The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7 (1610.05369v1)

Abstract: We present the discovery of nine quasars at $z\sim6$ identified in the Sloan Digital Sky Survey (SDSS) imaging data. This completes our survey of $z\sim6$ quasars in the SDSS footprint. Our final sample consists of 52 quasars at $5.7<z\le6.4$, including 29 quasars with $z_{\rm AB}\le20$ mag selected from 11,240 deg$^2$ of the SDSS single-epoch imaging survey (the main survey), 10 quasars with $20\le z_{\rm AB}\le20.5$ selected from 4223 deg$^2$ of the SDSS overlap regions (regions with two or more imaging scans), and 13 quasars down to $z_{\rm AB}\approx22$ mag from the 277 deg$^2$ in Stripe 82. They span a wide luminosity range of $-29.0\le M_{1450}\le-24.5$. This well-defined sample is used to derive the quasar luminosity function (QLF) at $z\sim6$. After combining our SDSS sample with two faint ($M_{1450}\ge-23$ mag) quasars from the literature, we obtain the parameters for a double power-law fit to the QLF. The bright-end slope $\beta$ of the QLF is well constrained to be $\beta=-2.8\pm0.2$. Due to the small number of low-luminosity quasars, the faint-end slope $\alpha$ and the characteristic magnitude $M_{1450}^{\ast}$ are less well constrained, with $\alpha=-1.90_{-0.44}^{+0.58}$ and $M^{\ast}=-25.2_{-3.8}^{+1.2}$ mag. The spatial density of luminous quasars, parametrized as $\rho(M_{1450}<-26,z)=\rho(z=6)\,10^{k(z-6)}$, drops rapidly from $z\sim5$ to 6, with $k=-0.72\pm0.11$. Based on our fitted QLF and assuming an IGM clumping factor of $C=3$, we find that the observed quasar population cannot provide enough photons to ionize the $z\sim6$ IGM at $\sim90$\% confidence. Quasars may still provide a significant fraction of the required photons, although much larger samples of faint quasars are needed for more stringent constraints on the quasar contribution to reionization.

• The paper reports the final SDSS census of 52 z>5.7 quasars, including 9 new discoveries confirmed by color selection and spectroscopy.
• It employs a simulation-based method to derive a double power-law luminosity function, with a well-constrained bright-end slope of -2.8 ± 0.2.
• Findings imply that the observed quasar population is insufficient for cosmic reionization, underscoring the need for deeper surveys to detect fainter quasars.

Overview of the Final SDSS High-Redshift Quasar Sample

The paper "The Final SDSS High-Redshift Quasar Sample of 52 Quasars at $z > 5.7$" by Linhua Jiang et al. presents the culmination of the Sloan Digital Sky Survey (SDSS) efforts to identify high-redshift quasars, specifically those at $z > 5.7$. The authors report the discovery of nine new quasars, bringing the total to 52 quasars within this redshift range, and provide a comprehensive analysis of the quasar luminosity function (QLF) at $z \sim 6$.

Quasar Sample and Methodology

The research utilized the SDSS imaging data to complete the survey of $z \sim 6$ quasars, identifying these celestial objects through a combination of color selection and spectroscopic confirmation. The final sample comprises 52 quasars, spanning a wide luminosity range ($-29.0 \leq M_{1450} \leq -24.5$).

The sample is divided into three complete sub-categories based on the survey region:

1. Main Survey: Includes 29 quasars from 11,240 deg$^2$ of single-epoch imaging, focusing on the most luminous populations.
2. Overlap Regions: Consists of 10 quasars from 4,223 deg$^2$ where the imaging data overlap, allowing for slightly fainter detections.
3. Stripe 82: Provides 13 quasars from 277 deg$^2$, benefitting from deeper co-added images.

The team applied a robust simulation-based approach to assess the completeness of the selection function, vital for accurately estimating the QLF.

Findings on the Quasar Luminosity Function

The authors derive a binned QLF and provide a double power-law fit to the observed data. A significant result of the paper is the bright-end slope $\beta = -2.8 \pm 0.2$, which is well-constrained. The QLF paper also integrates findings from additional faint quasars in the literature and cautious interpretation of the faint-end slope $\alpha = -1.90_{-0.44}^{+0.58}$ and characteristic magnitude $M_{1450}^\ast = -25.2_{-3.8}^{+1.2}$.

Implications for Cosmic Reionization

The paper explores the contribution of these high-redshift quasars to cosmic reionization. Assuming a clumping factor $C = 3$, the authors suggest that the current quasar population is unlikely to provide sufficient ionizing photons to account for reionization at $z \sim 6$. Their analysis indicates that a substantial number of faint quasars (with a steeper faint-end slope than previously observed) would be required to match the ionizing photon budget needed for reionization.

Future Prospects

The paper underscores the importance of deeper and wider-area surveys in addressing uncertainties in the QLF, particularly at the faint end. It predicts that upcoming surveys like LSST and continued efforts from existing projects such as Pan-STARRS and VISTA will be crucial in detecting fainter quasars and significantly improving our understanding of quasar evolution and the early universe's reionization history.

In conclusion, Jiang et al.'s work marks a significant milestone in the census of high-redshift quasars, providing valuable insights while paving the way for future studies that will probe the early universe with unprecedented detail.