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

¹ Purple Mountain Observatory, Key Lab of Radio Astronomy, 2 # West Beijing Road, 210008 Nanjing, PR China
e-mail: pmozhang@gmail.com
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ University of Chinese Academy of Sciences, 19A Yuquan Road, PO Box 3908, 100039 Beijing, PR China
⁴ Astron. Dept., King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁵ School of Physics and Astronomy Cardiff University, CF24 3AA, Queen’s Buildings The Parade, Cardiff, UK
⁶ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
⁷ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK

Received: 10 September 2013
Accepted: 4 June 2014

Abstract

We report mapping observations of the ¹²CO J = 3 → 2, 4 → 3, 6 → 5, and 7 → 6 transitions and the Ci ³P₁ → ³P₀ (Ci) 492GHz transition toward the central 40′′ × 40′′ region of the Circinus galaxy, using the Atacama Pathfinder EXperiment (APEX) telescope. We also detected ¹³COJ = 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_H2~10^3.2 cm^-3, T_kin~ 200 K, and dν/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_H2~ 10^4.2 cm^-3 and 10^3.0 cm^-3, T_kin~ 60 K and 30 K, and M_H2~2.3 × 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′′ 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(H₂) /I_{CO 1 → 0} = 0.37 × 10²⁰cm^-2(K km s^-1)^-1, about 1/5 of the Galactic disk value. The luminosity ratios of Ci and ¹²COJ = 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.