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′ $\times$ 3′ (or 20kpc $\times$ 20kpc at a distance of 23Mpc) 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 14kpc 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 H $\alpha $ . The global CO(3-2) luminosity, $L’_{\rm CO(3-2)}$ , of NGC 986 was determined to be (5.4 $\pm$ 1.1) $\times$ 10 $^8$ Kkms $^{-1}$ pc $^2$ . The CO(3-2) $/$ CO(1-0) integrated intensity ratio was found to be 0.60 $\pm$ 0.13 at a spatial resolution of 44 $”$ or 5kpc, and the CO(3-2) $/$ CO(2-1) ratio was 0.67 $\pm$ 0.14 at a beam size of $\sim $ 25 $”$ or $\sim $ 2.8kpc. These line ratios suggest moderate excitation conditions of CO lines ( $n_{\rm H_2}$ $\sim $ 10 $^{3-4}$ cm $^{-3}$ ) in a few kiloparsec region of central NGC 986.