With an antenna diameter of 12 m, the observations reach an angular resolution[^3] of $19.2''$ at this wavelength. The submm continuum emission mainly represents thermal radiation from cool dust, which is generally optically thin and, therefore, an excellent tracer of the amount of interstellar material on the line of sight. The ATLASGAL survey reaches an average rms noise level of $\sim 50$ mJy/beam, which translates in a $3\sigma$ detection limit of $\sim 4~M_{\sun}$ of total molecular mass (for a nominal distance of 2 kpc and a dust temperature of $T_\rd = 20$ K).

In the infrared, we primarily use two large scale surveys that cover the inner Galactic plane: The Two Micron All Sky Survey [2MASS, @Skrutskie2006] which provides near-infrared (NIR) images of the whole sky, in the $J$ (1.25 ), $H$ (1.65 ), and $K_s$ (2.16 ) filters, with an angular resolution of $\sim 2.5''$; and the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire [GLIMPSE, @Benjamin2003; @Churchwell2009], which is a set of various mid-infrared (MIR) surveys of the Galactic plane carried out with the InfraRed Array Camera [IRAC, @Fazio2004], on board of the *Spitzer Space Telescope* [@Werner2004]. Here we use the GLIMPSE I and II surveys which cover the $(\ell,b)$ 