[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 α_([C I](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_([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.

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