budget,
  and X factors
  This paper presents ground-based 12CO and 13CO measurements of 126 nearby
galaxy centers in various J transitions. More than 60 galaxies were measured in
at least four lines. The average relative intensities of the first four 12CO
transitions are 1.00 : 0.92 : 0.70 : 0.57. The average 12CO-to-13CO ratios are
13.0, 11.6, and 12.8 for the first three transitiions. The sizes of central CO
concentrations are well defined in maps, but poorly determined by
multi-aperture photometry. Using radiative transfer models (RADEX), we derived
model gas parameters, where 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, Co, and C+ each represent about
one-third of the gas-phase carbon in the molecular interstellar medium in
galaxy centers Their mean beam-averaged central molecular hydrogen column
density is N(H2) = (1.5+/-0.2) x 10^(21) cm^(2) and CO-to-H2 conversion factors
are typically ten times lower than the `standard' Milky Way disk value, with a
mean X(CO) = (1.9+/-0.2) x 10^(19) cm^(2)/K km/s. The corresponding [CI]-to-H2
factor is five times higher than X(CO), with X[CI] = (9+/-2) x 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 higher central gas-phase carbon abundances,
elevated gas temperatures, and larger gas velocity dispersions than found in
galaxy disks.
