An estimate of the local ISW signal, and its impact on CMB anomalies (0909.2495v3)

Abstract: We estimate the local density field in redshift shells to a maximum redshift of z=0.3, using photometric redshifts for the 2MASS galaxy catalogue, matched to optical data from the SuperCOSMOS galaxy catalogue. This density-field map is used to predict the Integrated Sachs-Wolfe (ISW) CMB anisotropies that originate within the volume at z<0.3. We investigate the impact of this estimated ISW foreground signal on large-scale anomalies in the WMAP CMB data. We find that removal of the foreground ISW signal from WMAP data reduces the significance of a number of reported large-scale anomalies in the CMB, including the low quadrupole power and the apparent alignment between the CMB quadrupole and octopole.

• The paper demonstrates that removing the estimated ISW signal increases quadrupole power and reduces the alignment of CMB multipoles.
• It employs a two-dimensional Wiener filtering method on photometric redshift data from 2MASS and SuperCOSMOS to reconstruct local density fields.
• The study shows that subtracting ISW contributions refines CMB analyses, while anomalies like octopole planarity persist, prompting further investigation.

An Estimate of the Local ISW Signal and its Impact on CMB Anomalies

The research by Francis and Peacock diligently addresses the influence of the Integrated Sachs-Wolfe (ISW) effect on cosmic microwave background (CMB) anomalies, a subject that has garnered attention in cosmology for its potential implications on our understanding of the universe. The paper's objective is to estimate the local ISW signal up to a redshift of $z=0.3$ utilizing photometric redshifts derived from the 2MASS galaxy catalog and matched optical data from SuperCOSMOS. This estimation aims to scrutinize its impact on observed CMB data anomalies, as reported by the Wilkinson Microwave Anisotropy Probe (WMAP).

Methodology

The research employs a two-dimensional Wiener reconstruction to project density fields in specified redshift shells. This is followed by the calculation of the predicted ISW CMB temperature fluctuations, utilizing the reconstructed density field. The density field is obtained by leveraging photometric redshift data in three distinct redshift slices, allowing the researchers to attempt a comprehensive recovery of the field while mitigating radial smearing effects caused by inherent uncertainties in galaxy radial positioning.

The applied technique hinges on Wiener filtering under a masked dataset condition, allowing an extrapolation of galaxy distribution characteristics across the Galactic plane. This methodological approach is highly suited to handle systematic imperfections in the data, such as those introduced by photometric redshifts.

Findings and Numerical Results

Upon successfully estimating the local ISW signal, the researchers examined its effects on several well-documented CMB anomalies:

1. Quadrupole Power: There is a noticeable increase in quadrupole power after removing the ISW foreground from the WMAP data. This adjustments render the quadrupole power from being statistically anomalous against expected values derived from model predictions. Specifically, the probability of the quadrupole power aligning with model expectations rises significantly post-ISW subtraction.
2. Alignment of CMB Multipoles: Analysis demonstrates that the alignment previously observed between the quadrupole and octopole is considerably reduced when the estimated ISW signal is accounted for. The relative alignment, as measured through scalar product and separation degrees, aligns more closely with distributions expected under Gaussian random fields.
3. Planarity of the Octopole: Despite expectations, the octopole's planarity persists and appears accentuated with ISW signal removal. This raises intriguing questions regarding the nature of these residual anomalies.
4. North-South Asymmetry: While the asymmetry itself remains, the alignment of the ecliptic plane with a node in the temperature distribution ceases post-subtraction, suggesting some of the noted asymmetry could be attributed to the ISW effect.
5. Cold Spot Analysis: The cold spot's anomaly is slightly reduced in significance after removing the local ISW contribution, though it remains an area of interest that may necessitate further investigation.

Implications and Future Directions

The paper's implication extends beyond a procedural advancement in CMB data analysis; it posits that ISW subtraction could serve as a pivotal step toward aligning observed CMB characteristics with theoretical Gaussian randomness and isotropy. This underlines the necessity of foreground assessments before overt commitments to model revisions regarding the primordial CMB.

Moreover, the paper raises crucial dialog around the potential interaction between intrinsic CMB signals and ISW foregrounds. While the removal of estimated ISW signals appears to alleviate various reported anomalous behaviors, it compels a re-examination of the assumptions on the intrinsic nature of CMB and its interaction with surrounding large-scale structures.

In proceeding years, improved surveys, such as LSST or future Euclid missions, may grant deeper insights, thus refining our ability to disentangle secondary foreground impacts, like those from the ISW effect, from the intrinsic CMB. This line of inquiry may provide vital clarity in addressing whether these observations point towards anomalies needing new physics-based explanations or are attributable to current cosmic structures' interactions.

In conclusion, Francis and Peacock's research contributes significantly to the discourse on CMB analysis by illustrating that calculated interventions in foreground estimation can sharpen the clarity with which cosmologists interpret cosmic signals, presenting a cleaner link between theory and observation.