The clustering and evolution of optically selected quasi-stellar objects

Abstract

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 <B < 20.9).Such behaviour in the n(B) relation thus precludes a pure density evolution model from being an adequate representation for the evolution of the QSO luminosity function (LF). On explicit calculation of the QSO LP in discrete redshift intervals, we find that this evolution can, most simply, be parameterised by a pure luminosity evolution model in which the luminosities of QSOs increase as a power law with look-back time. The form of the QSO LP itself is best represented by a 2 power law function which, at z=0, is similar in shape and number to the Seyfert galaxy LF, QSOs observed in the spectroscopic survey are also used to provide estimates for f(r), the 2-point correlation function, employed to investigate the spatial clustering of QSOs. We find that, at small comoving separations (r<10h- ‘Mpc), QSOs exhibit strong clustering. The amplitude of this clustering appears to be stronger than that observed for galaxies but not as strong as that observed for Abell clusters. At large scales (r >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.