Improved Measurements of the CMB Power Spectrum with ACBAR

TL;DR

ACBAR delivers improved measurements of the CMB temperature power spectrum at high multipoles by analyzing un-differenced maps and employing RCW38-based cross-calibration to the WMAP/BOOMERANG scale. The results support a spatially flat $\\Lambda$CDM cosmology with lensing, including a damping tail consistent with photon diffusion, and reveal a small excess at $\\ell>2000$ with a spectrum consistent with the thermal SZ effect when combined with CBI data. Extending the analysis to include SZ contributions and a running spectral index shows only modest shifts in cosmological inferences, with final constraints strengthened by lensing and cross-surveys; the final ACBAR data, with far greater integration time and sky coverage, will yield significantly tighter parameter bounds. Overall, the study demonstrates robust control of systematics and foregrounds, confirms the coherence of the acoustic structure across experiments, and highlights the SZ background as a key secondary anisotropy at small angular scales.

Abstract

We report improved measurements of temperature anisotropies in the cosmic microwave background (CMB) radiation made with the Arcminute Cosmology Bolometer Array Receiver (ACBAR). In this paper, we use a new analysis technique and include 30% more data from the 2001 and 2002 observing seasons than the first release to derive a new set of band-power measurements with significantly smaller uncertainties. The planet-based calibration used previously has been replaced by comparing the flux of RCW38 as measured by ACBAR and BOOMERANG to transfer the WMAP-based BOOMERANG calibration to ACBAR. The resulting power spectrum is consistent with the theoretical predictions for a spatially flat, dark energy dominated LCDM cosmology including the effects of gravitational lensing. Despite the exponential damping on small angular scales, the primary CMB fluctuations are detected with a signal-to-noise ratio of greater than 4 up to multipoles of l=2000. This increase in the precision of the fine-scale CMB power spectrum leads to only a modest decrease in the uncertainties on the parameters of the standard cosmological model. At high angular resolution, secondary anisotropies are predicted to be a significant contribution to the measured anisotropy. A joint analysis of the ACBAR results at 150 GHz and the CBI results at 30 GHz in the multipole range 2000 < l < 3000 shows that the power, reported by CBI in excess of the predicted primary anisotropy, has a frequency spectrum consistent with the thermal Sunyaev-Zel'dovich effect and inconsistent with primary CMB. The results reported here are derived from a subset of the total ACBAR data set; the final ACBAR power spectrum at 150 GHz will include 3.7 times more effective integration time and 6.5 times more sky coverage than is used here.

Figures (9)

• Figure 1: The de-correlated ACBAR band-powers for two alternate binnings. These two binnings are not independent, therefore only one set is shown with error bars, which correspond to 1-$\sigma$ uncertainties calculated from the offset lognormal fits to the likelihood function. Both the overall features and the damping scale are in good agreement with predictions from a flat, low baryon density $\Lambda$CDM Universe. The third acoustic peak (around $\ell=800$) is clearly seen. Small scale primary CMB fluctuations are detected with high signal-to-noise ratio ($>4$) up to $l=2000$. The plotted model line is the best fit to the WMAP3 and ACBAR bandpowers.
• Figure 2: Systematic tests performed on the ACBAR data. Top: Power spectrum produced from the difference of maps made with left- and right-going chopper sweeps. Middle: Power spectrum (diamonds) calculated from the difference of maps made from the first and second halves of the data for each field (CMB2, CMB5, CMB6, and CMB7), compared with Monte Carlo simulations (error bars). Bottom: Power spectrum calculated from the LMT sum maps (squares), compared with the joint power spectrum (pre-decorrelated, filled circles). The consistency between two sets of band-powers demonstrates that the residual chopper offsets are below noise. See text for more detail on this test.
• Figure 3: The ACBAR band powers plotted with those from WMAP3 hinshaw06 and the 2003 flight of BOOMERANG jones06. The three experiments show excellent agreement in the overlapped region.
• Figure 4: ACBAR results on the high-$\ell$ anisotropies. Left: The ACBAR band-powers at $\ell >1000$, plotted on a logarithmic scale with the latest CBI data taken at a frequency of $30\,$GHz. All the ACBAR bins at $\ell >2000$ are lower than the CBI band-power measurement. Right: The likelihood distribution for the ratio of the "excess" power, observed by CBI at $30\,$GHz and ACBAR at $150\,$GHz. The excess for each experiment is defined as the difference of the measured band-powers and the model band-powers at $\ell>2000$. The vertical dashed line represents the expected ratio (4.3) for the excess being due to the SZ effect. If the excess power seen in CBI is caused by non-standard primordial processes, the ratio will be unity (blackbody), indicated by the dotted line. We conclude that it is 4.5 times more likely that the excess seen by CBI and ACBAR is caused by the thermal SZ effect than a primordial source. In addition, because of the weak detection of excess power in ACBAR ($1.2\sigma$), it is about 3 times more likely that the excess is due to the SZ effect than radio source contamination of the lower frequency CBI data, assuming no contaminations from dusty proto-galaxies.
• Figure 5: Basic parameter marginalized 1-dimensional likelihood distributions for the following data combinations; WMAP3 only (black, solid), ACBAR + WMAP3 (red, dashed), CMBall (green, long-dashed), and CMBall + LSS (blue, dash-dot). All runs include lensing.