likely sites of future star formation.

For a few regions within the ATLASGAL Galactic range not covered by the GLIMPSE survey, we use data from the Wide-field Infrared Survey Explorer [WISE, @Wright2010], which mapped the entire sky in four infrared bands centered at 3.4, 4.6, 12, and 22 , with an angular resolution of $\sim 6''$ in the first three bands. Despite the lower sensitivity and coarser resolution as compared with GLIMPSE, bright PAH emission and prominent IRDCs can still be identified in the WISE images, specially at 12  (see Section \[sec:MIR-morphology\]).

“Stellar cluster” definitions {#sec:cluster-definition}
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In this paper, we define:

-   an *embedded cluster* (EC) as any stellar group recently born and still containing an important fraction of residual gas within and surrounding its volume, keeping in mind that it may never become a bound open cluster on its own. Since star formation takes place in molecular clouds, this definition is equivalent to the concept of a *correlated star formation event* introduced by @Kroupa2011; we keep the term “cluster” in order to match older designations in the literature.

-   an *open cluster* (OC) as any agglomerate of spatially correlated stars, and relatively free of the remaining gas. We 