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Register a new userCross-correlation cosmic shear with the SDSS and VLA FIRST surveys
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Constantinos Demetroullas
Publication date
2015
fields
Physics
and research's language is
English
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We measure the cosmic shear power spectrum on large angular scales by cross-correlating the shapes of ~9 million galaxies measured in the optical SDSS survey with the shapes of ~2.7x10^5 radio galaxies measured by the overlapping VLA-FIRST survey. Our measurements span the multipole range 10 < l < 130, corresponding to angular scales 2deg < {theta} < 20deg. On these scales, the shear maps from both surveys suffer from significant systematic effects that prohibit a measurement of the shear power spectrum from either survey alone. Conversely we demonstrate that a power spectrum measured by cross-correlating the two surveys is unbiased. We measure an E-mode power spectrum from the data that is inconsistent with zero signal at the 99% confidence (~2.7{sigma}) level. The odd-parity B-mode signal and the EB cross- correlation are both found to be consistent with zero (within 1{sigma}). These constraints are obtained after a careful error analysis that accounts for uncertainties due to cosmic variance, random galaxy shape noise and shape measurement errors, as well as additional errors associated with the observed large-scale systematic effects in the two surveys. Our constraints are consistent with the expected signal in the concordance cosmological model assuming recent estimates of the cosmological parameters from the Planck satellite, and literature values for the median redshifts of the SDSS and FIRST galaxy populations. The cross-power spectrum approach described in this paper represents a powerful technique for mitigating shear systematics and will be ideal for extracting robust results, with the exquisite control of systematics required, from future cosmic shear surveys with the SKA, LSST, Euclid and WFIRST-AFTA.
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Stripe 82 in the Sloan Digital Sky Survey was observed multiple times, allowing deeper images to be constructed by coadding the data. Here we analyze the ellipticities of background galaxies in this 275 square degree region, searching for evidence of distortions due to cosmic shear. The E-mode is detected in both real and Fourier space with $>5$-$sigma$ significance on degree scales, while the B-mode is consistent with zero as expected. The amplitude of the signal constrains the combination of the matter density $Omega_m$ and fluctuation amplitude $sigma_8$ to be $Omega_m^{0.7}sigma_8 = 0.252^{+0.032}_{-0.052}$.
We present a new determination of the large-scale clustering of the CIV forest (i.e., the absorption due to all CIV absorbers) using its cross-correlation with quasars in the Sloan Digital Sky Survey (SDSS) Data Release 12 (DR12). We fit a linear bias model to the measured cross-correlation. We find that the transmission bias of the CIV forest, $b_{Fc}$, at a mean redshift of $z=2.3$, obeys the relation $(1+beta_c)b_{F c} = -0.024 pm 0.003$. Here, $beta_{c}$ is the linear redshift space distortion parameter of the CIV absorption, which can only be poorly determined at $beta_c=1.1pm 0.6$ from our data. This transmission bias is related to the bias of CIV absorbers and their host halos through the effective mean optical depth of the CIV forest, $bartau_c$. Estimating a value $bar tau_c(z) simeq 0.01$ from previous studies of the CIV equivalent width distribution, our measurement implies a CIV absorber bias near unity, with a large error due to uncertainties in both $beta_c$ and $bartau_c$. This makes it compatible with the higher DLA bias $b_{rm DLA}simeq 2$ measured previously from the cross-correlation of DLAs and the Lyman-$alpha$ forest. We discuss the implications of the CIV absorber bias for the mass distribution of their host halos. More accurate determinations of $bar tau_c(z)$ and $beta_c$ are necessary to obtain a more robust measurement of this CIV absorber bias.
We present measurements of cosmic shear two-point correlation functions (TPCFs) from Hyper Suprime-Cam Subaru Strategic Program (HSC SSP) first-year data, and derived cosmological constraints based on a blind analysis. The HSC first-year shape catalog is divided into four tomographic redshift bins ranging from $z=0.3$ to 1.5 with equal widths of $Delta z =0.3$. The unweighted galaxy number densities in each tomographic bin are 5.9, 5.9, 4.3, and 2.4 arcmin$^{-2}$ from lower to higher redshifts, respectively. We adopt the standard TPCF estimators, $xi_pm$, for our cosmological analysis, given that we find no evidence of the significant B-mode shear. The TPCFs are detected at high significance for all ten combinations of auto- and cross-tomographic bins over a wide angular range, yielding a total signal-to-noise ratio of 19 in the angular ranges adopted in the cosmological analysis, $7<theta<56$ for $xi_+$ and $28<theta<178$ for $xi_-$. We perform the standard Bayesian likelihood analysis for cosmological inference from the measured cosmic shear TPCFs, including contributions from intrinsic alignment of galaxies as well as systematic effects from PSF model errors, shear calibration uncertainty, and source redshift distribution errors. We adopt a covariance matrix derived from realistic mock catalogs constructed from full-sky gravitational lensing simulations that fully account for survey geometry and measurement noise. For a flat $Lambda$ cold dark matter model, we find $S_8 equiv sigma_8sqrt{Omega_m/0.3}=0.804_{-0.029}^{+0.032}$, and $Omega_m=0.346_{-0.100}^{+0.052}$. We carefully check the robustness of the cosmological results against astrophysical modeling uncertainties and systematic uncertainties in measurements, and find that none of them has a significant impact on the cosmological constraints.
We present a semi-analytic model for the shear two-point correlation function of a cosmic shear survey with non-uniform depth. Ground-based surveys are subject to depth variations that primarily arise through varying atmospheric conditions. For a survey like the Kilo-Degree Survey (KiDS), we find that the measured depth variation increases the amplitude of the observed shear correlation function at the level of a few percent out to degree-scales, relative to the assumed uniform-depth case. The impact on the inferred cosmological parameters is shown to be insignificant for a KiDS-like survey. For next-generation cosmic shear experiments, however, we conclude that variable depth should be accounted for.
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