Hu Zhan

Effect of Hot Baryons on the Weak-Lensing Shear Power Spectrum

We investigate the impact of the intracluster medium on the weak-lensing shear power spectrum (PS). Using a halo model we find that, compared to the dark matter-only case, baryonic pressure leads to a suppression of the shear PS on the order of a few percent or more for l 1000. Cooling/cooled baryons and the intergalactic medium can further alter the shear PS. Therefore, the interpretation of future precision weak-lensing data at high multipoles must take into account the effects of baryons.

Using Galaxy Two-Point Correlation Functions to Determine the Redshift Distributions of Galaxies Binned by Photometric Redshift

We investigate how well the redshift distributions of galaxies sorted into photometric redshift bins can be determined from the galaxy angular two-point correlation functions. We find that the uncertainty in the reconstructed redshift distributions depends critically on the number of parameters used in each redshift bin and the range of angular scales used, but not on the number of photometric redshift bins. Using six parameters for each photometric redshift bin, and restricting ourselves to angular scales over which the galaxy number counts are normally distributed, we find that errors in the reconstructed redshift distributions are large; i.e., they would be the dominant source of uncertainty in cosmological parameters estimated from otherwise ideal weak lensing or baryon acoustic oscillation data. However, either by reducing the number of free parameters in each redshift bin or by (unjustifiably) applying our Gaussian analysis into the non-Gaussian regime, we find that the correlation functions can be used to reconstruct the redshift distributions with moderate precision; e.g., with mean redshifts determined to ~0.01. We also find that dividing the galaxies into two spectral types, thereby doubling the number of redshift distribution parameters, can result in a reduction in the errors in the combined redshift distributions.

Cosmic tomographies: baryon acoustic oscillations and weak lensing

We explore the complementarity between two tomographic probes of the universe: baryon acoustic oscillations (in the galaxy power spectrum) and weak gravitational lensing. The galaxy power spectrum characterizes the density fluctuations, whereas the weak lensing shear power spectrum is a direct measure of the potential fluctuations. We find that photometric measurements of baryon oscillations alone do not provide very tight constraints on the dark energy equation of state parameters, partially due to our uncertain knowledge of the galaxy clustering bias. Weak lensing, on the other hand, is adversely impacted by the uncertainties of the probability distribution of photometric redshift errors. A joint analysis of the two, however, is more robust to these uncertainties and leads to a remarkable improvement over the results from either probe alone. Forecasts of cosmological constraints with baryon oscillations and weak lensing are provided for four proposed multiband imaging surveys in combination with measurements of the cosmic microwave background from Planck. In particular, we find that the joint analysis of galaxy and shear power spectra with the Large Synoptic Survey Telescope can tighten the 1σ error bounds on the dark energy equation of state (at the pivot expansion factor ap = 0.63) and its rate of change, respectively, to 0.016 and 0.16 (marginalized over 131 other parameters). With supernovae and cluster counting as well as higher-order statistics of the same galaxy and shear data, one can further improve the constraints.

DISTANCE, GROWTH FACTOR, AND DARK ENERGY CONSTRAINTS FROM PHOTOMETRIC BARYON ACOUSTIC OSCILLATION AND WEAK LENSING MEASUREMENTS

Baryon acoustic oscillations (BAOs) and weak lensing (WL) are complementary probes of cosmology. We explore the distance and growth factor measurements from photometric BAO and WL techniques, and investigate the roles of the distance and growth factor in constraining dark energy. We find for WL that the growth factor has a great impact on dark energy constraints, but is much less powerful than the distance. Dark energy constraints from WL are concentrated in considerably fewer distance eigenmodes than those from BAO, with the largest contributions from modes that are sensitive to the absolute distance. Both techniques have some well-determined distance eigenmodes that are not very sensitive to the dark energy equation-of-state parameters w0 and wa, suggesting that they can accommodate additional parameters for dark energy and for the control of systematic uncertainties. A joint analysis of BAO and WL is far more powerful than either technique alone, and the resulting constraints on the distance and growth factor will be useful for distinguishing dark energy and modified gravity models. The Large Synoptic Survey Telescope (LSST) will yield both WL and angular BAO over a sample of several billion galaxies. Joint LSST BAO and WL can yield 0.5% level precision on ten comoving distances evenly spaced in log(1 + z) between redshift 0.3 and 3 with cosmic microwave background priors from Planck. In addition, since the angular diameter distance, which directly affects the observables, is linked to the comoving distance solely by the curvature radius in the Friedmann-Robertson-Walker metric solution, the LSST can achieve a pure metric constraint of 0.017 on the mean curvature parameter Ω k of the universe simultaneously with the constraints on the comoving distances.

MORPHOLOGY OF GALAXY CLUSTERS: A COSMOLOGICAL MODEL-INDEPENDENT TEST OF THE COSMIC DISTANCE-DUALITY RELATION

Aiming at comparing different morphological models of galaxy clusters, we use two new methods to make a cosmological model-independent test of the distance-duality (DD) relation. The luminosity distances come from the Union2 compilation of Supernovae Type Ia. The angular diameter distances are given by two cluster models (De Filippis et al. and Bonamente et al.). The advantage of our methods is that they can reduce statistical errors. Concerning the morphological hypotheses for cluster models, it is mainly focused on the comparison between the elliptical ?-model and spherical ?-model. The spherical ?-model is divided into two groups in terms of different reduction methods of angular diameter distances, i.e., the conservative spherical ?-model and corrected spherical ?-model. Our results show that the DD relation is consistent with the elliptical ?-model at 1? confidence level (CL) for both methods, whereas for almost all spherical ?-model parameterizations, the DD relation can only be accommodated at 3? CL, particularly for the conservative spherical ?-model. In order to minimize systematic uncertainties, we also apply the test to the overlap sample, i.e., the same set of clusters modeled by both De Filippis et al. and Bonamente et al. It is found that the DD relation is compatible with the elliptically modeled overlap sample at 1? CL; however, for most of the parameterizations the DD relation cannot be accommodated even at 3? CL for any of the two spherical ?-models. Therefore, it is reasonable that the marked triaxial ellipsoidal model is a better geometrical hypothesis describing the structure of the galaxy cluster compared with the spherical ?-model if the DD relation is valid in cosmological observations.

Baryon Oscillations and Consistency Tests for Photometrically Determined Redshifts of Very Faint Galaxies

We investigate the impact of uncertainties in the photometric redshift error probability distribution on dark energy constraints from detection of baryon acoustic oscillations in galaxy power spectra. We also consider the prospects for determining the photometric redshift error probability distribution from the galaxy power spectra themselves. We find that although a sufficiently precise determination of redshift biases δz and variances σ is not possible, a strong consistency test is. For example, if σz is known to 1%, then any redshift bias δz greater than 0.008 will be detectable in the proposed survey of the Large Synoptic Survey Telescope through baryon oscillations. We speculate about the utility of this test for interpretation of cosmic shear data.

THE CORRELATION FUNCTION OF GALAXY CLUSTERS AND DETECTION OF BARYON ACOUSTIC OSCILLATIONS

We calculate the correlation function of 13,904 galaxy clusters of z ≤ 0.4 selected from the cluster catalog of Wen et al. The correlation function can be fitted with a power-law model ξ(r) = (r/R 0)–γ on the scales of 10 h –1 Mpc ≤ r ≤ 50 h –1 Mpc, with a larger correlation length of R 0 = 18.84 ± 0.27 h –1 Mpc for clusters with a richness of R ≥ 15 and a smaller length of R 0 = 16.15 ± 0.13 h –1 Mpc for clusters with a richness of R ≥ 5. The power-law index of γ = 2.1 is found to be almost the same for all cluster subsamples. A pronounced baryon acoustic oscillations (BAO) peak is detected at r ~ 110 h –1 Mpc with a significance of ~1.9σ. By analyzing the correlation function in the range of 20 h –1 Mpc ≤ r ≤ 200 h –1 Mpc, we find that the constraints on distance parameters are Dv (zm = 0.276) = 1077 ± 55(1σ) Mpc and h = 0.73 ± 0.039(1σ), which are consistent with the cosmology derived from Wilkinson Microwave Anisotropy Probe (WMAP) seven-year data. However, the BAO signal from the cluster sample is stronger than expected and leads to a rather low matter density Ω m h 2 = 0.093 ± 0.0077(1σ), which deviates from the WMAP7 result by more than 3σ. The correlation function of the GMBCG cluster sample is also calculated and our detection of the BAO feature is confirmed.

Weighing the Universe with Photometric Redshift Surveys and the Impact on Dark Energy Forecasts

With a wariness of Occams razor awakened by the discovery of cosmic acceleration, we abandon the usual assumption of zero mean curvature and ask how well it can be determined by planned surveys. We also explore the impact of uncertain mean curvature on forecasts for the performance of planned dark energy probes. We find that weak lensing and photometric baryon acoustic oscillation data, in combination with cosmic microwave background (CMB) data, can determine the mean curvature well enough that the residual uncertainty does not degrade constraints on dark energy. We also find that determinations of curvature are highly tolerant of photometric redshift errors.

The Local Power Spectrum and Correlation Hierarchy of the Cosmic Mass Field

We analyze the power spectrum of a QSOs Lyα-transmitted flux in the discrete wavelet transform (DWT) representation. Although the mean DWT power spectrum is consistent with its counterpart in Fourier representation, the spatial distribution of the local power varies greatly; i.e., the local DWT power spectra show remarkably spiky structures on small scales. To measure these spiky features, we introduce the quantities roughness of the local power spectrum and correlation between spikes on different scales. We then test the predictions made by the correlation hierarchy model on the roughness and the scale-scale correlations of the local power spectrum. Using the Lyα-transmitted flux of the QSO HS 1700, we find that the underlying cosmic mass field of the transmitted flux at redshift around z 2.2 can be described by the hierarchical clustering model on physical scales from 2.5 h-1 Mpc to a few tens h-1 kpc in an Einstein-de Sitter universe. However, the nonlinear features of the clustering show differences on different scale ranges: (1) On physical scales larger than ~1.3 h-1 Mpc, the field is almost Gaussian. (2) On scales 1.3-0.3 h-1 Mpc, the field is consistent with the correlation hierarchy with a constant value for the coefficient Q4. (3) On scales less than 300 h-1 kpc, the field is no longer Gaussian but essentially intermittent. In this case, the field can still be fitted by the correlation hierarchy, but the coefficient, Q4, should be scale dependent. These three points are strongly supported by the following result: the scale dependencies of Q4 given by two statistically independent measures, i.e., Q by roughness and Q by scale-scale correlation, are the same in the entire scale range considered.

CHARACTERIZING AND PROPAGATING MODELING UNCERTAINTIES IN PHOTOMETRICALLY-DERIVED REDSHIFT DISTRIBUTIONS

The uncertainty in the redshift distributions of galaxies has a significant potential impact on the cosmological parameter values inferred from multi-band imaging surveys. The accuracy of the photometric redshifts measured in these surveys depends not only on the quality of the flux data, but also on a number of modeling assumptions that enter into both the training set and spectral energy distribution (SED) fitting methods of photometric redshift estimation. In this work we focus on the latter, considering two types of modeling uncertainties: uncertainties in the SED template set and uncertainties in the magnitude and type priors used in a Bayesian photometric redshift estimation method. We find that SED template selection effects dominate over magnitude prior errors. We introduce a method for parameterizing the resulting ignorance of the redshift distributions, and for propagating these uncertainties to uncertainties in cosmological parameters.