Introducing constrained matched filters for improved separation of point sources from galaxy clusters

Abstract

Matched filters (MFs) are elegant and widely used tools to detect and measure signals that resemble a known template in noisy data. However, they can perform poorly in the presence of contaminating sources of similar or smaller spatial scale than the desired signal, especially if signal and contaminants are spatially correlated. We introduce new multicomponent MF and matched multifilter (MMF) techniques that allow for optimal reduction of the contamination introduced by sources that can be approximated by templates. The application of these new filters is demonstrated by applying them to microwave and X-ray mock data of galaxy clusters with the aim of reducing contamination by point-like sources, which are well approximated by the instrument beam. Using microwave mock data, we show that our method allows for unbiased photometry of clusters with a central point source but requires sufficient spatial resolution to reach a competitive noise level after filtering. A comparison of various MF and MMF techniques is given by applying them to Planck multifrequency data of the Perseus galaxy cluster, whose brightest cluster galaxy hosts a powerful radio source known as Perseus A. We also give a brief outline how the constrained MF (CMF) introduced in this work can be used to reduce the number of point sources misidentified as clusters in X-ray surveys like the upcoming eROSITA all-sky survey. A PYTHON implementation of the filters is provided by the authors of this manuscript at https://github.com/j-erler/pymf.

Measurement of the mean central optical depth of galaxy clusters via the pairwise kinematic Sunyaev-Zel’dovich effect with SPT-3G and DES

“…The first filter is a minimum variance multifrequency matched filter (MF-MF) with the 95, 150, and 220 GHz maps from SPT-3G. The second filter is a constrained minimum variance version (MF-TSZ), with the nonrelativistic TSZ effect nulled, following [50]. Third, following [23], we build a single-frequency matched filter for the 150 GHz map (MF-150 GHz); the 150 GHz map has the lowest noise level of the three frequency bands at 3.9 μK-arcmin.…”

Understanding matched filters for precision cosmology

“…Other potential sources of bias include, but are not limited to, the wrong modelling of the signal to be extracted, i.e., the matched filter not matching the true cluster tSZ signal, both spatially and spectrally, the latter possible if the relativistic SZ effect (e.g., Sazonov & Sunyaev 1998;Challinor & Lasenby 1998;Chluba et al 2012) is non-negligible, and the presence of other, 'foreground' signals such as the CMB, the kSZ effect, radio emission from the cluster's galaxies, and point sources (see, e.g., Mroczkowski et al 2019;Erler et al 2019). Any assessment of these potential sources of systematics is beyond the scope of this paper.…”

Validating Planck SZ2 clusters with optical counterparts

“…The colorbar at the bottom of the figure has units in mK. The masked sources are shown in blue.2005, Haehnelt and Tegmark 1996, Erler et al 2019 to estimate the signal-to-noise for SZ galaxy cluster candidates that are not included in the Planck PSZ2 catalog(Ade et al 2016) but are detected in one or more of the optical catalogs discussed above. The MMF code is only run on cluster candidates with a…”