Astron. Astrophys. 325, 433-438 (1997)

4. Conclusions

We showed that the abundance-plus-correlation test can serve as a promising discrimination among models of cosmic structure formation using the high redshift () QSO number density and their clustering on large scales (10 h^-1 Mpc). We found that the SCDM and LCDM model are consistent with the observations of abundance and two-point correlation, while the CHDM models is difficult to fit with the observed numbers.

The contributions of variously biasing mechanisms have been investigated. A bias, in fact, is a model of the environment suitable to form objects, or a phenomenological relationship between the cosmic density field and QSOs. We have studied possible biasing processes, including both gravitational and non-gravitational origins. None of them seems to give large characteristic scale and clustering amplitude required to make the CHDM model success.

One should also consider the possibility that each halo may host more than one QSOs. In this case, the correlation function can be stronger than that of one-QSO/one-halo model. In order to fit with observed abundance of QSOs in the CHDM model, one can assumed that each halo of host more than 10 QSOs. In this case, halos of in the CHDM can also fit with the observed two-point correlation function of QSOs (Fig. 2c). However, this apparently leads to the paradox that we did not found so many bright QSOs concentrating in galaxy groups. A possible way to explain it is that the lifetime of QSOs is just longer than the cosmic time at but shorter than the cosmic time at the present. (If QSOs have a life time shorter than the cosmic time at , we require even more low mass halos, this makes the situation even worse.) However, the difference between the cosmic time at and is only a factor of 8 ( in the universe). Therefore, we may predict that on average, each of 8 galaxy group will have a bright QSO. This is not true. Otherwise, we should require a very particular mechanism to make the QSO formation at .

It is theoretically possible to explain any correlations if we are allowed to introduce unknown inhomogeneity into the density field, and assume that the correlation of QSOs absorbers is given by these inhomogeneities. However, to plan these inhomogeneities is equal to put desired structures in the initial perturbations. Such models will, however, no longer be the SCDM, LCDM or CHDM models, which are based on initial fluctuations produced at the inflationary era. However, even in this case the CHDM will still be difficult to produce enough halos with reasonable velocity dispersions to fit with the abundance of high redshift QSOs.

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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