The clustering of galaxies at z~0.5 in the SDSS-III Data Release 9 BOSS-CMASS sample: a test for the LCDM cosmology

TL;DR

The paper tests LCDM by clustering 282,068 BOSS CMASS galaxies at z≈0.5 against predictions from a large-volume MultiDark N-body simulation using Halo Abundance Matching. Across scales from ~0.5 to ~90 h^{-1} Mpc, the model broadly matches the observed two- and three-point statistics without free parameters beyond abundance matching, but shows ~10–20% deviations on small and intermediate scales that motivate more sophisticated modeling and larger data sets. It also derives the mean halo occupancy, satellite fraction (~12%), and a high large-scale bias (~b≈2) for CMASS-like haloes, along with a scale-dependent bias that exhibits BAO-related dips. The results support ΛCDM with WMAP7-inspired parameters and provide a framework for interpreting CMASS clustering through HAM in a cosmological context, while highlighting areas where non-linear effects and stochasticity need refinement. Overall, the work links galaxy clustering measurements to the underlying halo population, informing theoretical modeling of massive, high-redshift galaxies and the interpretation of BAO features in the clustering signal.

Abstract

We present results on the clustering of 282,068 galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS) sample of massive galaxies with redshifts 0.4<z<0.7 which is part of the Sloan Digital Sky Survey III project. Our results cover a large range of scales from ~0.5 to ~90 Mpc/h. We compare these estimates with the expectations of the flat LCDM cosmological model with parameters compatible with WMAP7 data. We use the MultiDark cosmological simulation together with a simple halo abundance matching technique, to estimate galaxy correlation functions, power spectra, abundance of subhaloes and galaxy biases. We find that the LCDM model gives a reasonable description to the observed correlation functions at z~0.5, which is a remarkably good agreement considering that the model, once matched to the observed abundance of BOSS galaxies, does not have any free parameters. However, we find a deviation (>~10%) in the correlation functions for scales less than ~1 Mpc/h and ~10-40 Mpc/h. A more realistic abundance matching model and better statistics from upcoming observations are needed to clarify the situation. We also estimate that about 12% of the "galaxies" in the abundance-matched sample are satellites inhabiting central haloes with mass M>~1e14 M_sun/h. Using the MultiDark simulation we also study the real space halo bias b(r) of the matched catalogue finding that b=2.00+/-0.07 at large scales, consistent with the one obtained using the measured BOSS projected correlation function. Furthermore, the linear large-scale bias depends on the number density n of the abundance-matched sample as b=-0.048-(0.594+/-0.02)*log(n/(h/Mpc)^3). Extrapolating these results to BAO scales we measure a scale-dependent damping of the acoustic signal produced by non-linear evolution that leads to ~2-4% dips at ~3 sigma level for wavenumbers k>~0.1 h/Mpc in the linear large-scale bias.

Figures (17)

• Figure 1: Sky area covered by the DR9 BOSS-CMASS sample shown in Aitoff projection colour-coded by completeness (see text). The upper and lower maps display the northern and southern galactic caps respectively.
• Figure 2: The comoving number density of galaxies in the DR9 BOSS-CMASS sample both for the north and south subsamples in the redshift range $0.4<z<0.7$. Dashed lines show the smoothed distributions used to create the Poisson distribution of particles when computing the correlation functions (see text).
• Figure 3: Projected correlation function times the projected distance for the DR9 BOSS-CMASS galaxy sample in the redshift range $0.43<z<0.7$. The blue and red shaded areas correspond to the north and south subsamples and give an estimate of their standard deviation. The dot-dashed lines display their mean value. The result of combining both subsamples is shown as filled circles. Standard deviation for the projected correlations of all samples are estimated using an ensemble of 600 mock catalogues (see Section \ref{['sec:clustering']}). For comparison the projected correlation inferred from the first semester of the BOSS-CMASS data is also shown White2011.
• Figure 4: Bottom panel: The cumulative number density of distinct haloes (dashed line) and subhaloes (dotted line) in the MultiDark simulation at $z=0.53$ as a function of maximum circular velocity. The cumulative number for all haloes is also shown as a solid line. Top panel: The cumulative subhalo fraction as a function of halo maximum circular velocity. As a reference we indicate in both panels the mean number density of the BOSS-CMASS galaxy sample and as vertical lines the corresponding maximum circular velocity threshold ($V_{\rm cut}$) used in the HAM procedure.
• Figure 5: Contours of the two-dimensional correlation function $\xi(\sigma,\pi)$ estimated from the DR9 BOSS-CMASS north galaxy sample (dashed contours) at $0.4<z<0.7$ and for our MultiDark halo catalogue constructed using the HAM technique at $z=0.53$ (solid contours).