The clustering of X-ray and sub-mm sources
  It is becoming clear that luminous extragalactic X-ray and sub-mm sources are essentially distinct populations. Thus, if high redshift sub-mm sources represent massive spheroids in formation, there must be a time lag between the major epoch of star-formation and the appearance of a visible quasar. Despite this distinction, I find tentative evidence for a puzzling angular cross-correlation between X-ray sources and bright sub-mm sources in two independent fields. If this signal is due to large-scale structure it would argue for a low redshift (z<2) for many of the SCUBA sources. Alternatively, I suggest that the effect may be enhanced by gravitational lensing. The exceptionally steep slope of the bright sub-mm counts makes this population particularly prone to lensing bias. An apparent correlation may therefore be produced if X-ray sources trace the intervening large scale structure. 
O. Almaini