[C i](1–0) and [C i](2–1) in Resolved Local Galaxies*

Abstract

We present resolved [C i ] 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 up  = 1 to 7 to interpret what phase of the interstellar medium the [C i ] lines trace within typical local galaxies. A tight, linear relation is found between the intensities of the CO(4–3) and [C i ](2–1) lines; we hypothesize this is due to the similar upper level temperature of these two lines. We modeled the [C i ] and CO line emission using large-velocity gradient models combined with an empirical template. According to this modeling, the [C i ](1–0) line is clearly dominated by the low-excitation component. We determine [C i ] to molecular mass conversion factors for both the [C i ](1–0) and [C i ](2–1) lines, with mean values of α [C i ](1−0)  = 7.3 M ⊙ K −1 km −1 s pc −2 and α [C i ](2−1)  = 34 M ⊙ 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 to (derived using the CO(2–1) line) suggests that [C i ](1–0) 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 α [C i ](2−1) and the tight relation found between [C i ](2–1) and CO(4–3) suggest that much of the [C i ](2–1) emission may originate in warmer molecular gas.