The ATLASGAL survey: distribution of cold dust in the Galactic plane.
  Combination with Planck data
  Sensitive ground-based submillimeter surveys, such as ATLASGAL, provide a
global view on the distribution of cold dense gas in the Galactic plane. Here
we use the 353 GHz maps from the Planck/HFI instrument to complement the
ground-based APEX/LABOCA observations with information on larger angular
scales. The resulting maps reveal the distribution of cold dust in the inner
Galaxy with a larger spatial dynamic range. We find examples of elongated
structures extending over angular scales of 0.5 degree. Corresponding to >30 pc
structures in projection at a distance of 3 kpc, these dust lanes are very
extended and show large aspect ratios. Furthermore, we assess the fraction of
dense gas ($f_{\rm DG}$), and estimate 2-5% (above A$_{\rm{v}}>$7 mag) on
average in the Galactic plane. PDFs of the column density reveal the typically
observed log-normal distribution for low- and exhibit an excess at high column
densities. As a reference for extragalactic studies, we show the line-of-sight
integrated N-PDF of the inner Galaxy, and derive a contribution of this excess
to the total column density of $\sim2.2$%, above $N_{\rm H_2} =
2.92\times10^{22}$ cm$^{-2}$. Taking the total flux density, we provide an
independent estimate of the mass of molecular gas in the inner Galaxy of
$\sim1\times10^9\,M_{\odot}$, which is consistent with previous estimates using
CO emission. From the mass and $f_{\rm DG}$ we estimate a Galactic SFR of $\dot
M = 1.3\,M_{\odot}$ yr$^{-1}$. While the distribution of diffuse gas is
homogenous in the inner Galaxy, the CMZ stands out with a higher dense gas
fraction. The low star formation efficiency of the Milky Way is well explained
by the low $f_{\rm DG}$ in the Galactic ISM, while the high $f_{\rm DG}$
towards the CMZ, despite its low star formation activity, suggests that, in
that particular region of our Galaxy, high-density gas is not the bottleneck
for star formation.
