[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 <SUB>up</SUB> = 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 <SUB>[C I](1-0)</SUB> = 7.3 M <SUB>☉</SUB> K<SUP>-1</SUP> km<SUP>-1</SUP> s pc<SUP>-2</SUP> and α <SUB>[C I](2-1)</SUB> = 34 M <SUB>☉</SUB> K<SUP>-1</SUP> km<SUP>-1</SUP> s pc<SUP>-2</SUP> with logarithmic root-mean-square spreads of 0.20 and 0.32 dex, respectively. The similar spread of {α }<SUB>[{{C</SUB>}{{I}}](1{--}0)} to {α }<SUB>CO</SUB>} (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 <SUB>[C I](2-1)</SUB> 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. <P />Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.