ASTE CO(3--2) Observations of the Southern Barred Spiral Galaxy NGC 986: a Large Gaseous Bar Filled with Dense Molecular Medium

TL;DR

This study presents CO(3-2) observations of NGC 986, revealing a large dense molecular gas bar that could fuel future star formation, with findings indicating moderate excitation conditions and a strong correlation between dense gas and star formation regions.

Contribution

First detailed CO(3-2) imaging of NGC 986 showing the largest dense-gas rich bar, highlighting its potential role in star formation and galaxy evolution.

Findings

Discovered a 14 kpc dense molecular gas bar in NGC 986.

Found a good spatial correlation between dense gas and star formation.

Measured moderate CO line excitation conditions in the galaxy's central region.

Abstract

We present CO(3-2) emission observations toward the 3'x3' (or 20x20kpc 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 14 kpc in total length) gaseous bar filled with 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 the 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 the 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 $(5.4 \pm 1.1) \times 10^8$ K km s$^{-1}$ pc$^2$. 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 a 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 ($n_{\rm H_2} \sim 10^{3-4}$ cm$^{-3}$) in the central a few kpc region of NGC 986.