High-resolution mapping of the physical conditions in two nearby active galaxies based on ¹²CO(1–0), (2–1), and (3–2) lines ^{⋆,⋆⋆,⋆⋆⋆}

¹ Observatoire de Paris, LERMA, 61 avenue de l’Observatoire 75014 Paris, France
e-mail: frederic.boone@obspm.fr
² Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
³ Observatorio Astronómico Nacional (OAN) – Observatorio de Madrid, C/ Alfonso XII 3, 28014 Madrid, Spain
⁴ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan
⁵ Department of Physics and Astronomy, Rutgers, the State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA
⁶ Joint ALMA Office, Av. El Golf 40, Piso 18, Las Condes, Santiago, Chile
⁷ Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁸ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 19 June 2009
Accepted: 4 October 2010

Abstract

We present a detailed analysis of high resolution observations of the three lowest CO transitions in two nearby active galaxies, NGC 4569 and NGC 4826. The CO(1–0) and (2–1) lines were observed with the Plateau de Bure Interferometer and the CO(3–2) line with the Submillimeter Array. Combining these data allows us to compare the emission in the three lines and to map the line ratios, R₂₁ = I_{CO(2 − 1)}/I_{CO(1 − 0)} and R₃₂ = I_{CO(3 − 2)}/I_{CO(1 − 0)} at a resolution of  ~2″, i.e., a linear resolution of 160 pc for NGC 4569 and 40 pc for NGC 4826. In both galaxies the emission in the three lines is similarly distributed spatially and in velocity, and CO is less excited (R₃₂ < 0.6) than in the Galactic Center or the centers of other active galaxies studied so far. According to a pseudo-LTE model the molecular gas in NGC 4569 is cold and mainly optically thick in the CO(1–0) and (2–1) lines; less than 50% of the gas is optically thin in the CO(3–2) line. LVG modeling suggests the presence of an elongated ring of cold and dense gas coinciding with the inner Lindblad resonance (ILR) of the stellar bar in agreement with a previous analysis of the kinematics.

More excited gas is resolved in the circumnuclear disk of NGC 4826. According to our pseudo-LTE model this corresponds to warmer gas with a  ~20% of the CO(3–2) emission being optically thin. LVG modeling indicates the presence of a semicircular arc of dense and cold gas centered on the dynamical center and  ~70 pc in radius. The gas temperature increases and its density decreases toward the center. A near side/far side asymmetry noticeable in the CO, R₃₂ and Paα maps suggests that opacity effects play a role. Examining published CO maps of nearby active galaxies we find similar asymmetries suggesting that this could be a common phenomenon in active galaxies. These mainly qualitative results open new perspectives for the study of active galaxies with the future Atacama Large Millimeter/submillimeter Array.