We describe the state-of-the art status of multifrequency detection techniques for compact sources in microwave astronomy. From the simplest cases where the spectral behaviour is well known (i.e., thermal SZ clusters) to the more complex cases where there is little a priori information (i.e., polarized radio sources) we will review the main advances and the most recent results in the detection problem. 1. Introduction Extragalactic foregrounds play a crucial role in microwave astronomy, not only by their effect as contaminants of the Cosmic Microwave Background (CMB) but also by their own right as cosmological probes. Galaxies and galaxy clusters, if not properly identified and taken into account, can seriously affect the measurement of the CMB anisotropies angular power spectrum in temperature [1–3] and polarization [4, 5], CMB non-Gaussianity tests [6–12], and even the performance of component separation methods used for the study of Galactic foregrounds (on the other hand, the opposite is also true: foregrounds can affect the performance of compact source detection algorithms. In general, compact source detection algorithms find it easier to deal with diffuse foregrounds than diffuse component separation techniques to deal with compact sources, so the typical CMB analysis pipeline includes the detection of compact sources as a previous step to diffuse component separation) [13, 14]. On the other hand, galaxy and galaxy cluster surveys in the submm regime of the electromagnetic spectrum are powerful tools for cosmology [15–17]. This is the motivation of the considerable number of works on extragalactic foreground detection that have appeared in the literature on recent years. As opposed to Galactic foregrounds, that are typically extended as diffuse clouds over large areas of the sky, individual extragalactic objects appear as compact blobs that subtend very small angular scales. For this reason, both galaxies and galaxy clusters are often referred to as compact sources and their detection/separation is typically treated as a problem apart from the one posed by the separation of Galactic diffuse components. It is precisely the compactness of extragalactic sources that makes it possible to detect them against the fluctuations of the diffuse components (CMB included) in single-frequency (channel) settings. Most of the detection methods that have been proposed in the literature make use of this scale diversity. The well-known SExtractor package [18], for example, is particularly good at estimating and then subtracting the background at coarse scales and
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