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Register a new userThe Effect of Large-Scale Structure on the SDSS Galaxy Three-Point Correlation Function
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We present measurements of the normalised redshift-space three-point correlation function (Q_z) of galaxies from the Sloan Digital Sky Survey (SDSS) main galaxy sample. We have applied our npt algorithm to both a volume-limited (36738 galaxies) and magnitude-limited sample (134741 galaxies) of SDSS galaxies, and find consistent results between the two samples, thus confirming the weak luminosity dependence of Q_z recently seen by other authors. We compare our results to other Q_z measurements in the literature and find it to be consistent within the full jack-knife error estimates. However, we find these errors are significantly increased by the presence of the ``Sloan Great Wall (at z ~ 0.08) within these two SDSS datasets, which changes the 3-point correlation function (3PCF) by 70% on large scales (s>=10h^-1 Mpc). If we exclude this supercluster, our observed Q_z is in better agreement with that obtained from the 2dFGRS by other authors, thus demonstrating the sensitivity of these higher-order correlation functions to large-scale structures in the Universe. This analysis highlights that the SDSS datasets used here are not ``fair samples of the Universe for the estimation of higher-order clustering statistics and larger volumes are required. We study the shape-dependence of Q_z(s,q,theta) as one expects this measurement to depend on scale if the large scale structure in the Universe has grown via gravitational instability from Gaussian initial conditions. On small scales (s <= 6h^-1 Mpc), we see some evidence for shape-dependence in Q_z, but at present our measurements are consistent with a constant within the errors (Q_z ~ 0.75 +/- 0.05). On scales >10h^-1 Mpc, we see considerable shape-dependence in Q_z.
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We study the effect of large scale tangled magnetic fields on the galaxy two-point correlation function in the redshift space. We show that (a) the magnetic field effects can be comparable the gravity-induced clustering for present magnetic field strength $B_0 simeq 5 times 10^{-8}$ G, (b) the absence of this signal from the present data gives an upper bound $B_0 la 3 times 10^{-8}$ G, (c) the future data can probe the magnetic fields of $simeq 10^{-8}$ G. A comparison with other constraints on the present magnetic field shows that they are marginally compatible.However if the magenetic fields corresponding to $B_0 simeq 10^{-8}$ G existed at the last scattering surface they will cause unacceptably large CMBR anisotropies.
The Large Scale Structure (LSS) in the galaxy distribution is investigated using the Sloan Digital Sky Survey Early Data Release (SDSS EDR). Using the Minimal Spanning Tree technique we have extracted sets of filaments, of wall-like structures, of galaxy groups, and of rich clusters from this unique sample. The physical properties of these structures were then measured and compared with the expectations from Zeldovich theory. The measured characteristics of galaxy walls were found to be consistent with those for a spatially flat $Lambda$CDM cosmological model with $Omega_mapprox$ 0.3 and $Omega_Lambda approx$ 0.7, and for Gaussian initial perturbations with a Harrison -- Zeldovich power spectrum. Furthermore, we found that the mass functions of groups and of unrelaxed structure elements generally fit well with the expectations from Zeldovich theory, although there was some discrepancy for lower mass groups which may be due to incompleteness in the selected sample of groups. We also note that both groups and rich clusters tend to prefer the environments of walls, which tend to be of higher density, rather than the environments of filaments, which tend to be of lower density. Finally, we note evidence of systematic differences in the properties of the LSS between the Northern Galactic Cap stripe and the Southern Galactic Cap stripe -- in particular, in the physical properties of the walls, their spatial distribution, and the relative numbers of clusters embedded in walls. Because the mean separation of walls is $approx$ 60 -- 70$h^{-1}$ Mpc, each stripe only intersects a few tens of walls. Thus, small number statistics and cosmic variance are the likely drivers of these systematic differences.
We analyse the large-scale angular correlation function (ACF) of the CMASS luminous galaxies (LGs), a photometric-redshift catalogue based on the Data Release 8 (DR8) of the Sloan Digital Sky Survey-III. This catalogue contains over $600 , , 000$ LGs in the range $0.45 leq z leq 0.65$, which was split into four redshift shells of constant width. First, we estimate the constraints on the redshift-space distortion (RSD) parameters $bsigma_8$ and $fsigma_8$, where $b$ is the galaxy bias, $f$ the growth rate and $sigma_8$ is the normalization of the perturbations, finding that they vary appreciably among different redshift shells, in agreement with previous results using DR7 data. When assuming constant RSD parameters over the survey redshift range, we obtain $fsigma_8 = 0.69 pm 0.21$, which agrees at the $1.5sigma$ level with Baryon Oscillation Spectroscopic Survey DR9 spectroscopic results. Next, we performed two cosmological analyses, where relevant parameters not fitted were kept fixed at their fiducial values. In the first analysis, we extracted the baryon acoustic oscillation peak position for the four redshift shells, and combined with the sound horizon scale from 7-year textit{Wilkinson Microwave Anisotropy Probe} $(WMAP7)$ to produce the constraints $Omega_{m}=0.249 pm 0.031$ and $w=-0.885 pm 0.145$. In the second analysis, we used the ACF full shape information to constrain cosmology using real data for the first time, finding $Omega_{m} = 0.280 pm 0.022$ and $f_b = Omega_b/Omega_m = 0.211 pm 0.026$. These results are in good agreement with $WMAP7$ findings, showing that the ACF can be efficiently applied to constrain cosmology in future photometric galaxy surveys.
We obtain constraints on cosmological parameters from the spherically averaged redshift-space correlation function of the CMASS Data Release 9 (DR9) sample of the Baryonic Oscillation Spectroscopic Survey (BOSS). We combine this information with additional data from recent CMB, SN and BAO measurements. Our results show no significant evidence of deviations from the standard flat-Lambda CDM model, whose basic parameters can be specified by Omega_m = 0.285 +- 0.009, 100 Omega_b = 4.59 +- 0.09, n_s = 0.96 +- 0.009, H_0 = 69.4 +- 0.8 km/s/Mpc and sigma_8 = 0.80 +- 0.02. The CMB+CMASS combination sets tight constraints on the curvature of the Universe, with Omega_k = -0.0043 +- 0.0049, and the tensor-to-scalar amplitude ratio, for which we find r < 0.16 at the 95 per cent confidence level (CL). These data show a clear signature of a deviation from scale-invariance also in the presence of tensor modes, with n_s <1 at the 99.7 per cent CL. We derive constraints on the fraction of massive neutrinos of f_nu < 0.049 (95 per cent CL), implying a limit of sum m_nu < 0.51 eV. We find no signature of a deviation from a cosmological constant from the combination of all datasets, with a constraint of w_DE = -1.033 +- 0.073 when this parameter is assumed time-independent, and no evidence of a departure from this value when it is allowed to evolve as w_DE(a) = w_0 + w_a (1 - a). The achieved accuracy on our cosmological constraints is a clear demonstration of the constraining power of current cosmological observations.
We investigate the properties of the 2-point galaxy correlation function at very large scales, including all geometric and local relativistic effects -- wide-angle effects, redshift space distortions, Doppler terms and Sachs-Wolfe type terms in the gravitational potentials. The general three-dimensional correlation function has a nonzero dipole and octupole, in addition to the even multipoles of the flat-sky limit. We study how corrections due to primordial non-Gaussianity and General Relativity affect the multipolar expansion, and we show that they are of similar magnitude (when f_NL is small), so that a relativistic approach is needed. Furthermore, we look at how large-scale corrections depend on the model for the growth rate in the context of modified gravity, and we discuss how a modified growth can affect the non-Gaussian signal in the multipoles.
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