Atacama Cosmology Telescope Research Articles

Prospects for ACT: Simulations, power spectrum, and non-Gaussian analysis

A new generation of instruments will reveal the microwave sky at high resolution. We focus on one of these, the Atacama Cosmology Telescope, which probes scales 1000<l<10,000, where both primary and secondary anisotropies are important. Including gravitational lensing, thermal and kinetic Sunyaev–Zeldovich (SZ) effects, and extragalactic point sources, we simulate the telescope’s observations of the CMB in three channels, then extract the power spectra of these components in a multifrequency analysis. We present results for various cases, differing in assumed knowledge of the contaminating point sources. We find that both radio and infrared point sources are important, but can be effectively eliminated from the power spectrum given three (or more) channels and a good understanding of their frequency dependence. However, improper treatment of the scatter in the point source frequency dependence relation may introduce a large systematic bias. Even if all thermal SZ and point source effects are eliminated, the kinetic SZ effect remains and corrupts measurements of the primordial slope and amplitude on small scales. We discuss the non-Gaussianity of the one-point probability distribution function as a way to constrain the kinetic SZ effect, and we develop a method for distinguishing this effect from the CMB in a window where they overlap. This method provides an independent constraint on the variance of the CMB in that window and is complementary to the power spectrum analysis.

Reconstructing Sunyaev-Zel'dovich clusters in future cosmic microwave background experiments

We present a new method for component separation aimed at extracting Sunyaev-Zel'dovich (SZ) galaxy clusters from multifrequency maps of cosmic microwave background (CMB) experiments. This method is designed to recover non-Gaussian, spatially localized and sparse signals. We first characterize the cluster non-Gaussianity by studying it on simulated SZ maps. We then apply our estimator on simulated observations of the Planck and Atacama Cosmology Telescope (ACT) experiments. The method presented here outperforms multifrequency Wiener filtering, both in the reconstructed average intensity for given input and in the associated error. In the absence of point source contamination, this technique reconstructs the ACT (Planck) bright (big) cluster central y parameter with an intensity that is about 84 (43) per cent of the original input value. The associated error in the reconstruction is about 12 and 27 per cent for the 50 (12) ACT (Planck) clusters considered. For ACT, the error is dominated by beam smearing. In the Planck case, the error in the reconstruction is largely determined by the noise level: a noise reduction by a factor of 7 would imply almost perfect reconstruction and 10 per cent error for a large sample of clusters. We conclude that the selection function of Planck clusters will strongly depend on the noise properties in different sky regions, as well as the specific cluster extraction method assumed.

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 per cent of the observed Cosmic Background Imager (CBI) and Berkeley Illinois Maryland Association (BIMA) array power spectrum at l > 2000. As a consequence, the inferred σ SZ 8 value could be biased high by up to 6-7 per cent. As for all-sky experiments, we find that the contribution of accretion-flow sources to the Wilkinson Microwave Anisotropy Probe (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 the radio luminosity and accretion-flow properties of early-type galaxies. These sources may also contribute up to the 30 per cent level to the residual radio sources power spectrum in future high-resolution Sunyaev-Zeldovich (SZ) surveys (like the Atacama Cosmology Telescope or the Atacama Pathfinder Experiment) reaching mJy flux limits.

The effect of cluster magnetic field on the Sunyaev-Zel'dovich power spectrum

Precision measurements of the Sunyaev–Zel'dovich (SZ) effect in upcoming blank sky surveys require theoretical understanding of all physical processes with ≳10 per cent effects on the SZ power spectrum. We show that the observed cluster magnetic field could reduce the SZ power spectrum by ∼20 per cent at l∼ 4000, where the SZ power spectrum will be precisely measured by the Sunyaev–Zel'dovich array (SZA) and the Atacama Cosmology Telescope (ACT). At smaller scale, this effect is larger and could reach a factor of several. Such an effect must be considered for an unbiased interpretation of the SZ data. Although the magnetic effect on the SZ power spectrum is very similar to that of radiative cooling, it is measurable by multiband cosmic microwave background polarization measurement.

Point Sources in the Context of Future SZ Surveys

We look at the impact of infrared (IR) and radio point sources on upcoming large-yield Sunyaev-Zel'dovich (SZ) cluster surveys such as those to be undertaken by the Atacama Pathfinder Experiment (APEX), the South Pole Telescope (SPT), and the Atacama Cosmology Telescope (ACT). The IR and radio point-source counts are based on observations by the Submillimeter Common-User Bolometric Array (SCUBA) and the Wilkinson Microwave Anisotropy Probe (WMAP) instruments, respectively. We show that the contributions from IR-source counts, when extrapolated from the SCUBA frequency of 350 GHz to the operating frequencies of these surveys (~100-300 GHz), can be a significant source of additional noise, which needs to be accounted for in order to extract the optimal science from these surveys. These surveys give us an opportunity to study IR sources, their numbers and clustering properties, opening a new window to the high-redshift universe. For the radio point sources, the contribution depends on a more uncertain extrapolation from 40 GHz, but is comparable to the IR near 200 GHz. However, the radio signal may be correlated with clusters of galaxies and have a disproportionately larger effect.

Precision era of the kinetic Sunyaev-Zel'dovich effect: simulations, analytical models and observations and the power to constrain reionization

The kinetic SZ effect, which is the dominant CMB source at arc-minute scales and $\nu \sim 217$ Ghz, probes the ionized gas peculiar momentum up to the epoch of reionization and is a sensitive measure of the reionization history. We ran high resolution self-similar and $\Lambda$CDM hydro simulations and built an analytical model to study this effect. Our model reproduces the $\Lambda$CDM simulation results to several percent accuracy, passes various tests against self-similar simulations, and shows a wider range of applicability than previous analytical models. Our model in its continuous version is free of simulation limitations such as finite simulation box and finite resolution and allows an accurate prediction of the kinetic SZ power spectrum $C_l$. For the WMAP cosmology, we find $l^2C_l/(2\pi)\simeq 0.91 \times 10^{-12} [(1+z_{\rm reion})/10]^{0.34}(l/5000)^{0.23-0.015(z_{\rm reion}-9)}$ for the reionization redshift $6<z_{\rm reion}<20$ and $3000<l<9000$. The corresponding temperature fluctuation is several $\mu$K at these ranges. The dependence of $C_l$ on the reionization history allows an accurate measurement of the reionization epoch. For the Atacama cosmology telescope experiment, $C_l$ can be measured with $\sim 1%$ accuracy. $C_l$ scales as $(\Omega_b h)^2 \sigma_8^{4\sim 6}$. Given cosmological parameters, ACT would be able to constrain $z_{\rm reion}$ with several percent accuracy. Some multi-reionization scenarios degenerate in the primary CMB temperature and TE measurement can be distinguished with $\sim 10 \sigma$ confidence.

The Atacama Cosmology Telescope

The proposed Atacama Cosmology Telescope (ACT) project is described. This multi-institution collaboration aims to produce arcminute-resolution and micro-Kelvin sensitivity maps of the microwave background temperature over 200 square degree of the sky in three frequency bands. We give a brief overview of the scientific motivations for such a map, followed by a design outline of our 6-m custom telescope, an overview of our proposed bolometer array detector technology, and site considerations and scan strategy. We also describe associated optical and X-ray galaxy cluster surveys.

Dark energy and cosmic microwave background bispectrum

We compute the cosmic microwave background bispectrum arising from the cross-correlation of primordial, lensing, and Rees-Sciama signals. The amplitude of the bispectrum signal is sensitive to the matter density parameter ${\ensuremath{\Omega}}_{0}$ and the equation of state of the dark energy, which we parametrize by ${w}_{Q}.$ We conclude that the data set of the Atacama Cosmology Telescope, combined with MAP 2-year data, or the Planck data set alone will allow us to break the degeneracy between ${\ensuremath{\Omega}}_{0}$ and ${w}_{Q}$ that arises from the analysis of the CMB power spectrum. In particular a joint measurement of ${\ensuremath{\Omega}}_{0}$ and ${w}_{Q}$ with 10% and 30% error on the two parameters, respectively, at the 90% confidence level can realistically be achieved.