A new X-ray-selected sample of very extended galaxy groups from the ROSAT All-Sky Survey

Context. Some indications for tension have long been identified between cosmological constraints obtained from galaxy clusters and primary cosmic microwave background (CMB) measurements. Typically, assuming the matter density and fluctuations, as parameterized with Omega(m) and sigma(8), estimated from CMB measurements, many more clusters are expected than those actually observed. This has been reinforced recently by the Planck collaboration. One possible explanation could be that certain types of galaxy groups or clusters were missed in samples constructed in previous surveys, resulting in a higher incompleteness than estimated. Aims. In this work, we aim to determine if a hypothetical class of very extended, low-surface-brightness galaxy groups or clusters have been missed in previous X-ray cluster surveys based on the ROSAT All-Sky Survey (RASS). Methods. We applied a dedicated source-detection algorithm sensitive also to more unusual group or cluster surface-brightness distributions. It includes a multiresolution filtering, a source-detection algorithm, and a maximum-likelihood fitting procedure. To optimize parameters, this algorithm is calibrated using extensive simulations before it is used to reanalyze the RASS data. In addition, the cross-correlation of the candidates with optical/infrared surveys is used for cluster identification and redshift estimation. Results. We found many known groups but also a number of new group candidates, which are not included in any previous X-ray or SZ cluster catalogs. In this paper, we present a pilot sample of 13 very extended groups discovered in the RASS at positions where no X-ray source has been detected previously and with clear optical counterparts. The X-ray fluxes of at least 5 of these are above the nominal flux-limits of previous RASS cluster catalogs ( greater than or similar to 3 x 10(-12) erg s(-1) cm(-2) in the 0.1-2.4 keV energy band). They have low mass (10(13) M-circle dot less than or similar to M-500 less than or similar to 10(14) M-circle dot; i.e., they are galaxy groups), are at low redshift (z < 0.08), and exhibit flatter surface-brightness distributions than usual. Conclusions. We demonstrate that galaxy groups were missed in previous RASS surveys, possibly due to the flat surface-brightness distributions of this potential new population. Analysis of the full sample will show if this might have a significant effect on previous cosmological parameter constraints based on RASS cluster surveys.