We report on the small scale (0.5 < r < 40h −1 Mpc) clustering of 78895 massive (M * ∼ 10 11.3 M ) galaxies at 0.2 < z < 0.4 from the first two years of data from the Baryon Oscillation Spectroscopic Survey (BOSS), to be released as part of SDSS Data Release 9 (DR9). We describe the sample selection, basic properties of the galaxies, and caveats for working with the data. We calculate the real-and redshift-space two-point correlation functions of these galaxies, fit these measurements using Halo Occupation Distribution (HOD) modeling within dark matter cosmological simulations, and estimate the errors using mock catalogs. These galaxies lie in massive halos, with a mean halo mass of 5.2 × 10 13 h −1 M , a large scale bias of ∼ 2.0, and a satellite fraction of 12 ± 2%. Thus, these galaxies occupy halos with average masses in between those of the higher redshift BOSS CMASS sample and the original SDSS I/II LRG sample. c 0000 RAS arXiv:1211.3976v1 [astro-ph.CO]

The two-point clustering of dark matter halos is influenced by halo properties besides mass, a phenomenon referred to as halo assembly bias. Using the depth of the gravitational potential well, V max , as our secondary halo property, in this paper we present the first study of the scale-dependence assembly bias. In the large-scale linear regime, r 10Mpc/h, our findings are in keeping with previous results. In particular, at the low-mass end (M vir < M coll ≈ 10 12.5 M /h), halos with high-V max show stronger large-scale clustering relative to halos with low-V max of the same mass; this trend weakens and reverses for M vir M coll . In the nonlinear regime, assembly bias in low-mass halos exhibits a pronounced scale-dependent "bump" at 500kpc/h − 5Mpc/h, a new result. This feature weakens and eventually vanishes for halos of higher mass. We show that this scale-dependent signature can primarily be attributed to a special subpopulation of ejected halos, defined as present-day host halos that were previously members of a higher-mass halo at some point in their past history. A corollary of our results is that galaxy clustering on scales of r ∼ 1 − 2Mpc/h can be impacted by up to ∼ 15% by the choice of the halo property used in the halo model, even for stellar mass-limited samples.

Recent constraints on the splashback radius around optically selected galaxy clusters from the redMaPPer cluster-finding algorithm in the literature have shown that the observed splashback radius is ${\sim}20\%$ smaller than that predicted by N-body simulations. We present analyses on the splashback features around ∼ 3000 optically selected galaxy clusters detected by the independent cluster-finding algorithm CAMIRA over a wide redshift range of 0.1 &lt; zcl &lt; 1.0 from the second public data release of the Hyper Suprime-Cam (HSC) Subaru Strategic Program covering ∼427 deg2 for the cluster catalog. We detect the splashback feature from the projected cross-correlation measurements between the clusters and photometric galaxies over the wide redshift range, including for high-redshift clusters at 0.7 &lt; zcl &lt; 1.0, thanks to deep HSC images. We find that constraints from red galaxy populations only are more precise than those without any color cut, leading to 1σ precisions of ${\sim}15\%$ at 0.4 &lt; zcl &lt; 0.7 and 0.7 &lt; zcl &lt; 1.0. These constraints at 0.4 &lt; zcl &lt; 0.7 and 0.7 &lt; zcl &lt; 1.0 are more consistent with the model predictions (≲1σ) than their $20\%$ smaller values as suggested by the previous studies with the redMaPPer (∼2σ). We also investigate selection effects of the optical cluster-finding algorithms on the observed splashback features by creating mock galaxy catalogs from a halo occupation distribution model, and find such effects to be sub-dominant for the CAMIRA cluster-finding algorithm. We also find that the redMaPPer-like cluster-finding algorithm induces a smaller inferred splashback radius in our mock catalog, especially at lower richness, which can well explain the smaller splashback radii in the literature. In contrast, these biases are significantly reduced when increasing its aperture size. This finding suggests that aperture sizes of optical cluster finders that are smaller than splashback feature scales can induce significant biases on the inferred location of a splashback radius.

We critically examine the methodology behind the claimed observational detection of halo assembly bias using optically selected galaxy clusters by and . We mimic the optical cluster detection algorithm and apply it to two different mock catalogs generated from the Millennium simulation galaxy catalog, one in which halo assembly bias signal is present, while the other in which the assembly bias signal has been expressly erased. We split each of these cluster samples into two using the average cluster-centric distance of the member galaxies to measure the difference in the clustering strength of the subsamples with respect to each other. We observe that the subsamples split by cluster-centric radii show differences in clustering strength, even in the catalog where the true assembly bias signal was erased. We show that this is a result of contamination of the member galaxy sample from interlopers along the line-of-sight. This undoubtedly shows that the particular methodology adopted in the previous studies cannot be used to claim a detection of the assembly bias signal. We figure out the telltale signatures of such contamination, and show that the observational data also shows similar signatures. Furthermore, we also show that projection effects in optical galaxy clusters can bias the inference of the 3-dimensional edges of galaxy clusters (splashback radius), so appropriate care should be taken while interpreting the splashback radius of optical clusters.