Publication Date:
2020-07-16
Description:
We present high-quality Atacama Large Millimeter/submillimeter Array (ALMA) Band 8 observations of the [C i] 3P1–3P0 line and 609-μm dust continuum emission towards the nearby luminous infrared galaxy (LIRG) IRAS F18293-3413, as well as matched resolution (300-pc scale) Band 3 CO J = 1–0 data, which allow us to assess the use of the [C i] 3P1–3P0 line as a total gas mass estimator. We find that the [C i] line basically traces structures detected in CO (and dust) and a mean (median) [C i]/CO luminosity ($L^{prime }_{
m [C, {small I}]}$/$L^{prime }_{
m CO}$) ratio of 0.17 (0.16) with a scatter of 0.04. However, a pixel-by-pixel comparison revealed that there is a radial $L^{prime }_{
m [C, {small I}]}$/$L^{prime }_{
m CO}$ gradient and a superlinear $L^{prime }_{
m CO}$ versus $L^{prime }_{
m [C, {small I}]}$ relation (slope = 1.54 ± 0.02) at this spatial scale, which can be explained by radial excitation and/or line opacity gradients. Based on the molecular gas masses converted from the dust continuum emission, we found that the CO-to-H2 and [C i]-to-H2 conversion factors are relatively flat across the molecular gas disc with a median value of 3.5$^{+1.9}_{-1.3}$ and 20.7$^{+9.2}_{-4.9}$ M⊙ (K km s−1 pc2)−1, respectively. A non-LTE calculation yields that typical molecular gas properties seen in nearby (U)LIRGs ($n_{
m H_2}$ = 103−4 cm−3, Tkin ∼ 50 K, and $X_{
m C, {small I}}$ = (0.8–2.3) × 10−5) can naturally reproduce the derived [C i]-to-H2 conversion factor. However, we caution that a careful treatment of the physical gas properties is required in order to measure H2 gas mass distributions in galaxies using a single [C i] line. Otherwise, a single [C i] line is not a good molecular gas estimator in a spatially resolved manner.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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