Physical conditions of molecular gas in the Circinus galaxy: Multi-J CO and CI 1-0 observations

Abstract

We report mapping observations of the $^{12}$CO $J=3-2$, $4-3$, $6-5$, and $7-6$ transitions and the CI 492 GHz transition toward the central 40$''\times$40$''$ region of the Circinus galaxy, using the Atacama Pathfinder EXperiment (APEX) telescope. We also detected $^{13}$CO $J=3-2$ at the central position of Circinus. These observations are to date the highest CO transitions reported in Circinus. With large velocity gradient (LVG) modeling and likelihood analysis we try to obtain density, temperature, and column density of the molecular gas in three regions: the nuclear region ($D< 18''\sim$ 360 pc), the entire central 45$''$ ($D<45''\sim$ 900pc) region, and the star-forming (S-F) ring (18$''<D<45''$). In the nuclear region, we can fit the CO excitation with a single excitation component, yielding an average condition of $n_{\rm H_2} \sim 10^{3.2}$cm$^{-3}$, $T_{\rm kin}$ $\sim$ 200 K, and d$v$/d$r \sim$ 3 km s$^{-1}$pc$^{-1}$. In the entire 45$''$ region, two excitation components are needed with $n_{\rm H_2}$ $\sim$ 10$^{4.2}$ and 10$^{3.0}$ cm$^{-3}$, $T_{\rm kin}\sim$ 60 K and 30 K, and $M_{\rm H_2}\sim$ $2.3\times 10^7$ M$_\odot$ and $6.6 \times 10^7$ M$_\odot$, respectively. The gas excitation in the S-F ring can also be fitted with two LVG components, after subtracting the CO fluxes in the 18$''$ region. The S-F ring region contributes 80\% of the molecular mass in the 45$''$ region. For the 45$''$ region, we find a conversion factor of $N({\rm H_2})/I_{\rm CO\ 1-0}$ = $0.37\times 10^{20}$ cm$^{-2} ({\rm K\ km\ s}^{-1})^{-1}$, about 1/5 of the Galactic disk value. The luminosity ratios of CI and $^{12}$CO $J=3-2$ ($R_{\rm CI/CO\ J=3-2}$) in Circinus basically follow a linear trend. The average $R_{\rm CI/CO\ J=3-2}$ in Circinus is $\sim$ 0.2, lying at an intermediate value between non-AGN nuclear region and high-redshift galaxies.