Left. Contour map of the surface density of IRAC sources, computed as explained in the text. Only sources detected in at least the first two Spitzer/IRAC bands, with photometric errors <0.3 mag, and up to [3.6] = 12.25 are considered. The contours are: 25 stars/arcmin², 36 stars/arcmin² (red line), 47 stars/arcmin², ranging in steps of 3σ (11 stars/arcmin²) from the estimated average surface density (14 stars/arcmin²) of field stars plus 3σ, overlaid on the image at 3.6 μm (grey-scale). Also labelled, the tentatively identified subclusters (A is also known as AH03J1725–34.4). Right. Same as left, but with the positions of identified Class II sources (full blue squares) and Class I sources (open red triangles) superimposed. Other YSO concentrations are enclosed in dashed-line circles. A colour version of this figure is available in the on-line edition.