LRG Sample Research Articles

Stringent σ8 constraints from small-scale galaxy clustering using a hybrid MCMC + emulator framework

ABSTRACT We present a novel simulation-based hybrid emulator approach that maximally derives cosmological and Halo Occupation Distribution (HOD) information from non-linear galaxy clustering, with sufficient precision for DESI Year 1 (Y1) analysis. Our hybrid approach first samples the HOD space on a fixed cosmological simulation grid to constrain the high-likelihood region of cosmology + HOD parameter space, and then constructs the emulator within this constrained region. This approach significantly reduces the parameter volume emulated over, thus achieving much smaller emulator errors with fixed number of training points. We demonstrate that this combined with state-of-the-art simulations result in tight emulator errors comparable to expected DESI Y1 LRG sample variance. We leverage the new abacussummit simulations and apply our hybrid approach to CMASS non-linear galaxy clustering data. We infer constraints on σ8 = 0.762 ± 0.024 and fσ8(zeff = 0.52) = 0.444 ± 0.016, the tightest among contemporary galaxy clustering studies. We also demonstrate that our fσ8 constraint is robust against secondary biases and other HOD model choices, a critical first step towards showcasing the robust cosmology information accessible in non-linear scales. We speculate that the additional statistical power of DESI Y1 should tighten the growth rate constraints by at least another 50–60 ${{\ \rm per\ cent}}$, significantly elucidating any potential tension with Planck. We also address the ‘lensing is low’ tension, which we find to be in the same direction as a potential tension in fσ8. We show that the combined effect of a lower fσ8 and environment-based bias accounts for approximately $50{{\ \rm per\ cent}}$ of the discrepancy.

Luminosity-environment relation in the redshift region of 0.60 ≤ z ≤ 0.75

In this work, I construct a LRG sample with the redshift of 0.6 ≤ z ≤ 0.75 from the Sloan Digital Sky Survey Data Release 15 (SDSS DR15), which contains 184172 CMASS LRGs and 27158 eBOSS LRGs and examine the environmental dependence of the u-, g-, r-, i- and z-band luminosities in this galaxy sample. I divide this LRG sample into subsamples with a redshift binning size of Δz=0.01, and analyze the environmental dependence of all five band luminosities for these subsamples in each redshift bin. Overall, u-band luminosity is nearly independent of the local environment in all redshift bins. Though the environmental dependence of g-band luminosity is weak, a substantial luminosity-density correlation can be observed in some high redshift bins. The environmental dependence of r-, i- and z-band luminosities still is minimal in low redshift bins, like u- and g-band luminosities do. It is noteworthy that in high redshift bins, r-, i- and z-band luminosities have a strongly abnormal correlation with environments: luminous galaxies tend to reside in low density regions, while faint galaxies tend to reside in dense environments, like u-band luminosity in the apparent-magnitude limited Main galaxy sample does.

The 2QDES Pilot: the luminosity and redshift dependence of quasar clustering

We present a new redshift survey, the 2dF Quasar Dark Energy Survey pilot (2QDESp), which consists of ${\approx}10000$ quasars from ${\approx}150$ deg$^2$ of the southern sky, based on VST-ATLAS imaging and 2dF/AAOmega spectroscopy. Combining our optical photometry with the WISE (W1,W2) bands we can select essentially contamination free quasar samples with $0.8{<}z{<}2.5$ and $g{<}20.5$. At fainter magnitudes, optical UVX selection is still required to reach our $g{\approx}22.5$ limit. Using both these techniques we observed quasar redshifts at sky densities up to $90$ deg$^{-2}$. By comparing 2QDESp with other surveys (SDSS, 2QZ and 2SLAQ) we find that quasar clustering is approximately luminosity independent, with results for all four surveys consistent with a correlation scale of $r_{0}{=}6.1{\pm}0.1 \: h^{-1}$Mpc, despite their decade range in luminosity. We find a significant redshift dependence of clustering, particularly when BOSS data with $r_{0}{=}7.3{\pm}0.1 \: h^{-1}$Mpc are included at $z{\approx}2.4$. All quasars remain consistent with having a single host halo mass of ${\approx}2{\pm}1{\times}10^{12} \: h^{-1}M_\odot$. This result implies that either quasars do not radiate at a fixed fraction of the Eddington luminosity or AGN black hole and dark matter halo masses are weakly correlated. No significant evidence is found to support fainter, X-ray selected quasars at low redshift having larger halo masses as predicted by the `hot halo' mode AGN model of Fanidakis et al. 2013. Finally, although the combined quasar sample reaches an effective volume as large as that of the original SDSS LRG sample, we do not detect the BAO feature in these data.

The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: the low-redshift sample

We report on the small scale (0.5<r<40h^-1 Mpc) clustering of 78895 massive (M*~10^11.3M_sun) 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 5.2x10^13 h^-1 M_sun, 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.

Cross-correlatingWMAP5 with 1.5 million LRGs: a new test for the ISW effect

We present the cross-correlation of the density map of LRGs and the temperature fluctuation in the CMB as measured by the WMAP5 observations. The LRG samples were extracted from imaging data of the SDSS based on two previous spectroscopic redshift surveys, the SDSS-LRG and the 2SLAQ surveys at average redshifts z~0.35 and z~0.55. In addition we have added a higher-redshift photometric LRG sample based on the selection of the AAOmega LRG redshift survey at z~0.7. The total LRG sample thus comprises 1.5 million galaxies, sampling a redshift range of 0.2 < z < 0.9 over ~7600 square degrees of the sky, probing a total cosmic volume of ~5.5 h^{-3} Gpc^3. We find that the new LRG sample at z~0.7 shows very little positive evidence for the ISW effect. Indeed, the cross-correlation is negative out to ~1 deg. The standard LCDM model is rejected at ~2-3% significance by the new LRG data. We then performed a new test on the robustness of the LRG ISW detections at z~0.35 and 0.55. We made 8 rotations through 360deg of the CMB maps with respect to the LRG samples around the galactic pole. We find that in both cases there are stronger effects at angles other than zero. This implies that the z~0.35 and 0.55 ISW detections may still be subject to systematic errors which combined with the known sizeable statistical errors may leave these ISW detections looking unreliable. We have further made the rotation test on several other samples where ISW detections have been claimed and find that they also show peaks when rotated. We conclude that in the samples we have tested the ISW effect may be absent and we argue that this result may not be in contradiction with previous results.

THE TWO-POINT CORRELATION OF 2QZ QUASARS AND 2SLAQ LRGS: FROM A QUASAR FUELING PERSPECTIVE

Public data from the 2dF quasar survey (2QZ) and 2dF/SDSS LRG & QSO (2SLAQ), with their vast reservoirs of spectroscopically located and identified sources, afford us the chance to more accurately study their real space correlations in the hopes of identifying the physical processes that trigger quasar activity. We have used these two public databases to measure the projected cross correlation, $\omega_p$, between quasars and luminous red galaxies. We find the projected two-point correlation to have a fitted clustering radius of $r_0, = 5.3 \pm 0.6 $ and a slope, $\gamma =1.83 \pm 0.42 $ on scales from 0.7-27$h^{-1}$Mpc. We attempt to understand this strong correlation by separating the LRG sample into 2 populations of blue and red galaxies. We measure at the cross correlation with each population. We find that these quasars have a stronger correlation amplitude with the bluer, more recently starforming population in our sample than the redder passively evolving population, which has a correlation that is much more noisy and seems to flatten on scales $< 5h^{-1}$Mpc. We compare this result to published work on hierarchical models. The stronger correlation of bright quasars with LRGs that have undergone a recent burst of starformation suggests that the physical mechanisms that produce both activities are related and that minor mergers or tidal effects may be important triggers of bright quasar activity and/or that bright quasars are less highly biased than faint quasars.

Large‐Scale Anisotropic Correlation Function of SDSS Luminous Red Galaxies

We study the large-scale anisotropic two-point correlation function using 46,760 luminous red galaxies at redshifts 0.16-0.47 from the Sloan Digital Sky Survey. We measure the correlation function as a function of separations parallel and perpendicular to the line of sight in order to take account of anisotropy of the large-scale structure in redshift space. We find a slight signal of baryonic features in the anisotropic correlation function, i.e., a baryon ridge corresponding to a baryon acoustic peak in the spherically averaged correlation function, which has already been reported using the same sample. The baryon ridge has primarily a spherical structure with a known radius in comoving coordinates. It enables us to divide the redshift distortion effects into dynamical and geometrical components and provides further constraints on cosmological parameters, including the dark energy equation-of-state. With an assumption of a flat ? cosmology, we find the best-fit values of -->?m = 0.218?0.037+0.047 and -->?b = 0.047 ? 0.016 (68% CL) when we use the overall shape of the anisotropic correlation function of -->40 h?1 Mpc including a scale of baryon acoustic oscillations. When an additional assumption of -->?bh2 = 0.024 is adopted, we obtain -->?DE = 0.770?0.040+0.051 and -->w = ? 0.93?0.35+0.45. These constraints are estimated only from our data of the anisotropic correlation function, and they agree quite well with values both from the cosmic microwave background (CMB) anisotropies and from other complementary statistics using the LRG sample. With the CMB prior from the 3 year WMAP results, we give stronger constraints on those parameters.

The Small‐Scale Clustering of Luminous Red Galaxies via Cross‐Correlation Techniques

We present the small-scale (0.2h^{-1} to 7h^{-1} Mpc) cross-correlations between 32,000 luminous early-type galaxies and a reference sample of 16 million normal galaxies from the Sloan Digital Sky Survey. Our method allows us to construct the spherically averaged, real-space cross-correlation function between the spectroscopic LRG sample and galaxies from the SDSS imaging. We report the cross-correlation as a function of scale, luminosity, and redshift. We find very strong luminosity dependences in the clustering amplitudes, up to a factor of 4 over a factor of 4 in luminosity, and measure this dependence with high signal-to-noise ratio. The luminosity dependence of bias is found to depend on scale, with more variation on smaller scales. The clustering as a function of scale is not a power law, but instead has a dip at 1h^{-1} Mpc and an excess on small scales. The fraction of red galaxies within the L* sample surrounding LRGs is a strong function of scale, as expected. However, the fraction of red galaxies evolves in redshift similarly on small and large scales, suggesting that cluster and field populations are changing in the same manner. The results highlight the advantage on small scales of using cross-correlation methods as a means of avoiding shot noise in samples of rare galaxies.