Molecular Gas and Star Formation Properties in the Central and Bar Regions of NGC 6946

TL;DR

This study examines molecular gas and star formation in NGC 6946's central and bar regions, revealing variations in gas properties and star formation efficiency influenced by galactic structures.

Contribution

It provides high-resolution molecular line observations and analyzes the physical conditions and star formation activity in different regions of NGC 6946, highlighting the impact of the galactic bar.

Findings

Central molecular gas is warmer and denser than offset ridges.

Dense gas fraction in the center is comparable to LIRGs/ULIRGs.

Star formation efficiency varies significantly across the inner disk.

Abstract

In this work, we investigate the molecular gas and star formation properties in the barred spiral galaxy NGC 6946 using multiple molecular lines and star formation tracers. High-resolution image (100 pc) of $^{13}$CO (1-0) is created by single dish NRO45 and interferometer CARMA for the inner 2 kpc disk, which includes the central region (nuclear ring and bar) and the offset ridges of the primary bar. Single dish HCN (1-0) observations were also made to constrain the amount of dense gas. Physical properties of molecular gas are inferred by (1) the Large Velocity Gradient (LVG) calculations using our observations and archival $^{12}$CO (1-0), $^{12}$CO(2-1) data, (2) dense gas fraction suggested by HCN to $^{12}$CO (1-0) luminosity ratio, and (3) infrared color. The results show that the molecular gas in the central region is warmer and denser than that of the offset ridges. Dense gas fraction of the central region is similar with that of LIRGs/ULIRGs, while the offset ridges are close to the global average of normal galaxies. The coolest and least dense region is found in a spiral-like structure, which was misunderstood to be part of the southern primary bar in previous low-resolution observations. Star formation efficiency (SFE) changes by ~ 5 times in the inner disk. The variation of SFE agrees with the prediction in terms of star formation regulated by galactic bar. We find a consistency between star-forming region and the temperature inferred by the infrared color, suggesting that the distribution of sub-kpc scale temperature is driven by star formation.