[CI](1-0) and [CI](2-1) in resolved local galaxies

Abstract: We present resolved [CI] line intensities of 18 nearby galaxies observed with the SPIRE FTS spectrometer on the Herschel Space Observatory. We use these data along with resolved CO line intensities from $J_\mathrm{up} = 1$ to 7 to interpret what phase of the interstellar medium the [CI] lines trace within typical local galaxies. A tight, linear relation is found between the intensities of the CO(4-3) and CI lines; we hypothesize this is due to the similar upper level temperature of these two lines. We modeled the [CI] and CO line emission using large velocity gradient models combined with an empirical template. According to this modeling, the CI line is clearly dominated by the low-excitation component. We determine [CI] to molecular mass conversion factors for both the CI and CI lines, with mean values of $\alpha_{\mathrm{CI}} = 7.3$ M${\mathrm{sun}}$ K$^{-1}$ km$^{-1}$ s pc$^{-2}$ and $\alpha{\mathrm{CI}} = 34 $ M${\mathrm{sun}}$ K$^{-1}$ km$^{-1}$ s pc$^{-2}$ with logarithmic root-mean-square spreads of 0.20 and 0.32 dex, respectively. The similar spread of $\alpha{\mathrm{CI}}$ to $\alpha_{\mathrm{CO}}$ (derived using the CO(2-1) line) suggests that CI may be just as good a tracer of cold molecular gas as CO(2-1) in galaxies of this type. On the other hand, the wider spread of $\alpha_{\mathrm{CI}}$ and the tight relation found between CI and CO(4-3) suggest that much of the CI emission may originate in warmer molecular gas.