High-resolution mapping of the physical conditions in two nearby active galaxies based on12CO(1–0), (2–1), and (3–2) lines

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 = ICO(2 − 1)/ICO(1 − 0) and R32 = ICO(3 − 2)/ICO(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.