The Carbon-to-H2, CO-to-H2 conversion factors, and carbon abundance on kiloparsec scales in nearby galaxies

Using the atomic carbon [C I] ($^{3} \rm P_{1} \rightarrow {\rm ^3 P}_{0}$) and [C I] ($^{3} \rm P_{2} \rightarrow {\rm ^3 P}_{1}$) emission {hereafter [C I] (1 - 0) and [C I] (2 - 1), respectively} maps observed with the Herschel Space Observatory, and CO (1 - 0), H I, infrared and submm maps from literatures, we estimate the [C I]-to-H₂ and CO-to-H₂ conversion factors of α_{[C I](1 - 0)}, α_{[C I](2 - 1)}, and α_CO at a linear resolution $\sim 1\,$kpc scale for six nearby galaxies of M 51, M 83, NGC 3627, NGC 4736, NGC 5055, and NGC 6946. This is perhaps the first effort, to our knowledge, in calibrating both [C I]-to-H₂ conversion factors across the spiral disks at spatially resolved $\sim 1\,$kpc scale though such studies have been discussed globally in galaxies near and far. In order to derive the conversion factors and achieve these calibrations, we adopt three different dust-to-gas ratio (DGR) assumptions that scale approximately with metallicity taken from precursory results. We find that for all DGR assumptions, the α_{[C I](1 - 0)}, α_{[C I](2 - 1)}, and α_CO are mostly flat with galactocentric radii, whereas both α_{[C I](2 - 1)} and α_CO show decrease in the inner regions of galaxies. And the central α_CO and α_{[C I](2 - 1)} values are on average ~2.2 and 1.8 times lower than its galaxy averages. The obtained carbon abundances from different DGR assumptions show flat profiles with galactocentric radii, and the average carbon abundance of the galaxies is comparable to the usually adopted value of 3 × 10^-5. We find that both metallicity and infrared luminosity correlate moderately with the α_CO, whereas only weakly with either the α_{[C I](1 - 0)} or carbon abundance, and not at all with the α_{[C I](2 - 1)}.