Physical conditions of molecular gas in the Circinus galaxy Multi-J CO and Ci 3P1 → 3P0 observations⋆

We report mapping observations of the 12CO J = 3 → 2, 4 → 3, 6 → 5, and 7 → 6 transitions and the C i P1 →P0 (C i) 492 GHz transition toward the central 40′′×40′′ region of the Circinus galaxy, using the Atacama Pathfinder EXperiment (APEX) telescope. We also detected 13CO 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′′ ∼360 pc), the entire central 45′′ (D < 45′′ ∼ 900 pc) 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 nH2∼ 103.2cm−3 , Tkin ∼ 200 K, and d3/dr∼ 3 km s−1pc−1. In the entire 45′′ region, which covers both the nucleus and the S-F ring, two excitation components are needed with nH2 ∼ 104.2 and 103.0 cm−3 , Tkin ∼ 60 K and 30 K, and MH2 ∼ 2.3 × 107 M⊙ and 6.6 × 107 M⊙, 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 entire 45′′ region, we find a standard conversion factor of N(H2)/ICO 1→0 = 0.37 × 1020 cm−2(K km s−1)−1, about 1/5 of the Galactic disk value. The luminosity ratios of C i and 12CO J = 3 → 2 (RCI/CO 3→2) in Circinus basically follow a linear trend, similar to that obtained in high-redshift galaxies. The average RCI/CO J=3→2 in Circinus is found to be ∼ 0.2, lying at an intermediate value between non-AGN nuclear region and high-redshift galaxies.