Search and analysis of small scale structures in two X-ray clusters of galaxies

Wepresent a refinement of the wavelet analysis technique for the detection and characterisation of small scale features embedded in a strongly varying background. This technique handles with particular care the side effects of nonorthogonality in the wavelet space which can cause spurious detections and lead to a biased estimate of source parameters. This novel technique is applied to two ROSAT PSPC pointed observations of nearby clusters of galaxies, A 1367 and A 194. We find evidence that the case of A 1367 is not unique and that galaxy-scale X-ray emission could be a quite common property of clusters of galaxies. We detect 28 sources in the field of A 1367 and 26 in the field of A 194. Since these numbers are significantly larger than those expected from the logN − logS relation in the field, most of the sources are expected to be associated with the cluster itself and indeed several identifications with galaxies are possible. In addition, CCD observations have revealed that two X-ray sources in the field of A 194, classified as extended by the multi-scale analysis, are very likely associated with two background galaxy clusters at intermediate redshift.