[Paper Review] Extragalactic point source detection in WMAP 7-year data at 61 and 94 GHz

L. Lanz, D. Herranz+5 more|arXiv (Cornell University)|Oct 31, 2011

Radio Astronomy Observations and Technology4 references3 citations

TL;DR

This paper presents a multi-frequency matched multifilter (MMF) method to detect extragalactic point sources in WMAP 7-year data at 61 and 94 GHz, leveraging spatial and spectral correlations across frequencies without assuming source spectral behavior. The method detects 129 $5\sigma$ sources at $|b|>5^\circ$, with 119 confirmed as extragalactic and 104 outside the WMAP Point Source Catalog mask, significantly improving upon prior catalogs like NEWPS-3year.

ABSTRACT

The detection of point sources in Cosmic Microwave Background maps is usually based on a single-frequency approach, whereby maps at each frequency are filtered separately and the spectral information on the sources is derived combining the results at the different frequencies. On the contrary, in the case of multi-frequency detection methods, source detection and spectral information are tightly interconnected in order to increase the source detection efficiency. In this work we apply the \emph{matched multifilter} method to the detection of point sources in the WMAP 7yr data at 61 and 94 GHz. This linear filtering technique takes into account the spatial and the cross-power spectrum information at the same time using the spectral behaviour of the sources without making any a priori assumption about it. We follow a two-step approach. First, we do a blind detection of the sources over the whole sky. Second, we do a refined local analysis at their positions to improve the signal-to-noise ratio of the detections. At 94 GHz we detect 129 5σ objects at |b|>5° (excluding the Large Magellanic Cloud region); 119 of them are reliable extragalactic sources and 104 of these 119 lie outside the WMAP Point Source Catalog mask. Nine of the total 129 detections are known Galactic sources or lie in regions of intense Galactic emission and one additional (weak) high-Galactic latitude source has no counterpart in low-frequency radio catalogues. Our results constitute a substantial improvement over the NEWPS-3year catalogue.

Motivation & Objective

• To improve extragalactic point source detection in CMB maps by overcoming limitations of single-frequency methods.
• To develop and apply a multi-frequency detection technique that jointly uses spatial and spectral information without assuming source spectral behavior.
• To produce a more complete and reliable source catalog at 61 and 94 GHz than previous WMAP-based catalogs.
• To assess the reliability and flux density accuracy of detections using cross-identification with low-frequency radio catalogs and Planck data.
• To characterize the spectral properties of detected sources, particularly the 61–94 GHz spectral index distribution.

Proposed method

• The matched multifilter (MMF) method is applied to WMAP 7-year maps at 61 and 94 GHz, using a linear filtering technique that incorporates both spatial and cross-power spectrum information.
• The method assumes no a priori spectral model for sources, relying instead on the known frequency dependence of the signal through the filter's design.
• A two-step detection strategy is used: first, blind detection over the entire sky; second, refined local analysis at detected source positions to enhance signal-to-noise ratio.
• The MMF method jointly optimizes detection significance across both frequency channels by modeling the covariance structure of the signal and noise.
• Source flux densities are estimated using the MMF filter response, with error estimates derived from the filter's noise properties.
• Results are validated by cross-identification with low-frequency radio catalogs and comparison with Planck ERCSC data.

Experimental results

Research questions

• RQ1 Can a multi-frequency, non-parametric filtering method detect more extragalactic point sources in WMAP data than single-frequency approaches?
• RQ2 What is the spectral index distribution of extragalactic radio sources between 61 and 94 GHz, and does it confirm spectral steepening at higher frequencies?
• RQ3 How do the flux density estimates from the MMF method compare with ground-based and Planck measurements, and what are the implications for measurement errors?
• RQ4 To what extent do the MMF detections improve upon existing catalogs like NEWPS-3year and the WMAP Point Source Catalog mask?
• RQ5 Are there sources detected by MMF that are not present in the Planck ERCSC, and what does this imply about source completeness and variability?

Key findings

• The MMF method detects 129 $5\sigma$ sources at $|b|>5^\circ$ in WMAP 7-year data at 61 and 94 GHz, excluding the Large Magellanic Cloud region.
• Of these, 119 are confirmed as extragalactic sources, with 104 lying outside the WMAP Point Source Catalog mask, indicating a substantial improvement in source completeness.
• The median 61–94 GHz spectral index for detected extragalactic sources is $\gamma_{61}^{94} = -0.65$, with a dispersion of 0.71, confirming spectral steepening above 70 GHz.
• The flux density estimates from MMF show generally good agreement with ground-based (ATCA, NRAO 12m) and Planck ERCSC data above 2 Jy, but with larger-than-expected rms differences, suggesting overestimated errors in MMF flux estimates.
• Below 2 Jy, MMF flux densities tend to be overestimated due to Eddington bias, consistent with statistical expectations for faint source populations.
• Three sources detected by MMF are not present in the Planck ERCSC, yet their flux densities are consistent with lower-frequency measurements, indicating potential for detecting previously missed sources.