Giant molecular clouds in the non-grand design spiral galaxy NGC 6946
  We present high spatial resolution observations of Giant Molecular Clouds
(GMCs) in the eastern part of the nearby spiral galaxy NGC 6946 obtained with
the Combined Array for Research in Millimeter-wave Astronomy (CARMA). We have
observed 12CO(1-0), 12CO(2-1) and 13CO(1-0), achieving spatial resolutions of
5.4" x 5.0", 2.5" x 2.0" and 5.6" x 5.4" respectively over a region of 6 x 6
kpc. This region extends from 1.5 kpc to 8 kpc galactocentric radius, thus
avoiding the intense star formation in the central kpc. We have recovered
short-spacing u-v components by using single dish observations from the
Nobeyama 45m and IRAM 30m telescopes. Using the automated CPROPS algorithm we
identified 44 CO cloud complexes in the 12CO(1-0) map and 64 GMCs in the
12CO(2-1) maps. The sizes, line widths, and luminosities of the GMCs are
similar to values found in other extragalactic studies. We have classified the
clouds into on-arm and inter-arm clouds based on the stellar mass density
traced by the 3.6 um map. On-arm clouds present in general higher star
formation rates than clouds located on inter-arm regions. Although the star
formation efficiency shows no systematic trend with galactocentric radius, some
on-arm clouds -- which are more luminous and more massive compared to inter-arm
GMCs -- are also forming stars more efficiently than the rest of the identified
GMCs. These structures appear to be located on two specific regions in the
spiral arms. One of them shows a strong gradient, suggesting that this region
of high star formation efficiency may be the result of gas flow convergence.
