2020A&A...635A.131I

We present ground-based measurements of 126 nearby galaxy centers in ¹²CO and 92 in ¹³CO in various low-J transitions. More than 60 galaxies were measured in at least four lines. The average relative intensities of the first four J ¹²CO transitions are 1.00:0.92:0.70:0.57. In the first three J transitions, the average ¹²CO-to-¹³CO intensity ratios are 13.0, 11.6, and 12.8, with individual values in any transition ranging from 5 to 25. The sizes of central CO concentrations are well defined in maps, but poorly determined by multi-aperture photometry. On average, the J=1-0 ¹²CO fluxes increase linearly with the size of the observing beam. 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(H₂)=(1.5±0.2)x10²¹cm^–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)x10¹⁹cm^–2/(K.km/s) and a dispersion 1.7. The corresponding [CI]-H₂ factor is five times higher than X(CO), with X[CI]=(9±2)x10¹⁹cm^–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.