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. 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).
We applied a dedicated source-detection algorithm sensitive also to more unusual group or cluster surface-brightness distributions. 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 talk, I will 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 (> 3x10^{-12} erg s^{-1} cm^{-2} in the 0.1-2.4 keV energy band). They have low mass (10^{13} Msun 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.
