ASTE CO (3-2) observations of the southern barred spiral galaxy NGC 986: A large gaseous bar filled with a dense molecular medium

We present CO (3-2) emission observations toward the 3′ × 3′ (or 20 kpc × 20 kpc at a distance of 23 Mpc) region of the southern barred spiral galaxy NGC 986 using the Atacama Submillimeter Telescope Experiment (ASTE). This effort is a part of our on-going extragalactic CO (3-2) imaging project, ADIoS (ASTE Dense gas Imaging of Spiral galaxies). Our CO (3-2) image revealed the presence of a large (the major axis is 14 kpc in total length) gaseous bar filled with a dense molecular medium along the dark lanes observed in optical images. This is the largest "dense-gas rich bar" known to date. The dense gas bar, discovered in NGC 986, could be a huge reservoir of possible "fuel" for future starbursts in the central region, and we suggest that star formation in the central region of NGC 986 could still be in a growing phase. We found a good spatial coincidence between the overall distributions of dense molecular gas traced by CO (3-2) and massive star formation depicted by Ha. The global CO (3-2) luminosity, L′_CO(3-2), of NGC 986 was determined to be (5.4 ± 1.1) × 10⁸ K km s^-1 pc². The CO(3-2)/CO(1-0) integrated intensity ratio was found to be 0.60 ± 0.13 at a spatial resolution of 44″ or 5 kpc, and the CO (3-2)/CO (2-1) ratio was 0.67 ± 0.14 at a beam size of ∼ 25″ or ∼ 2.8 kpc. These line ratios suggest moderate excitation conditions of CO lines (n_H2 ∼ 10^3-4 cm^-3) in a few kiloparsec region of central NGC 986.