[Paper Review] WMAP anomalous signal in the ecliptic plane

J. M. Diego, M. Cruz+4 more|arXiv (Cornell University)|Jan 27, 2009

Cosmology and Gravitation Theories1 references14 citations

TL;DR

The paper identifies a 7-sigma, large-scale anomalous signal in WMAP 5-year data correlated with the ecliptic plane, suggesting a combination of unresolved extragalactic sources and modified Zodiacal light emission (ZLE) with a quasi-blackbody spectrum. Neither ZLE nor unresolved sources alone explain the signal, but a model with a ZLE-like spatial distribution and blackbody-like spectrum can account for both the anomalous signal and the low-l cosmic microwave background anomalies, including the suppressed quadrupole and its alignment with the octupole.

ABSTRACT

We report the detection of a high Galactic latitude, large scale, 7-sigma signal in WMAP 5yr and spatially correlated with the ecliptic plane. Two possible candidates are studied, namely unresolved sources and Zodiacal light emission. We determine the strength of the Zodiacal light emission at WMAP frequencies and estimate the contribution from unresolved extragalactic sources. Neither the standard Zodiacal light emission nor the unresolved sources alone seem to be able to explain the observed signal. Other possible interpretations like Galactic foregrounds and diffuse Sunyaev-Zel'dovich effect also seem unlikely. We check if our findings could affect the low-l anomalies which have been reported in the WMAP data. Neither Zodiacal light emission nor unresolved point source residuals seem to affect significantly the quadrupole and octupole measurements. However, a signal with a quasi-blackbody spectrum and with a spatial distribution similar to the Zodiacal light emission, could explain both the anomalous signal and the low-ell anomalies. Future data (Planck) will be needed in order to explain the origin of this signal.

Motivation & Objective

• To identify and characterize a previously undetected large-scale anomalous signal in WMAP 5-year data at high Galactic latitudes.
• To determine whether the signal arises from unresolved extragalactic sources or Zodiacal light emission (ZLE).
• To assess whether this signal could explain the low-l cosmic microwave background anomalies, such as the suppressed quadrupole and its alignment with the octupole.
• To evaluate the impact of residual ZLE and unresolved sources on the measurement of the CMB quadrupole and octupole in WMAP data.

Proposed method

• Analyzed WMAP 5-year data using the combination map V+W−2Q to isolate large-scale residuals.
• Modeled the frequency dependence of the signal as ν^2.1, approximating a blackbody spectrum, and tested its spatial correlation with the ecliptic plane.
• Estimated the expected ZLE intensity at WMAP frequencies using modified dust emissivity laws, accounting for deviations from the λ^−2 law at long wavelengths.
• Quantified the contribution of unresolved extragalactic sources via statistical modeling of residual point-source emission.
• Used internal linear combination (ILC) maps with standard WMAP weights to simulate the signal's effect on CMB multipoles.
• Compared the quadrupole and octupole power spectra of the hypothetical signal with observed WMAP values to assess consistency.

Experimental results

Research questions

• RQ1 Can the observed 7-sigma anomalous signal in the WMAP 5-year data be explained by standard foreground components such as ZLE or unresolved extragalactic sources?
• RQ2 Does the spatial and spectral behavior of the anomalous signal match that of ZLE or a modified dust emission model?
• RQ3 Could a signal with a quasi-blackbody spectrum and ZLE-like spatial distribution explain both the anomalous signal and the low-l CMB anomalies (e.g., suppressed quadrupole)?
• RQ4 To what extent do residual ZLE or unresolved source emissions affect the measured CMB quadrupole and octupole in WMAP data?

Key findings

• A high-significance (7-sigma) large-scale signal is detected in WMAP 5-year data, spatially correlated with the ecliptic plane and strongest at high Galactic latitudes.
• Neither standard ZLE emission nor unresolved extragalactic sources alone can account for the observed signal amplitude, which exceeds predictions by a factor of several.
• A hypothetical signal with a quasi-blackbody spectrum (ν^2.1 dependence) and ZLE-like spatial distribution can explain the observed redder zones in the residual map.
• This hypothetical signal suppresses the measured CMB quadrupole by nearly a factor of two, explaining its anomalously low amplitude in WMAP data.
• The signal is anti-correlated with the observed CMB quadrupole, and its inclusion would restore the quadrupole amplitude to near-theoretical expectations.
• The signal affects only even multipoles significantly and may contribute to the observed alignment between the quadrupole and octupole, though odd multipoles like the octupole are minimally impacted.