Physical conditions of molecular gas in the Circinus galaxy Multi-JCO and Ci³PP₀observations

Abstract

We report mapping observations of the 12 CO J = 3 → 2, 4 → 3, 6 → 5, and 7 → 6 transitions and the C i 3 P 1 → 3 P 0 (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 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 ∼ 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 n H 2 ∼ 10 3.2 cm −3 , T kin ∼ 200 K, and d /dr ∼ 3 km s −1 pc −1 . In the entire 45 region, which covers both the nucleus and the S-F ring, two excitation components are needed with n H 2 ∼ 10 4.2 cm −3 and 10 3.0 cm −3 , T kin ∼ 60 K and 30 K, and M H 2 ∼ 2.3 × 10 7 M and 6.6 × 10 7 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 nuclear 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(H 2 )/I CO 1→0 = 0.37 × 10 20 cm −2 (K km s −1 ) −1 , about 1/5 of the Galactic disk value. The luminosity ratios of C i and 12 CO J = 3 → 2 (R CI/CO 3→2 ) in Circinus basically follow a linear trend, similar to that obtained in high-redshift galaxies. The average R CI/CO J = 3 → 2 in Circinus is found to be ∼0.2, lying at an intermediate value between non-AGN nuclear regions and high-redshift galaxies.

The Molecular Gas Environment in the 20 km s−1 Cloud in the Central Molecular Zone

“…An example is shown in Appendix A while details of this procedure can be found in Zhang et al (2014).…”

Neutral carbon and CO in 76 (U)LIRGs and starburst galaxy centers

“…The data on M 82 were taken from Ward et al (2006) and cover the inner lobes. The data on NGC 4945 and the Circinus galaxy were taken from Hitschfeld et al (2008: Nanten2, 38 ) and Zhang et al (2014;APEX, 14 , corrected to 22 ). For the nearby extended galaxies, the [C I] and 12 CO fluxes closely represent the emission from the bright central concentrations of gas and dust quite well, but not at all the more extended disk emission.…”

Influence of velocity dispersions on star-formation activities in galaxies

“…The value of CO-to-H 2 conversion factor X CO is affected by the physical conditions of molecular gas and could be different in different galaxies. We used the conversion factor derived from previous studies and adopted X CO = 2.4 × 10 20 for NGC 628 (Sandstrom et al 2013), X CO = 0.8 × 10 20 for NGC 3627 (Sandstrom et al 2013), X CO = 1.2 × 10 20 for NGC 1365 (Tabatabaei et al 2013), X CO = 0.4 × 10 20 = for the Circinus galaxy (Zhang et al 2014b), and X CO = 0.8 × 10 20 for NGC 1808 (Salak et al 2014). We did not find the CO-to-H 2 conversion factor for NGC 4568; we thus adopted the X CO = 1.8 × 10 20 for NGC 4568 following Kaneko et al (2017).…”