[Paper Review] Can the WMAP Haze really be a signature of annihilating neutralino dark matter?

Daniel T. Cumberbatch, Joe Zuntz+2 more|ArXiv.org|Jan 31, 2009

Dark Matter and Cosmic Phenomena2 references16 citations

TL;DR

This paper re-evaluates the hypothesis that the WMAP Haze—excess microwave emission from the Galactic center—arises from synchrotron radiation produced by relativistic electrons and positrons from annihilating neutralino dark matter. Using Gibbs and ILC CMB estimators with multi-linear foreground fitting, it finds that a regional variation in soft synchrotron emission's spectral index better explains the haze than dark matter annihilation, which would require extreme boost factors (300–1700) to match observations.

ABSTRACT

Observations by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite have identified an excess of microwave emission from the centre of the Milky Way. It has been suggested that this WMAP haze emission could potentially be synchrotron emission from relativistic electrons and positrons produced in the annihilations of one (or more) species of dark matter particles. In this paper we re-calculate the intensity and morphology of the WMAP haze using a multi-linear regression involving full-sky templates of the dominant forms of galactic foreground emission, using two different CMB sky signal estimators. The first estimator is a posterior mean CMB map, marginalized over a general foreground model using a Gibbs sampling technique, and the other is the ILC map produced by the WMAP team. Earlier analyses of the WMAP haze used the ILC map, which is more contaminated by galactic foregrounds than the Gibbs map. In either case, we re-confirm earlier results that a statistically significant residual emission remains after foreground subtraction that is concentrated around the galactic centre. However, we find that the significance of this emission can be significantly reduced by allowing for a subtle spatial variation in the frequency dependence of soft synchrotron emission in the inner and outer parts of the galaxy. We also re-investigate the prospect of a neutralino dark matter interpretation of the origin of the haze, and find that significant boosting in the dark matter annihilation rate is required, relative to that obtained with a smooth galactic dark matter distribution, in order to reproduce the inferred residual emission, contrary to that deduced in several recent studies.

Motivation & Objective

• To reassess whether the WMAP Haze can be explained by synchrotron emission from dark matter annihilations.
• To investigate whether the residual microwave emission after foreground subtraction is better explained by spatially varying soft synchrotron emission.
• To evaluate the viability of neutralino dark matter as the source of the haze using updated CMB estimators and multi-template fitting.
• To quantify the required boost factors in the dark matter annihilation rate to match observed haze emission.
• To compare results using two distinct CMB signal estimators: Gibbs sampling and ILC, assessing their impact on residual emission significance.

Proposed method

• Employed a multi-linear regression with full-sky templates for free-free, dust, and synchrotron emission to model Galactic foregrounds.
• Used two CMB sky signal estimators: a posterior mean CMB map from Gibbs sampling (more robust) and the WMAP team's ILC map.
• Split the synchrotron template into inner and outer Galactic regions with independent spectral indices to model regional spectral variations.
• Fitted the angular distribution of synchrotron emission from dark matter annihilation to the residual haze emission in the K-band (23 GHz).
• Calculated required boost factors in the dark matter annihilation cross-section to match observed synchrotron flux, using four benchmark SUSY neutralino models.
• Applied HEALPix for sky mapping and LAMBDA for data access, ensuring high-fidelity analysis of WMAP data.

Experimental results

Research questions

• RQ1 Can the WMAP Haze be explained by a regional variation in the spectral index of soft synchrotron emission?
• RQ2 Does the use of a more robust CMB estimator (Gibbs) reduce the significance of the residual haze emission compared to the ILC estimator?
• RQ3 What boost factors in the dark matter annihilation rate are required to reproduce the observed haze emission in neutralino annihilation models?
• RQ4 How do different dark matter density profiles and neutralino compositions affect the predicted synchrotron emission morphology?
• RQ5 Is the angular distribution of the haze emission consistent with predictions from neutralino annihilation, given the required boost factors?

Key findings

• Allowing for a spatially varying spectral index in soft synchrotron emission reduced the residual K-band emission by approximately 46% in the inner 50° of the Galactic center when using the Gibbs CMB estimator.
• The residual emission significance was significantly reduced when accounting for regional differences in synchrotron spectral behavior, suggesting a conventional astrophysical origin over exotic dark matter.
• Boost factors of 372 (model 1) to 1701 (model 4) were required in neutralino dark matter models to reproduce the observed haze emission, indicating extreme enhancements in annihilation rates.
• The angular dependence of the synchrotron emission from dark matter was well-fit by the data across all four neutralino models, with χ² values ranging from 82.1% to 98.5% of model 1’s value.
• The boost factor dependence on neutralino mass was consistent with expectations: lighter neutralinos required smaller boost factors due to higher intrinsic annihilation rates scaling as m_χ⁻².
• The results were largely insensitive to the choice of CMB estimator, but the Gibbs map was deemed more trustworthy due to its statistical robustness and lower foreground contamination.