The clustering of galaxies in the completed SDSS-III Baryon Oscillation Spectroscopic Survey: Observational systematics and baryon acoustic oscillations in the correlation function

TL;DR

This work delivers a rigorous, end-to-end BAO analysis of the final BOSS DR12 galaxy sample by explicitly modeling the angular selection function and its observational systematics, especially stellar density and seeing. It demonstrates that BAO scale measurements are robust to these systematics, while providing a quantitative assessment of any residual uncertainties via mock-based covariance and systematic tests. The study achieves precise transverse and radial BAO distance constraints in multiple redshift bins and validates the results against other DR12 analyses and Planck LCDM, contributing to an integrated cosmological constraint framework. The methodologies and robustness checks established here underpin the reliable use of BOSS DR12 BAO measurements in subsequent combined analyses (e.g., Acacia) and future large-scale structure surveys.

Abstract

We present baryon acoustic oscillation (BAO) scale measurements determined from the clustering of 1.2 million massive galaxies with redshifts 0.2 < z < 0.75 distributed over 9300 square degrees, as quantified by their redshift-space correlation function. In order to facilitate these measurements, we define, describe, and motivate the selection function for galaxies in the final data release (DR12) of the SDSS III Baryon Oscillation Spectroscopic Survey (BOSS). This includes the observational footprint, masks for image quality and Galactic extinction, and weights to account for density relationships intrinsic to the imaging and spectroscopic portions of the survey. We simulate the observed systematic trends in mock galaxy samples and demonstrate that they impart no bias on baryon acoustic oscillation (BAO) scale measurements and have a minor impact on the recovered statistical uncertainty. We measure transverse and radial BAO distance measurements in 0.2 < z < 0.5, 0.5 < z < 0.75, and (overlapping) 0.4 < z < 0.6 redshift bins. In each redshift bin, we obtain a precision that is 2.7 per cent or better on the radial distance and 1.6 per cent or better on the transverse distance. The combination of the redshift bins represents 1.8 per cent precision on the radial distance and 1.1 per cent precision on the transverse distance. This paper is part of a set that analyses the final galaxy clustering dataset from BOSS. The measurements and likelihoods presented here are combined with others in Alam et al. (2016) to produce the final cosmological constraints from BOSS.

Figures (17)

• Figure 1: The number density as a function of redshift for the three different LOWZ selections, in the North Galactic Cap (NGC). The LOWZE2 and LOWZE3 selections were applied to early BOSS observations.
• Figure 2: The number density as a function of redshift for CMASS (solid curves) and LOWZ (dashed curves) selections, in the North and South Galactic Caps (NGC, colored 'forestgreen'; and SGC, colored 'darkkhaki'). The overall offset between densities in the two regions is due to calibration offsets in the imaging data between the two regions.
• Figure 3: Projected BOSS galaxy density versus stellar density, measured as the number of $17.5 < i < 19.9$ stars in Healpix pixels with Nside=128. Top panel: the relationships for CMASS and the three LOWZ selections. Middle panel: The relationships for CMASS, split into bins of $i_{\rm fib2}$ magnitude. These are the measurements used to define the stellar density weights applied to clustering measurements. Bottom panel: The relationships for CMASS, split by redshift, before (curves) and after (points with error-bars) stellar density weights are applied. The relationships before any weighting is applied are slightly dependent on redshift, due to a weak correlation between $i_{\rm fib2}$ and redshift. Weighting based on $i_{\rm fib2}$ (illustrated in the middle panel) removes this dependency.
• Figure 4: The relationship between observed density of BOSS galaxies and $i$-band seeing. Top panel: The relationships for CMASS and the three LOWZ selections. Middle panel: The relationships for CMASS NGC and SGC. The dashed curves display the best-fit relationship used to define the weights that correct for the observed trends. The solid curve displays the measured relationship for the combined NGC+SGC sample, after the weights have been applied. Bottom panel: The relationships for the LOWZE3 sample, split into four bins by $i_{\rm mod}$ magnitude. These relationships are used to define the weights applied the LOWZE3 sample.
• Figure 5: The relationship between galaxy density observed density and sky background (in nanomaggies per square arc second), Galactic extinction (in E(B-V)), and airmass, for CMASS and LOWZ. The dashed lines display the predicted relationship with Galactic extinction, based on the difference between the extinction coefficients applied to BOSS imaging data and those found in Schlafly & Finkbeiner (2011).