WFCAM, Spitzer/IRAC and SCUBA observations of the massive star-forming region DR21/W75 - II. Stellar content and star formation

Wide-field near-infrared observations and Spitzer Space Telescope Infrared Array Camera (IRAC) observations of the DR21/W75 star formation regions are presented. The photometric data are used to analyse the extinction, stellar content and clustering in the entire region by using standard methods. A young stellar population is identified all over the observed field, which is found to be distributed in embedded clusters that are surrounded by a distributed halo population extending over a larger projected area. The Spitzer/IRAC data are used to compute a spectral index value, alpha, for each young stellar object in the field. We use these data to separate pure photospheres from disc excess sources. We find a small fraction of sources with alpha in excess of 2 to 3 (plus a handful with alpha similar to 4), which is much higher than the values found in the low-mass star-forming region IC 348 (alpha <= 2). The sources with high values of alpha spatially coincide with the densest regions of the filaments and also with the sites of massive star formation. Star formation is found to be occurring in long filaments stretching to few parsecs that are fragmented over a scale of similar to 1 pc. The spatial distribution of young stars are found to be correlated with the filamentary nebulae that are prominently revealed by 8- and 850-mu m observations. Five filaments are identified that appear to converge on a centre that includes the DR21/DR21(OH) regions. The morphological pattern of filaments and clustering compare well with numerical simulations of star cluster formation by Bate et al.