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Astron. Astrophys. 353, 440-446 (2000)

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4. Discussion and conclusions

In the previous section we have discussed the possibility of a dependence on redshift of the AGN2/AGN1 ratio. In particular, this ratio remains constant or slightly increases up to [FORMULA]2, and then decreases. This would suggest that AGN2 are a later evolutionary stage of the AGN phenomenon, a possibility worth to be explored theoretically.

Another possibility is that the decreasing fraction of AGN2 for z[FORMULA]2 is only apparent, and that in reality there is an increase of the fraction of sources with [FORMULA] cm-2, i.e. completely hidden at all X-ray energies. This could be linked with the star formation rate history, which is observed to increase with the redshift up to z[FORMULA]2, and than stays constant (Madau et al. 1996; Rowan-Robinson 1999). A high star formation rate would imply a large amount of dust and gas, and then a large absorption.

A different approach in fitting the XRB consists in a luminosity-dependent number ratio [FORMULA], as has already been done by Gilli et al. (1999b).

The direct way to discriminate between different evolutionary models is to study the AGN2 XLF, a task within the capabilities of the new generation X-ray missions (Chandra and XMM).

In Table 1 and Table 2 we report the AGN1 and AGN2 densities for different flux limits corresponding to the [FORMULA] models without and with the inclusion of the 30% increase in the normalization of the XRB. The effect of [FORMULA] shows up in an AGN2 percentage decreasing at lower fluxes, a consequence of the sampling at higher redshifts where the AGN2/AGN1 number ratio decreases. It is worth noting that [FORMULA] erg cm-2 s- 1 and [FORMULA] erg cm- 2 s-1 are the flux limits expected for the Deep observations of the Lockman Hole and the Hubble Deep Field scheduled for Chandra and XMM, respectively.


[TABLE]

Table 1. AGN2 percentage prediction as a function of the sampling flux in the 5-10 keV band. The second and third column indicate the number density of AGN1 and AGN2.



[TABLE]

Table 2. Same as in Table 1, but introducing a 30% higher normalization to the Gruber (1992) data.


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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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