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The SDSS-III Baryon Oscillation Spectroscopic Survey: Quasar Target Selection for Data Release Nine (1105.0606v1)

Published 3 May 2011 in astro-ph.CO

Abstract: The SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), a five-year spectroscopic survey of 10,000 deg2, achieved first light in late 2009. One of the key goals of BOSS is to measure the signature of baryon acoustic oscillations in the distribution of Ly-alpha absorption from the spectra of a sample of ~150,000 z>2.2 quasars. Along with measuring the angular diameter distance at z\approx2.5, BOSS will provide the first direct measurement of the expansion rate of the Universe at z > 2. One of the biggest challenges in achieving this goal is an efficient target selection algorithm for quasars over 2.2 < z < 3.5, where their colors overlap those of stars. During the first year of the BOSS survey, quasar target selection methods were developed and tested to meet the requirement of delivering at least 15 quasars deg-2 in this redshift range, out of 40 targets deg-2. To achieve these surface densities, the magnitude limit of the quasar targets was set at g <= 22.0 or r<=21.85. While detection of the BAO signature in the Ly-alpha absorption in quasar spectra does not require a uniform target selection, many other astrophysical studies do. We therefore defined a uniformly-selected subsample of 20 targets deg-2, for which the selection efficiency is just over 50%. This "CORE" subsample will be fixed for Years Two through Five of the survey. In this paper we describe the evolution and implementation of the BOSS quasar target selection algorithms during the first two years of BOSS operations. We analyze the spectra obtained during the first year. 11,263 new z>2.2 quasars were spectroscopically confirmed by BOSS. Our current algorithms select an average of 15 z > 2.2 quasars deg-2 from 40 targets deg-2 using single-epoch SDSS imaging. Multi-epoch optical data and data at other wavelengths can further improve the efficiency and completeness of BOSS quasar target selection. [Abridged]

Citations (266)

Summary

  • The paper's main contribution is developing robust multi-algorithm methods, including XDQSO, to effectively distinguish high-redshift quasars from stars.
  • It employs comprehensive survey techniques with SDSS photometry and auxiliary UKIDSS and GALEX data to achieve a high density of z > 2.2 quasars.
  • The research demonstrates that optimized target selection enhances BAO measurements, refining our understanding of the Universe's expansion and distance scale.

Overview of the SDSS-III BOSS Quasar Target Selection for DR9

The paper addresses the methodology used in selecting quasar targets for the Baryon Oscillation Spectroscopic Survey (BOSS) within the Sloan Digital Sky Survey (SDSS-III) Data Release Nine (DR9). The primary aim is to identify high-redshift quasars efficiently to paper the Lyman-alpha forest and measure baryon acoustic oscillations (BAO), providing insights into the cosmic distance scale and the expansion rate of the Universe.

The research outlines the challenges in targeting quasars in the redshift range 2.2 < z < 3.5 due to their similar photometric colors to stars, thus complicating their distinction in the SDSS imaging data. A multifaceted approach was taken, involving Kernel Density Estimation (KDE), likelihood methods, Neural Networks (NN), and spectral observations from auxiliary sources like FIRST and UKIDSS. This comprehensive strategy aimed to achieve a quasar density of at least 15 per square degree to meet the survey's scientific requirements.

Key Findings and Methodology

  • Targeting Algorithms: The team developed and refined several algorithms to improve the efficiency of quasar selection. The combination of KDE, likelihood, NN, and XDQSO (Extreme Deconvolution) contributed to the robust discrimination between quasars and stars, with XDQSO ultimately chosen as the CORE method.
  • Survey Methodology and Data Utilization: The BOSS survey utilized a targeted area of 10,000 square degrees, employing the SDSS photometric data and additional resources like UKIDSS near-infrared and GALEX ultraviolet data to enhance quasar detection capabilities. The innovative use of these datasets helped in targeting faint and high-redshift quasars efficiently.
  • Performance Evaluation: Over 54,000 spectra were acquired, with over 13,000 confirmed quasars having z > 2.2. The first-year observations yielded a significant increase in the density of high-redshift quasars compared to previously known samples.
  • Results on Quasar Selection Efficiency: The paper quantifies the improved efficiency and completeness of the quasar selection methods. By leveraging auxiliary datasets and refining selection algorithms, the team reached an aggregate efficiency that aligned closely with their target density goals.

Implications and Future Work

The paper provides a roadmap for balancing efficiency and uniformity in large-scale spectroscopic surveys with a specific focus on high-redshift targets. By optimizing target selection and incorporating multi-wavelength data, the SDSS-III BOSS survey lays a foundation for future astronomical surveys in maximizing data yield and scientific return.

Future work could further exploit variability in photometric datasets to enhance quasar selection or integrate deeper photometric surveys. The paper suggests the potential for collaborations with upcoming surveys like the Wide-field Infrared Survey Explorer (WISE), which could augment the target selection process through mid-infrared data.

The research contributes to the broader understanding of quasar selection biases and data completeness, serving as a critical reference for cosmological studies reliant on high-redshift observations. The improvements in target selection are expected to yield better cosmological measurements, thereby refining our understanding of the Universe's expansion history.