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