[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 [CI](2-1) 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](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_[CI](1−0) = 7.3 M⊙/K km/ s pc^−2 and α[CI](2−1) = 34 M⊙/ K km/ s pc^−2 with logarithmic root-mean-square spreads of 0.20 and 0.32 dex, respectively. The similar spread of α_[CI](1−0) to α_CO (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_[CI](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.