The Quasar‐Quasar Correlation Function in the Palomar Transit Grism Survey

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Abstract
The quasar-quasar correlation function is measured in four strips of the Palomar Transit Grism Survey, covering an effective area of 50 square degrees at high Galactic latitude. The quasar sample contains 232 objects with well-defined line flux and equivalent width selection criteria. It spans the redshift range 0.7 < z < 4.7 and the magnitude range 16 < r4 < 22. Two estimators are used to measure the covariance function of quasars, ξqq = DD/RR - 1 and ξqq = (DD - 2DR + RR)/RR. Comparison samples are constructed by scrambling of redshifts and angular coordinates, and by randomly placing quasars within the survey volume using the smoothed redshift distribution of the observed sample. These methods give consistent results, indicating that survey geometry and selection effects are taken into account properly. Significant clustering is detected at comoving scales r < 20 h-1 Mpc. Despite the fact that only six pairs of quasars contribute to the clustering signal, the null hypothesis that they are drawn from an unclustered population is rejected at 99% confidence for pairs with separations r < 10 h-1 Mpc, and at 95% confidence for pairs with separations r < 20 h-1 Mpc. The clustering amplitude ratio of the high- and low-redshift subsamples in the Ω = 1 model is ξqq(z > 2)/ξqq(z < 2)=1.8+ 2.5−1.2, significantly higher than the prediction of the linear theory.