Elena Pierpaoli

Power-spectrum normalization from the local abundance of rich clusters of galaxies

The number density of rich galaxy clusters still provides the most robust way of normalizing the power spectrum of dark matter perturbations on scales relevant to large-scale structure. We revisit this constraint in light of several recent developments: (1) the availability of well-defined samples of local clusters with relatively accurate X-ray temperatures; (2) new theoretical mass functions for dark matter haloes which provide a good fit to large numerical simulations; (3) more accurate mass-temperature relations from larger catalogs of hydrodynamical simulations; (4) the requirement to consider closed as well as open and flat cosmologies to obtain full multi-parameter likelihood constraints for CMB and SNe studies. We present a new sample of clusters drawn from the literature and use this sample to obtain improved results on sigma_8, the normalization of the matter power spectrum on scales of 8 h^{-1} Mpc, as a function of the matter density and cosmological constant in a Universe with general curvature. We discuss our differences with previous work, and the remaining major sources of uncertainty. Final results on the 68 per cent confidence region, approximately independent of power spectrum shape, can be expressed as constraints on sigma at an appropriate cluster normalization scale R_Cl. We provide fitting formulas for R_Cl and sigma(R_Cl) for general cosmologies, as well as for sigma_8 as a function of cosmology and shape parameter Gamma. For flat models we find approximately sigma_8 simeq 0.495^{+0.034}_{-0.037}) Omega_M^{-0.60} for Gamma=0.23, where the error bar is dominated by uncertainty in the mass-temperature relation.

On determining the cluster abundance normalization

Different determinations currently suggest scattered values for the power spectrum normalization on the scale of galaxy clusters, σ 8 . Here we concentrate on the constraints coming from the X-ray temperature and luminosity functions (XTF and XLF), and investigate several possible sources of discrepancies in the results. We conclude that the main source of error in both methods is the mass scaling relation involved, in particular the way its intrinsic scatter and systematic normalization are treated. For temperature-derived constraints, we use a sample adapted from the Highest X-ray Flux Galaxy Cluster Sample (HIFLUGCS), and test for several sources of systematic error. We parametrize the mass-temperature relation with an overall factor T * , which varies between approximately 1.5 and 1.9 in the literature, with simulations typically giving lower results than empirically derived estimates. After marginalizing over this range of 7 * , we obtain a 68 per cent confidence range of σ 8 = 0.77 + 0 . 0 5 - 0 . 0 4 for a standard A-cold dark matter (ACDM) model. Most other determinations have chosen a single value for T * , and hence have neglected an important source of uncertainty. For luminosity-derived constraints we use the XLF from the REFLEX survey and explore how sensitive the final results are to the details of the mass-luminosity, M-L, conversion. Assuming a uniform systematic uncertainty of ′20 per cent in the amplitude of the mass-luminosity relation by Reiprich & Bohringer, we derive σ 8 = 0.79 + 0 . 0 6 - 0 . 0 7 for the same standard ACDM model. Although the XTF- and XLF-derived constraints agree very well with each other. we emphasize that such results can change by approximately 10-15 per cent, depending on how uncertainties in the L-T-M conversions are interpreted and included in the analysis. We point out that in order to achieve precision cosmology on σ 8 using cluster abundance, it is first important to separate the uncertainty in the scaling relation into its intrinsic and overall normalization parts. Careful consideration of all sources of scatter is also important, as is the use of the most accurate formulae and full consideration of dependence on cosmology. A significant improvement will require the simultaneous determination of mass using a variety of distinct methods, such as X-ray observations, weak lensing, Sunyaev-Zeldovich measurements and velocity dispersions of member galaxies, for a moderately large sample of clusters.

Boomerang returns unexpectedly

Experimental study of the anisotropy in the cosmic microwave background (CMB) is gathering momentum. The eagerly awaited Boomerang results have lived up to expectations. They provide convincing evidence in favor of the standard paradigm: the Universe is close to flat and with primordial fluctuations which are redolent of inflation. Further scrutiny reveals something even more exciting however -- two hints that there may be some unforeseen physical effects. Firstly the primary acoustic peak appears at slightly larger scales than expected. Although this may be explicable through a combination of mundane effects, we suggest it is also prudent to consider the possibility that the Universe might be marginally closed. The other hint is provided by a second peak which appears less prominent than expected. This may indicate one of a number of possibilities, including increased damping length or tilted initial conditions, but also breaking of coherence or features in the initial power spectrum. Further data should test whether the current concordance model needs only to be tweaked, or to be enhanced in some fundamental way.

Constraints on the cosmic neutrino background

The radiative component of the Universe has a characteristic impact on both large-scale structure (LSS) and the cosmic microwave background radiation (CMB). We use the recent WMAP data, together with previous Cosmic Background Imager (CBI) data and 2dF matter power spectrum, to constrain the effective number of neutrino species N e f f in a general cosmology. We find that N e f f = 4.31 with a 95 per cent confidence limit 1.6 ≤ N e f f ≤ 7.1. If we include the H 0 prior from the HST project we find the best fit N e f f = 4.08 and 1.90 ≤ N e f f ≤ 6.62 for a 95 per cent confidence limit. The curvature we derive is still consistent with flat, but assuming a flat Universe from the beginning implies a bias toward lower N e f f , as well as artificially smaller error bars. Adding the supernova constraint does not improve the result. We analyse and discuss the degeneracies with other parameters, and point out that probes of the matter power spectrum on smaller scales and accurate independent σ 8 measurements, together with better independent measurement of H 0 , would help in breaking the degeneracies.

Decaying particles and the reionization history of the universe.

We investigate the possibility that the Universe is significantly reionized by the decay products of heavy particles. The ionization produced by decaying particles implies a high optical depth even if the maximum level of ionization ever produced is low (10(-2)). As a consequence, a high ionization fraction (x approximately equal to 0.5) at high redshifts (z approximately equal to 20) fails to fit the cosmic microwave background (CMB) spectra at l> or =30. Recent CMB data limit the primordial abundance of the decaying particles, favoring long decay times. Other significant sources of reionization are still needed at z approximately equal to 13. The decay process heats up the medium, bringing the expected y distortion to unobservable levels.

Microwave background anisotropies and large scale structure constraints on isocurvature modes in a two-field model of inflation

In this paper we study the isocurvature m ode contribution to the cosm ic m icrowave background anisotropiesand thelargescalestructurepowerspectrum ,foratwo-�eld m odelofination proposed by Lindeand M ukhanov.W eprovideconstraintson theparam etersofthem odelby com paring its predictionswith observationsofthe m icrowave background anisotropies,large scale structure data

Measurement of the Sunyaev–Zel'dovich increment in massive galaxy clusters

We have detected the Sunyaev-Zeldovich (SZ) increment at 850µm in two galaxy clus- ters (Cl0016 + 16 and MS1054.4 0321) using SCUBA (Sub-millimetre Common User Bolometer Array) on the James Clerk Maxwell Telescope. Fits to the isothermalmodel yield a central Compton y parameter of (2.2 ± 0.7) × 10 4 and a central 850µm flux of �I0 = 2.2 ± 0.7 mJy beam 1 in Cl0016. This can be combined with decrement measure- ments to infer y = (2.38± 0.36 0.34 ) × 10 4 and vpec = 400± 1900 1400 km s 1 . In MS1054 we find a peak850µm flux ofI0 = 2.0±1.0 mJy beam 1 andy = (2.0±1.0)×10 4 . To be success- ful such measurements require large chop throws and non-standard data analysis techniques. In particular, the 450µm data are used to remove atmospheric variations in the 850µm data. An explicit annular model is fit to the SCUBA difference data i n order to extract the radial profile, and separately fit to the model differences to minimi ze the effect of correlations in- duced by our scanning strategy. We have demonstrated that with sufficient care, SCUBA can be used to measure the SZ increment in massive, compact galaxy clusters.

Radio emission from early‐type galaxies and cosmic microwave background experiments

We investigate the possible contribution from the emission of accretion flows around supermassive black holes in early type galaxies to current measurements of the Cosmic Microwave Background (CMB) at radio frequencies. We consider a range of luminosities suggested by targeted radio observations and accretion models and compute the residual contribution of these sources to the spectrum and bispectrum of the observed CMB maps. As for high-resolution CMB experiments, we find that the unresolved component of these sources could make up to ~40-50% of the observed CBI and BIMA power spectrum at l>2000. As a consequence, the inferred sigma_8^{SZ} value could be biased high by up to 6-7%. As for all sky experiments, we find that the contribution of accretion-flow sources to the WMAP bispectrum is at the 2-3 per cent level at most. At the flux limit that Planck will achieve, however, these sources may contribute up to 15 per cent of the bispectrum in the 60-100 GHz frequency range. Moreover, Planck should detect hundreds of these sources in the 30-300 GHz frequency window. These detections, possibly coupled with galaxy type confirmation from optical surveys, will allow number counts to put tighter constraints on early-type galaxies radio luminosity and accretion flows properties. These sources may also contribute up to the 30 per cent level to the residual radio sources power spectrum in future high-resolution SZ surveys (like ACT or APEX) reaching mJy flux limits.

Point sources in the MAP sky maps

We discuss point sources foregrounds for the MAP experiment. We consider several possible strategies for removing them and we assess how the statistics of the CMB signal are affected by the residual sources. Assuming a power law distribution for the point sources, we propose a method aimed to determine the slope of the distribution from the analysis of the moments of the observed maps. The same method allows for a determination of the underlying CMB variance. We conclude that the best strategy for point sources finding is the simultaneous thresholding of the filtered map at all frequencies, with a relatively low threshold. With this strategy, we expect to find 70 (95)% of the sources above 3 (4)

Point Sources in the Wilkinson Microwave Anisotropy Probe Sky Maps

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