the symbols are coded to the hardness-to-soft X-ray ratio, as indicated in the color bar. \[fig:hardness\]](./f3.eps){width="5.5in"}

We identified a total of 1008 YSOs in W3, distributed as follows: 60 Class I/0 candidate sources, 780 Class II candidate sources and 167 Class III candidate sources. Our list contains a significantly larger number of YSO candidates compared to the list of [@Rivera-Ingraham:2011yq]. We suspect that the differences are mostly due to the addition of X-ray and near-infrared criteria which increased the number of candidates in each class. @Rivera-Ingraham:2011yq only list candidates with detections in all IRAC bands, which complicates a direct assessment of completeness.

In Figure \[fig:ysobd\] we show the K and \[3.6\] band brightness distributions of the three identified YSO classes in our sample. In the case of the \[3.6\] band, the three distributions show a sharp drop at a similar limit of about 15.5 mag. In contrast, for the $K$ band, the distribution for Class III sources drops at about 15.0 mag, while the distributions for Class I and Class II sources drop near 18.0 mag. These histograms show that, due to the high and non-uniform extinction, the brightness distributions for young sources are likely incomplete above the sensitivity limits. 