The 300-pc Scale ALMA View of [CI] ${}^3{P}_1$-${}^3{P}_0$, CO $J$=1-0, and 609 $\hat{I}¼$m Dust Continuum in A Luminous Infrared Galaxy

Abstract

We present high-quality ALMA Band 8 observations of the [CI] ${}^3{P}_1$-${}^3{P}_0$ line and 609 $\hat{I}¼$m dust continuum emission toward 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 [CI] ${}^3{P}_1$-${}^3{P}_0$ line as a total gas mass estimator. We find that the [CI] line basically traces structures detected in CO (and dust), and a mean (median) [CI]/CO luminosity ($L_{[\mathrm{CI}]}^{\prime }$/$L_{\mathrm{CO}}^{\prime }$) ratio of 0.17 (0.16) with a scatter of 0.04. However, a pixel-by-pixel comparison revealed that there is a radial $L_{[\mathrm{CI}]}^{\prime }$/$L_{\mathrm{CO}}^{\prime }$ gradient and a superlinear $L_{\mathrm{CO}}^{\prime }$ vs. $L_{[\mathrm{CI}]}^{\prime }$ relation (slope = 1.54 $\pm$ 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-H${}_2$ and [CI]-to-H${}_2$ conversion factors are relatively flat across the molecular gas disk with a median value of 3.5${}_{-1.3}^{+1.9}$ and 20.7${}_{-4.9}^{+9.2}$ $M_{\odot }$ (K km s${}^{-1}$ pc${}^2$)${}^{-1}$, respectively. A non-LTE calculation yields that typical molecular gas properties seen in nearby (U)LIRGs ($n_{{\mathrm{H}}_2}$ = 10${}^{3-4}$ cm${}^{-3}$, $T_{\mathrm{kin}}$ $\sim$ 50 K, and $X_{\mathrm{CI}}$ = (0.8-2.3) $\times$ 10${}^{-5}$) can naturally reproduce the derived [CI]-to-H${}_2$ conversion factor. However, we caution that a careful treatment of the physical gas properties is required in order to measure H${}_2$ gas mass distributions in galaxies using a single [CI] line. Otherwise, a single [CI] line is not a good molecular gas estimator in a spatially resolved manner.

11 pages, 6 figures, accepted for publication in MNRAS