The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Analysis of potential systematics (1203.6499v3)

Abstract: We analyze the density field of galaxies observed by the Sloan Digital Sky Survey (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS) included in the SDSS Data Release Nine (DR9). DR9 includes spectroscopic redshifts for over 400,000 galaxies spread over a footprint of 3,275 deg^2. We identify, characterize, and mitigate the impact of sources of systematic uncertainty on large-scale clustering measurements, both for angular moments of the redshift-space correlation function and the spherically averaged power spectrum, P(k), in order to ensure that robust cosmological constraints will be obtained from these data. A correlation between the projected density of stars and the higher redshift (0.43 < z < 0.7) galaxy sample (the `CMASS' sample) due to imaging systematics imparts a systematic error that is larger than the statistical error of the clustering measurements at scales s > 120h^-1Mpc or k < 0.01hMpc^-1. We find that these errors can be ameliorated by weighting galaxies based on their surface brightness and the local stellar density. We use mock galaxy catalogs that simulate the CMASS selection function to determine that randomly selecting galaxy redshifts in order to simulate the radial selection function of a random sample imparts the least systematic error on correlation function measurements and that this systematic error is negligible for the spherically averaged correlation function. The methods we recommend for the calculation of clustering measurements using the CMASS sample are adopted in companion papers that locate the position of the baryon acoustic oscillation feature (Anderson et al. 2012), constrain cosmological models using the full shape of the correlation function (Sanchez et al. 2012), and measure the rate of structure growth (Reid et al. 2012). (abridged)

• The paper mitigates systematic errors from redshift failures and fiber collisions by applying targeted weighting corrections to maintain unbiased clustering measurements.
• It examines photometric offsets between the Northern and Southern Galactic Caps and adjusts for stellar density variations to harmonize galaxy number densities.
• The study demonstrates that shuffling redshift assignments in the radial selection function minimizes bias, enhancing the reliability of BAO scale measurements.

Systematic Analysis of BOSS 3D Clustering

The paper "The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Analysis of potential systematics" by Ross et al. provides a comprehensive analysis of systematic uncertainties affecting the large-scale clustering measurements of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). The paper utilizes data from the ninth data release (DR9) of the Sloan Digital Sky Survey (SDSS), which includes spectroscopic redshifts for over 400,000 galaxies. The primary goal is to ensure the robustness of cosmological constraints derived from these data by identifying and mitigating systematic errors.

Key Findings and Methodological Approaches

1. Redshift Failures and Fiber Collisions: The authors address systematic errors arising from redshift failures and fiber collisions. They propose weights to correct for these issues: by up-weighting neighboring galaxies, they account for missed observations due to close fiber proximity or failed redshift acquisition. This method reduces potential biases in clustering measurements, especially at larger scales (s > 10h⁻¹Mpc).
2. Differences in Northern and Southern Galactic Caps: Photometric offsets between the Northern Galactic Cap (NGC) and Southern Galactic Cap (SGC) are examined. The paper finds systematic number density differences attributable to color offsets in the photometric calibration of these regions. When these offsets are applied to the selection criteria, the observed number density discrepancies become statistically insignificant. The paper recommends treating the NGC and SGC separately to account for these differences.
3. Angular Systematics: Stellar density is identified as the primary source of angular systematics affecting galaxy density. The authors use a set of weights based on the linear relationship between galaxy density and stellar density to mitigate this effect. This approach, applied specifically to CMASS galaxies, significantly improves the consistency and reliability of the clustering measurements.
4. Radial Selection Function: The paper explores different strategies for modeling the radial selection function of galaxy samples, finding that randomly assigning redshifts from the galaxy catalog to random catalogs (shuffling) provides the least biased results in comparison to spline fitting methods.
5. Clustering Measurements: The analysis includes comprehensive measurements of the monopole and quadrupole of the correlation function. The impact of various weighting schemes on these measurements is carefully assessed. The results demonstrate that systematic corrections are essential for robust cosmological analysis.

Implications and Future Directions

The results suggest that a rigorous treatment of systematic errors is vital for deriving accurate cosmological constraints from galaxy clustering data. The weighting strategies proposed in the paper prove effective in minimizing biases while maintaining the strength of the signal from the BAO scale. The paper's findings have implications for the analysis pipelines of ongoing and future galaxy surveys, emphasizing the need for detailed assessments of observational and instrumental systematics.

Additionally, the paper points out a feature at scales of approximately 200h⁻¹Mpc in the correlation function. Although similar features are found in simulations, the significance of this observation remains a topic for further investigation. This highlights an area where more detailed modeling could provide insights into potential new physics or systematic effects.

As BOSS progresses and the dataset expands, the approaches outlined in this paper will be essential for enhancing the precision and reliability of cosmological measurements. The meticulous attention to potential systematic influences sets a benchmark for future surveys, ensuring that the full statistical power of large-scale structure observations can be utilized effectively.