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Astron. Astrophys. 347, 1-20 (1999)

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10. Conclusion

In this article we have described an analytic model for structure formation in the universe which deals simultaneously with quasars, galaxies and Lyman-[FORMULA] clouds, within the framework of a hierarchical scenario. This allows us to study the reheating and reionization history of the universe consistently with the properties of these various classes of objects. We have shown that for both a critical and an open universe our predictions agree reasonably well with observations. However, as was noticed by Haiman et al. (1998) it appears that the observational constraints on the quasar luminosity function are already strong. Moreover, the Gunn-Peterson test for HeII provides stringent additional constraints on the quasar contribution to the UV radiation field and on the reionization redshift. Thus, although our model in its simplest version (i.e. as described here, with no additional cutoffs for the quasar multiplicity function) is marginally consistent with the data, further observations of the helium opacity and of the quasar number counts (e.g. with the NGST) could provide tight constraints on such models where reionization is produced by QSOs. On the other hand, since reionization occurs rather late [FORMULA] the damping of CMB anisotropies is quite small.

We can note that our predictions are similar to some results obtained by Gnedin & Ostriker (1997) with a numerical simulation (but for a different cosmology). Moreover, the fact that our model agrees reasonably with observations for Lyman-[FORMULA] clouds, galaxies, quasars and constraints on the reionization process, strongly suggests that its main characteristics are fairly realistic. Thus, it provides a simple description of structure formation in the universe, from high redshifts after recombination down to the present epoch.

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© European Southern Observatory (ESO) 1999

Online publication: June 18, 1999