Abstract
The combination of X-ray and Sunyaev–Zeldovich (SZ) observations can potentially improve the cluster detection efficiency, when compared to using only one of these probes, since both probe the same medium, the hot ionized gas of the intra-cluster medium. We present a method based on matched multifrequency filters (MMF) for detecting galaxy clusters from SZ and X-ray surveys. This method builds on a previously proposed joint X-ray–SZ extraction method and allows the blind detection of clusters, that is finding new clusters without knowing their position, size, or redshift, by searching on SZ and X-ray maps simultaneously. The proposed method is tested using data from the ROSAT all-sky survey and from the Planck survey. The evaluation is done by comparison with existing cluster catalogues in the area of the sky covered by the deep SPT survey. Thanks to the addition of the X-ray information, the joint detection method is able to achieve simultaneously better purity, better detection efficiency, and better position accuracy than its predecessor Planck MMF, which is based on SZ maps alone. For a purity of 85%, the X-ray–SZ method detects 141 confirmed clusters in the SPT region; to detect the same number of confirmed clusters with Planck MMF, we would need to decrease its purity to 70%. We provide a catalogue of 225 sources selected by the proposed method in the SPT footprint, with masses ranging between 0.7 and 14.5 ×1014 M⊙ and redshifts between 0.01 and 1.2.
Highlights
Galaxy clusters can be detected from observations at different bands of the electromagnetic spectrum, each of them probing a different component of the cluster
In these images we observe the emission of the hot gas of the intracluster medium (ICM), which accounts for 10–15% of the cluster mass
We should keep in mind that there could be some clusters in the transition region that are neither in SPT nor in MCXC
Summary
Galaxy clusters can be detected from observations at different bands of the electromagnetic spectrum, each of them probing a different component of the cluster. In our previous work (Tarrío et al 2016), we proposed a new analysis tool based on matched multifrequency filters (MMF) for extracting clusters from SZ and X-ray maps. If an object does not follow the expected relation, it could still pass the detection threshold if it has a very strong signal in the X-ray band This is similar to the strong infrared emissions that were detected with the classical SZ MMF used by the Planck Collaboration VIII (2011); Planck Collaboration XXIX (2014); Planck Collaboration XXVII (2016) even though their spectra did not fit the expected SZ spectrum. This provides a better S/N for the clusters because the SOFT band is dominated by the diffuse X-ray background of the local bubble This is a common choice in cluster detection surveys based on RASS data, such as REFLEX (Böhringer et al 2001, 2013)
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