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Register a new userClustering of luminous red galaxies II: small scale redshift space distortions
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This is the second paper of a series where we study the clustering of LRG galaxies in the latest spectroscopic SDSS data release, DR6, which has 75000 LRG galaxies covering over 1 $Gpc^3/h^3$ for $0.15<z<0.47$. Here we focus on modeling redshift space distortions in $xisp$, the 2-point correlation in separate line-of-sight and perpendicular directions, at small scales and in the line-of-sight. We show that a simple Kaiser model for the anisotropic 2-point correlation function in redshift space, convolved with a distribution of random peculiar velocities with an exponential form, can describe well the correlation of LRG at all scales. We show that to describe with accuracy the so called fingers-of-God (FOG) elongations in the radial direction, it is necessary to model the scale dependence of both bias $b$ and the pairwise rms peculiar velocity $sigma_{12}$ with the distance. We show how both quantities can be inferred from the $xisp$ data. From $r simeq 10$ Mpc/h to $r simeq 1$ Mpc/h, both the bias and $sigma_{12}$ are shown to increase by a factor of two: from $b=2$ to $b=4$ and from $sigma_{12}=400$ to 800 Km/s. The later is in good agreement, within a 5 percent accuracy in the recovered velocities, with direct velocity measurements in dark matter simulations with $Omega_m=0.25$ and $sigma_8$=0.85.
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This is the first paper of a series where we study the clustering of LRG galaxies in the latest spectroscopic SDSS data release, DR6, which has 75000 LRG galaxies covering over 1 $Gpc^3/h^3$ at $0.15<z<0.47$. Here we focus on modeling redshift space distortions in $xips$, the 2-point correlation in separate line-of-sight and perpendicular directions, on large scales. % and away from the line-of-sight. We use large mock simulations to study the validity of models and errors. We show that errors in the data are dominated by a shot-noise term that is 40% larger than the Poisson error commonly used. We first use the normalized quadrupole for the whole sample (mean z=0.34) to estimate $beta=f(Omega_m)/b=0.34 pm 0.03$, where $f(Omega_m)$ is the linear velocity growth factor and $b$ is the linear bias parameter that relates galaxy to matter fluctuations on large scales. We next use the full $xips$ plane to find $Omega_{0m}= 0.245 pm 0.020$ (h=0.72) and the biased amplitude $b sigma_8 = 1.56 pm 0.09$. For standard gravity, we can combine these measurements to break degeneracies and find $sigma_8=0.85 pm 0.06$, $b=1.85 pm 0.25$ and $f(Omega_m)=0.64 pm 0.09$. We present constraints for modified theories of gravity and find that standard gravity is consistent with data as long as $0.80<sigma_8<0.92$. We also calculate the cross-correlation with WMAP5 and show how both methods to measure the growth history are complementary to constrain non-standard models of gravity. Finally, we show results for different redshift slices, including a prominent BAO peak in the monopole at different redshifts. (Abridged)
We present the small-scale (0.01<r<8 h^{-1} Mpc) projected correlation function w_p(r_p) and real space correlation function xi(r) of 24520 luminous early-type galaxies from the Sloan Digital Sky Survey Luminous Red Galaxy (LRG) sample (0.16<z<0.36). ``Fiber collision incompleteness of the SDSS spectroscopic sample at scales smaller than 55 arcseconds prevents measurements of the correlation function for LRGs on scales smaller than ~0.3 Mpc by the usual methods. In this work, we cross-correlate the spectroscopic sample with the imaging sample, with a weighting scheme to account for the collisions, extensively tested against mock catalogs. We correct for photometric biases in the SDSS imaging of close galaxy pairs. We find that the correlation function xi(r) is surprisingly close to a r^{-2} power law over more than 4 orders of magnitude in separation r. This result is too steep at small scales to be explained in curre
The redshift-space distortion (RSD) in the observed distribution of galaxies is known as a powerful probe of cosmology. Observations of large-scale RSD have given tight constraints on the linear growth rate of the large-scale structures in the universe. On the other hand, the small-scale RSD, caused by galaxy random motions inside clusters, has not been much used in cosmology, but also has cosmological information because universes with different cosmological parameters have different halo mass functions and virialized velocities. We focus on the projected correlation function $w(r_p)$ and the multipole moments $xi_l$ on small scales ($1.4$ to $30 h^{-1}rm{Mpc}$). Using simulated galaxy samples generated from a physically motivated most bound particle (MBP)-galaxy correspondence scheme in the Multiverse Simulation, we examine the dependence of the small-scale RSD on the cosmological matter density parameter $Omega_m$, the satellite velocity bias with respect to MBPs, $b_v^s$, and the merger-time-scale parameter $alpha$. We find that $alpha=1.5$ gives an excellent fit to the $w(r_p)$ and $xi_l$ measured from the SDSS-KIAS value added galaxy catalog. We also define the ``strength of Fingers-of-God as the ratio of the parallel and perpendicular size of the contour in the two-point correlation function set by a specific threshold value and show that the strength parameter helps constraining $(Omega_m, b_v^s, alpha)$ by breaking the degeneracy among them. The resulting parameter values from all measurements are $(Omega_m,b_v^s)=(0.272pm0.013,0.982pm0.040)$, indicating a slight reduction of satellite galaxy velocity relative to the MBP. However, considering that the average MBP speed inside haloes is $0.94$ times the dark matter velocity dispersion, the main drivers behind the galaxy velocity bias are gravitational interactions, rather than baryonic effects.
A new determination of the sound horizon scale in angular coordinates is presented. It makes use of ~ 0.6 x 10^6 Luminous Red Galaxies, selected from the Sloan Digital Sky Survey imaging data, with photometric redshifts. The analysis covers a redshift interval that goes from z=0.5 to z=0.6. We find evidence of the Baryon Acoustic Oscillations (BAO) signal at the ~ 2.3 sigma confidence level, with a value of theta_{BAO} (z=0.55) = (3.90 pm 0.38) degrees, including systematic errors. To our understanding, this is the first direct measurement of the angular BAO scale in the galaxy distribution, and it is in agreement with previous BAO measurements. We also show how radial determinations of the BAO scale can break the degeneracy in the measurement of cosmological parameters when they are combined with BAO angular measurements. The result is also in good agreement with the WMAP7 best-fit cosmology. We obtain a value of w_0 = -1.03 pm 0.16 for the equation of state parameter of the dark energy, Omega_M = 0.26 pm 0.04 for the matter density, when the other parameters are fixed. We have also tested the sensitivity of current BAO measurements to a time varying dark energy equation of state, finding w_a = 0.06 pm 0.22 if we fix all the other parameters to the WMAP7 best-fit cosmology.
We present a clustering analysis of Luminous Red Galaxies in SDSS Stripe 82. We study the angular 2-point correlation function, of 130,000 LRG candidates via colour-cut selections in izK with the K band coverage coming from UKIDSS LAS. We have used the cross-correlation technique of Newman (2008) to establish the LRG redshift distribution. Cross-correlating with SDSS QSOs, MegaZ-LRGs and DEEP2 galaxies implies an average LRG redshift of z~1 with space density, n_g~3.2 +/-0.16 x10^-4 h^3 Mpc^-3. For theta<10, w(theta) significantly deviates from a single power-law. A double power-law with a break at r_b~2.4 h^-1 Mpc fits the data better, with best-fit scale length, r_0,1=7.63+/-0.27 h^-1Mpc and slope gamma_1=2.01 +/-0.02 at small scales and r_0,2=9.92 +/-0.40 h^-1 Mpc and gamma_2=1.64 +/-0.04 at large scales. Due to the flat slope at large scales, we find that a standard LambdaCDM linear model is accepted only at 2-3sigma, with the best-fit bias factor, b=2.74+/-0.07. We also fitted HOD models and estimate an effective halo mass of M_eff=3.3 +/-0.6x10^13 h^-1 M_sun. But at large scales, the current HOD models did not help explain the power excess in the clustering signal. We then compare the w(theta) results to those of Sawangwit et al. (2011) from 3 samples of LRGs at lower redshifts to measure clustering evolution. We find that a long-lived model may be a poorer fit than at lower redshifts, although this assumes that the Stripe 82 LRGs are luminosity-matched to the AAOmega LRGs. We find stronger evidence for evolution in the form of the z~1 LRG correlation function, with the above flat 2-halo slope maintaining to r>50 h^-1 Mpc. Applying the cross-correlation test of Ross et al. (2011), we find little evidence that the result is due to systematics. Otherwise it may provide evidence for primordial non-Gaussianity in the matter distribution, with f^local_NL=90+/-30.[Abridged]
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