The Atacama Cosmology Telescope: Cosmological Parameters from the 2008 Power Spectra

TL;DR

The paper presents cosmological constraints from ACT 2008 power spectra at 148 and 218 GHz, modeling primary CMB, thermal and kinetic SZ effects, and foregrounds to extract precise small-scale information. Using a joint ACT+WMAP analysis with BAO/H_0 priors, the authors constrain ΛCDM and extensions, measuring a negative running dn_s/dlnk and placing a rigorous upper bound on r, while obtaining a 6σ detection of primordial helium (Y_P=0.313±0.044) and a 4σ hint of extra relativistic species (N_eff=5.3±1.3); the data also limit cosmic string tensions (Gμ<1.6×10^-7). A multi-template SZ analysis yields SZ power B_{3000}^{SZ}=6.8±2.9 μK^2 with templates spanning gas physics, and a 5σ detection of IR clustering, highlighting the power of small-scale CMB data to probe cluster physics and dusty galaxy populations. Collectively, these results reinforce the ΛCDM framework, sharpen small-scale cosmological constraints, and demonstrate the synergy between ACT, WMAP, and low-redshift priors for precision cosmology.

Abstract

We present cosmological parameters derived from the angular power spectrum of the cosmic microwave background (CMB) radiation observed at 148 GHz and 218 GHz over 296 deg^2 with the Atacama Cosmology Telescope (ACT) during its 2008 season. ACT measures fluctuations at scales 500<l<10000. We fit a model for the lensed CMB, Sunyaev-Zel'dovich (SZ), and foreground contribution to the 148 GHz and 218 GHz power spectra, including thermal and kinetic SZ, Poisson power from radio and infrared point sources, and clustered power from infrared point sources. The power from thermal and kinetic SZ at 148 GHz is estimated to be B_3000 = 6.8+-2.9 uK^2, where B_l=l(l+1)C_l/2pi. We estimate primary cosmological parameters from the 148 GHz spectrum, marginalizing over SZ and source power. The LCDM cosmological model is a good fit to the data, and LCDM parameters estimated from ACT+WMAP are consistent with the 7-year WMAP limits, with scale invariant n_s = 1 excluded at 99.7% CL (3sigma). A model with no CMB lensing is disfavored at 2.8sigma. By measuring the third to seventh acoustic peaks, and probing the Silk damping regime, the ACT data improve limits on cosmological parameters that affect the small-scale CMB power. The ACT data combined with WMAP give a 6sigma detection of primordial helium, with Y_P = 0.313+-0.044, and a 4sigma detection of relativistic species, assumed to be neutrinos, with Neff = 5.3+-1.3 (4.6+-0.8 with BAO+H0 data). From the CMB alone the running of the spectral index is constrained to be dn/dlnk = -0.034 +- 0.018, the limit on the tensor-to-scalar ratio is r<0.25 (95% CL), and the possible contribution of Nambu cosmic strings to the power spectrum is constrained to string tension Gmu<1.6 \times 10^-7 (95% CL).

Figures (13)

• Figure 1: Thermal SZ templates for four different models considered in this analysis, and a single kSZ template, normalized at 148 GHz for cosmologies with $\sigma_8=0.8$. The 'TBO-1' template is from sehgal/etal:2010a, described further in *trac/bode/ostriker:prep together with 'TBO-2', derived from N-body simulations. The 'Battaglia' model is derived from hydrodynamic SPH simulations battaglia/etal:prep. The 'Shaw' model is based on an analytic halo model shaw/etal:prep. The 'kSZ' template is the kinetic SZ template in sehgal/etal:2010a. Two clustered IR source templates considered ('Src-1' and 'Src-2') are described in Sec 2.1.3 and normalized at $\ell=3000$. The IR source curves are multiplied by ten for clarity.
• Figure 2: The angular power spectrum measured by ACT at 148 GHz and 218 GHz das/etal:prep, with the theoretical model for CMB, SZ, and point sources best-fit to the three spectra. The lensed CMB corresponds to the $\Lambda$CDM model with parameters derived from WMAPkomatsu/etal:prep. It dominates at large scales, but falls exponentially due to Silk damping. The majority of power at $\ell>3000$ comes from extragalactic point sources below a $\approx$20 mJy flux cut after masking. The radio sources are sub-dominant, and are constrained by a source model fit to detected sources at 148 GHz marriage/etal:2010a. The infrared source emission, assumed to follow a power law, is dominated by Poisson power at small scale, but about 1/3 of the IR power at $\ell=3000$ is attributed to clustered source emission, assuming a template described in the text. The best-fit SZ (thermal and kinetic) contribution at 148 GHz (assuming the TBO-1 template, sehgal/etal:2010a) is $7 \mu {\rm K}^2$ at $\ell=3000$; the subdominant kinetic SZ also contributes at 218 GHz. The data spectra and errors have been scaled by best-fit calibration factors of $1.02^2$, $1.02\times1.09$ and $1.09^2$ for the $148\times148$, $148\times218$, and $218\times218$ spectra respectively.
• Figure 3: One-dimensional marginalized distributions for the estimated thermal SZ power in the ACT power spectra. There is evidence at the 95% CL level for non-zero SZ power. The value $A_{\rm tSZ}=1$ corresponds to the predicted thermal SZ amplitude in a universe with $\sigma_8=0.8$. The four curves correspond to the four SZ templates shown in Figure 1; the TBO-1 template results in a lower value, although all are consistent with $A_{\rm tSZ}=1$ at the 95% CL. The total SZ power (including kSZ) at 148 GHz and $\ell=3000$ for all the templates is consistent, with $\ell(\ell+1)C^{\rm SZ}_\ell/2\pi=7\pm3$$\mu {\rm K}^2$.
• Figure 4: Marginalized distributions (68% and 95% CL) for parameters describing the SZ and point source emission in the ACT power spectra. Left and center: The degeneracies between the total SZ power, ${\cal B}_{\ell}^{\rm SZ}\equiv \ell(\ell+1)C_\ell^{\rm SZ}$, and the infrared point source power, ${\cal B}_{\ell}^{\rm IR}$, at 148 GHz and $\ell=3000$ (solid unfilled contours), are broken with the addition of 218 GHz data (solid filled contours). Both the Poisson and clustered IR power are shown, for two different clustered source templates (solid and dashed contours). A clustered source component is required to fit the multi-frequency data at 5$\sigma$ significance. Right: The Poisson dust power and the index $\alpha_d=3.69\pm0.14$ (power law in flux between 148 GHz and 218 GHz, and unconstrained from 148 GHz alone) are anti-correlated; the index indicates a dust emissivity of $\beta\approx 1.7$.
• Figure 5: The power spectrum measured by ACT at 148 GHz, scaled by $\ell^4$, over the range dominated by primordial CMB ($\ell<3000$). The spectrum is consistent with the WMAP power spectrum over the scales $500<\ell<1000$, and gives a measure of the third to seventh acoustic peaks. The best-fit $\Lambda$CDM cosmological model is shown, and is a good fit to the two datasets. At $\ell>2000$ the contribution from point soures and SZ becomes significant (dashed shows the total best-fit theoretical spectrum; solid is lensed CMB). Three additional theoretical models for the primordial CMB are shown with $N_{\rm eff}$=10 relativistic species, $^4$He fraction $Y_p=0.5$, and running of the spectral index $dn_s/d \ln k=-0.075$. They are consistent with WMAP but are excluded at least at the 95% level by the ACT data.