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Hard X-ray surveys efficiently sample the obscured AGN population and in particular those with redder colors which would not be selected by UVX surveys. The discovery that a sizable fraction (~20%) of hard X-ray sources are also associated to extremely red objects (EROs) with optical to near-infrared colour R-K>5 is even more intriguing [1,2,1]. [ABSTRACT FROM AUTHOR]

Daddi et al. (2000) reported the detection of strong angular clustering of extremely red galaxies (EROs, defined as objects with R - K > 5) with $K \lesssim 19$. The EROs angular 2-point correlation function were remarkably well fitted by a "standard" power law with exponent $\delta = -0.8$ but with a very large amplitude, a factor of 10 larger than that of the field galaxy population at the same depth. The clustering strength was found to smoothly increase with the R - K color. This was interpreted as evidence that the bulk of EROs is made of ellipticals that produce as expected a strong clustering signal, and this important conclusion is confirmed by independent indications (Cimatti et al. 1999, Moriondo, Cimatti & Daddi 2000, Stiavelli & Treu 2000, Cimatti 2000). In fact, the use of simple test redshift distributions for the inversion of the Limber equation, shows that the ERO angular clustering necessarily imply a large correlation length r0: for example to obtain $r_0 \sim 5 h^{-1}$ Mpc (comoving units) EROs should be confined in an extremely narrow and unrealistic redshift range of $\Delta z \sim 0.1 - 0.2$. Therefore much larger r0 values are required, values unlikely to be due to a star-forming population of galaxies. [ABSTRACT FROM AUTHOR]