  We present the high redshift (3<z<5.3) 0.5-2 keV number counts and the 2-10
keV (rest frame) space density of X-ray selected AGNs detected in the Chandra
COSMOS survey. The sample comprises 81 X-ray detected sources with available
spectroscopic (31) and photometric (50) redshifts plus 20 sources with a formal
z_phot<3 but with a broad photometric redshift probability distribution, such
that z_phot+1sigma>3. 81 sources are selected in the 0.5-2 keV band, 14 are
selected in the 2-10 keV and 6 in the 0.5-10 keV bands. We sample the high
luminosity (log L_(2-10 keV)>44.15 erg/s) space density up to z~5 and a fainter
luminosity range (43.5<log L(2-10 keV)<44.15 erg/s) than previous studies, up
to z=3.5. We weighted the contribution to the number counts and the space
density of the sources with photometric redshift by using their probability of
being at z>3. We find that the space density of high-luminosity AGNs declines
exponentially at all the redshifts, confirming the trend observed for optically
selected quasars. At lower luminosity, the measured space density is not
conclusive, and a larger sample of faint sources is needed. Comparisons with
optical luminosity functions and BH formation models are presented together
with prospects for future surveys.
