The clustering and evolution of optically selected quasi-stellar objects
We investigate methods of selecting complete, redshift limited samples of QSOs, based on broadband search techniques applied to COSMOS machine measurements of UK Schmidt photographic plates. In particular we discuss the ultra-violet excess (UVX) technique, employed to compile unbiased samples of low redshift (z < 2.2) QSO candidates. Using both conventional and multi-object spectroscopy, we have obtained slit spectra for 450 faint (17.5 < B < 20.9) UVX stellar images, of which 200 are QSOs. The resulting QSO catalogue is therefore amongst the largest and faintest of its kind yet produced. From the QSOs observed in the survey, we confirm that the number magnitude relation, n(B), for low redshift (z <2.2) QSOs exhibits a steep power law slope (dlogn/dB=0,86) at bright magnitudes (17.5 < B <19 , 5 ) , with a sharp turnover to a much flatter slope (dlogn/dB = 0,35) at fainter magnitudes (19.5 50h-‘Mpc) the lack of clustering evident in the QSO correlation function places strong constraints on the homogeneity of the universe. Prom a study of the clustering properties of the UVX sample as a whole we find that, on the plane of the sky, UVX stellar objects are significantly anti-correlated with respect to galaxies in clusters. This observation is explained by postulating that dust, lying in foreground clusters of galaxies obscures the QSOs situated behind these clusters at distances implied by a cosmological interpretation of the QSO redshift. Only A(_B)=0.2 mag. of dust absorption is required to produce the observed anti-correlation. Broadband colour techniques have also been extended to search for QSOs at high (z >2.2) redshifts. From the surface densities of QSOs found in these searches, we tentatively conclude that, if significant absorption by dust at high redshifts can be ruled out, the rate of luminosity evolution observed for UVX (z < 2.2) QSOs is slowing down at z = 2.5. A physical model in which QSOs are long lived, gradually dimming in luminosity from their epoch of formation at z > 2.2, to become Seyfert galaxies at the present day, is shown to be compatible with the above observations.