SDSS-III Baryon Oscillation Spectroscopic Survey Data Release 12: galaxy target selection and large scale structure catalogues

TL;DR

The paper tackles systematic biases in the SDSS-III BOSS DR12 galaxy sample by detailing target selection for LOWZ and CMASS, along with a comprehensive framework to build large-scale structure catalogues. It introduces and publicizes the mksample code, and elaborates masks, random catalogs, and a robust weighting scheme to correct for fibre collisions, redshift failures, and angular systematics. The resulting CMASS+LOWZ combined catalogue enables precise BAO and RSD measurements and serves as a benchmark for future spectroscopic surveys. Together, these methods ensure unbiased clustering analyses over vast cosmic volumes and establish a reproducible pipeline for next-generation surveys.

Abstract

The Baryon Oscillation Spectroscopic Survey (BOSS), part of the Sloan Digital Sky Survey (SDSS) III project, has provided the largest survey of galaxy redshifts available to date, in terms of both the number of galaxy redshifts measured by a single survey, and the effective cosmological volume covered. Key to analysing the clustering of these data to provide cosmological measurements is understanding the detailed properties of this sample. Potential issues include variations in the target catalogue caused by changes either in the targeting algorithm or properties of the data used, the pattern of spectroscopic observations, the spatial distribution of targets for which redshifts were not obtained, and variations in the target sky density due to observational systematics. We document here the target selection algorithms used to create the galaxy samples that comprise BOSS. We also present the algorithms used to create large scale structure catalogues for the final Data Release (DR12) samples and the associated random catalogues that quantify the survey mask. The algorithms are an evolution of those used by the BOSS team to construct catalogues from earlier data, and have been designed to accurately quantify the galaxy sample. The code used, designated MKSAMPLE, is released with this paper.

Figures (12)

• Figure 1: BAO measurement errors predicted for various surveys as a function of the year of publication. In order to calculate these with a consistent methodology we plot "predictions" using the code of seo07 based on a single number of galaxies, and volume for each survey. The surveys plotted are 2dFGRS early Per01 and final Col05; SDSS-II LRGs Eis05; WiggleZ blake11; BOSS DR9 CMASS And12; BOSS DR11 LOWZ Toj14 and CMASS And14. In terms of survey volume, BOSS DR12 is very close to DR11 and we do not show it here. We also present approximate predictions for the eBOSS, DESI and Euclid future surveys (see text for details).
• Figure 2: Top panel: Black dots show median $c_{||}$ for LOWZ spectroscopically confirmed galaxies as a function of measured redshift, with the dashed lines showing the interquartile range. The efficiency of using this quantity to track redshift is clear. Bottom panel: Median $r_{\rm mod} - i_{\rm mod}$ as a function of redshift for confirmed CMASS galaxies, with interquartile range (dashed lines). The way in which we can track the high-redshift locus of galaxies using this colour, and select as a function of redshift, is clear.
• Figure 3: Density plot of LOWZ galaxies in the $(g-r, r-i)$ colour plane; red corresponds to higher density and dark blue to lower density, in an arbitrary normalisation and linear scale. Redshift increases rightwards and upwards along the galaxies locus, starting at $z\simeq0.1$ on the bottom left corner. The knee on the galaxy locus is caused by the 4000 Å break transitioning between the $g-$ and $r$-band filters, and happens at $z\approx 0.4$. The colours $c_{\perp}$ and $c_{||}$ are simple rotations of this colour plane, and trace the position of a target in parallel and perpendicular, respectively, to the data locus. The black thick line represents the passively evolving LRG model of Mar09. The green and red dashed lines are the colour and magnitude targeting cuts -- see the main text for details. The few targets seen outside of the selection cut are due to differences in the targeting and final photometry, see Section \ref{['sec:data']}.
• Figure 4: Both panels show density plots of CMASS galaxies; red corresponds to higher density and dark blue to lower density in an arbitrary normalisation and linear scale. The black thick line shows the passively evolving LRG model of Mar09. Top: redshift increases upwards, starting at $z\simeq0.4$ at $d_{\perp} = 0.55$. Bottom: the sliding cut in $d_{\perp}$ with $i-$band magnitude, designed to select an approximately stellar-mass complete sample. Stellar mass increases with the perpendicular distance to the sliding cut, represented here by the red dashed line - see Mar13 for details. The green dashed line shows the sliding cut adopted for the CMASS SPARSE sample (see Section \ref{['sec:cmass_sparse']}). Vertical solid lines show the magnitude limits. On both panels, the small fraction of targets that lie outside of the selection cut are due to differences in the targeting and final photometry, see Section \ref{['sec:data']}. Only chunks greater than 6 are shown.
• Figure 5: The distribution of spectroscopically confirmed stars (large blue points) and galaxies (small red points) in the psf-model vs model $i$-band (top) and $z$-band (bottom) planes selected in the CMASS sample of the commissioning data. The black lines are the linear cuts that remove the most spectroscopically confirmed stars whilst removing less than 1% of the galaxies. The $z-$band cut was added to the original $i-$band cut targeting from chunk 3 onwards.