implies a resolution near 0.2 pc at a distance of 1.4 kpc). In Figure \[fig:ysodist\] we see how the map clearly resolves the filamentary structure of the IRDC in the northern part of the field and the morphology of the dust pillar on which DR15 is located. The extinction contours are limited to a maximum level of $A_V$=30 mag and further smoothed on the figure with a factor 3 boxcar, in order to remove some spurious features at the IRDC region. The resultant map shown in Figure \[fig:ysodist\] is in good agreement with a column density map constructed from 250 to 500 micron dust emission images from Herschel/SPIRE[^1], which also allows a much larger dynamic range (up to $A_V\approx$150 mag; Schneider et al., private comm.)

Clearly, the regions with higher column density values are those hosting a majority of the youngest stars. Using our extinction map we determined how the Class I sources are distributed in the highest density regions. Using contours of constant extinction in s.pdf of 1.0 mag, we counted the number of Class I sources above each level, and found that 23 out of 24 sources in our region of study lie above A$_V$=13.0 mag, and 20 