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 0 � 3 0 (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 “densegas 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˛ .T heglobal CO (3–2) luminosity, L 0 .3� 2/ ,o fNGC 986 was determined t ob e (5.4˙ 1.1) � 10 8 Kk m s � 1 pc 2 .T he CO (3–2)=CO (1–0) integrated intensity ratio was found to be 0.60 ˙ 0.13 at a spatial resolution of 44 00 or 5 kpc, and the CO (3–2)=CO (2–1) ratio was 0.67 ˙ 0.14 at a beam size of � 25 00 or � 2.8 kpc. These line ratios suggest moderate excitation conditions of CO lines (nH2 � 10 3� 4 cm � 3 )i n a few kiloparsec region of central NGC 986.