Resolved neutral carbon emission in nearby galaxies: [CI] Lines as Total Molecular Gas Tracers

Abstract: We present maps of atomic carbon CI and CI at a linear resolution ~1kpc scale for a sample of one HII, six LINER, three Seyfert and five starburst galaxies observed with Herschel. We compare spatial distributions of two [CI] lines with that of CO(1-0) emission, and find that both [CI] lines distribute similarly to CO(1-0) emission in most galaxies. We present luminosity ratio maps of L'CI/L'_CO(1-0), L'CI/L'CO(1-0), L'CI/L'CI (hereafter R[CI]) and f_70/f_160. L'CI/L'_CO(1-0), R[CI] and f_70/f_160 are centrally peaked in starbursts; whereas remain relatively constant in LINERs, indicating that star-forming activity can enhance carbon emission, especially for CI. We explore the correlations between the luminosities of CO(1-0) and [CI] lines, and find that L'CO(1-0) correlates tightly and almost linearly with both L'CI and L'CI, suggesting that [CI] lines, similar as CO(1-0), can trace total molecular gas in our resolved galaxies on kpc scales. We investigate the dependence of L'CI/L'CO(1-0), L'CI/L'CO(1-0) and [CI] excitation temperature T_ex on dust temperature T_dust, and find non-correlation, a weak and modest correlation, respectively. The ratio of L'CI/L'CO(1-0) stays smooth distribution in most galaxies, indicating that the conversion factor of CI luminosity to H_2 mass (XCI) changes with CO(1-0) conversion factor (\alpha_CO) proportionally. Under optically thin and LTE assumptions, we derive a galaxy-wide average carbon excitation temperature T_ex ~ 19.7 \pm 0.5K and an average neutral carbon abundance X[CI]/X[H_2] ~2.5 \pm 1.0 * 10^{-5} in our resolved sample, which is comparable to the usually adopted value of 3*10^{-5}, but ~3 times lower than the carbon abundance in local (U)LIRGs. We conclude that the carbon abundance varies in different galaxy types.