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This paper presents ground-based 12CO and 13CO measurements of 126 nearbygalaxy centers in various J transitions. More than 60 galaxies were measured inat least four lines. The average relative intensities of the first four 12COtransitions are 1.00 : 0.92 : 0.70 : 0.57. The average 12CO-to-13CO ratios are13.0, 11.6, and 12.8 for the first three transitiions. The sizes of central COconcentrations are well defined in maps, but poorly determined bymulti-aperture photometry. Using radiative transfer models (RADEX), we derivedmodel gas parameters, where the assumed carbon elemental abundances and carbongas depletion onto dust are the main causes of uncertainty. The new CO data andpublished [CI] and [CII] data imply that CO, Co, and C+ each represent aboutone-third of the gas-phase carbon in the molecular interstellar medium ingalaxy centers Their mean beam-averaged central molecular hydrogen columndensity is N(H2) = (1.5+/-0.2) x 10^(21) cm^(2) and CO-to-H2 conversion factorsare typically ten times lower than the `standard' Milky Way disk value, with amean X(CO) = (1.9+/-0.2) x 10^(19) cm^(2)/K km/s. The corresponding [CI]-to-H2factor is five times higher than X(CO), with X[CI] = (9+/-2) x 10^(19) cm^(2)/Kkm/s. No unique conversion factor can be determined for [CII]. The lowmolecular gas content of galaxy centers relative to their CO intensities isexplained in roughly equal parts by higher central gas-phase carbon abundances,elevated gas temperatures, and larger gas velocity dispersions than found ingalaxy disks.

Central molecular zones in galaxies: 12CO-to-13CO ratios, carbon budget, and X factors