A double molecular disc in the triple-barred starburst galaxy NGC 6946:
  structure and stability
  The late-type spiral galaxy NGC 6946 is a prime example of molecular gas
dynamics driven by "bars within bars". Here we use data from the BIMA SONG and
HERACLES surveys to analyse the structure and stability of its molecular disc.
Our radial profiles exhibit a clear transition at distance R ~ 1 kpc from the
galaxy centre. In particular, the surface density profile breaks at R ~ 0.8 kpc
and is well fitted by a double exponential distribution with scale lengths R_1
~ 200 pc and R_2 ~ 3 kpc, while the 1D velocity dispersion sigma decreases
steeply in the central kpc and is approximately constant at larger radii. The
fact that we derive and use the full radial profile of sigma rather than a
constant value is perhaps the most novel feature of our stability analysis. We
show that the profile of the Q stability parameter traced by CO emission is
remarkably flat and well above unity, while the characteristic instability
wavelength exhibits clear signatures of the nuclear starburst and inner bar
within bar. We also show that CO-dark molecular gas, stars and other factors
can play a significant role in the stability scenario of NGC 6946. Our results
provide strong evidence that gravitational instability, radial inflow and disc
heating have driven the formation of the inner structures and the dynamics of
molecular gas in the central kpc.
