2004MNRAS.353.1025N

We present sub-arcsecond (0.35-0.9 arcsec), near-infrared J-, H- and K-band photometric observations of six fields along the W51 giant molecular cloud (W51 GMC). Our observations reveal four new, embedded clusters and provide a new high-resolution (0.35 arcsec) view of the W51 IRS2 (G49.5 - 0.4) region. The cluster associated with G48.9 - 0.3 is found to be a double cluster enclosed in a nest of near-infrared nebulosity. We construct stellar surface density maps for four major clusters in the W51 GMC. These unveil the underlying hierarchical structure. Colour-colour and colour-magnitude diagrams for each of these clusters show clear differences in the embedded stellar populations and indicate the relative ages of these clusters. In particular, the clusters associated with the HII regions G48.9 - 0.3 and G49.0 - 0.3 are found to have a high fraction of young stellar objects and are therefore considered the youngest of all the near-infrared clusters in the W51 GMC. The estimated masses of the individual clusters, when summed, yield a total stellar mass of ∼10⁴ M_☉in the W51 GMC, implying a star formation efficiency of 5-10 per cent. These results, in comparison with the CO observations of the W51 GMC, suggest for the first time that star formation in the W51 GMC is probably triggered by a galactic spiral density wave.