THE EVOLUTION OF QUASAR CLUSTERING

We have gathered several complete quasar surveys (greater than 1600 objects in total) to gain statistical information about the evolution of their clustering. We have computed both the conditional density and the two-point correlation functions. The quasar two-point correlation function does not evolve in comoving coordinates and the characteristic length scale turns out to be between 6 and 10 h-1 Mpc at the average redshifts of 0.70, 1.44, and 2.15. A steepening of the slope with redshift is however suggested by the conditional density. Moreover, a possible signature of quasar clustering is seen at redshift greater than 2.2. Clustering properties are not related to QSOs absolute luminosity, therefore our findings can be ascribed to the quasar population as a whole and not to a particular high luminosity class. This behavior strongly constrains both theories of galaxy formation predicting a strong evolution of clustering and those predicting a high contribution of unresolved QSOs to the X-ray background.