The 300-pc scale ALMA view of [C i] 3P1–3P0, CO J = 1–0, and 609-μm dust continuum in a luminous infrared galaxy. Issue 3 (16th July 2020)

• Record Type: Journal Article
• Authors: Saito, T; Michiyama, T; Liu, D; Ao, Y; Iono, D; Nakanishi, K; Schinnerer, E; Tadaki, K; Ueda, J; Yamashita, T
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 497:Issue 3(2020)
• Page Start: 3591
• Page End: 3600
• Publication Date: 2020-07-16
• Subjects: galaxies: individual: IRAS F18293-3413 -- galaxies: ISM -- submillimetre: ISM; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa2086
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We present high-quality Atacama Large Millimeter/submillimeter Array (ALMA) Band 8 observations of the [C i ] 3 P 1 – 3 P 0 line and 609- μ m 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 ] 3 P 1 – 3 P 0 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 ($L^{\prime }_{\rm [C\, {\small I}]}$ /$L^{\prime }_{\rm CO}$ ) ratio of 0.17 (0.16) with a scatter of 0.04. However, a pixel-by-pixel comparison revealed that there is a radial $L^{\prime }_{\rm [C\, {\small I}]}$ /$L^{\prime }_{\rm CO}$ gradient and a superlinear $L^{\prime }_{\rm CO}$ versus $L^{\prime }_{\rm [C\, {\small I}]}$ 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 pc 2 ) −1, respectively. A non-LTE calculation yields that typical molecular gas properties seen in nearby (U)LIRGs ($n_{\rm H_2}$ = 10 3−4 cm −3, T kin ∼ 50 K, and $X_{\rm C\, {\small I}}$ = (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.