2025A&A...700A.220S

Context. Galaxy cluster cosmology relies on complete and pure samples that cover a large range of masses and redshifts. In our previous ROSAT All-Sky Survey (RASS)-based works, we discovered an apparently new population of galaxy groups and clusters with, on average, flatter X-ray surface brightness profiles than all other known clusters; this population was missed in previous cluster surveys. The discovery of such a new class of objects could have a significant impact on cosmological applications of galaxy clusters.
Aims. We aim to characterize a subsample of these systems to assess whether they belong to a new population.
Methods. We followed up on three of these galaxy groups and clusters with high-quality XMM-Newton observations. We produced clean images and spectra and used them for model fitting. We also identified known galaxies, groups, and clusters in the field.
Results. The observations reveal that all three systems are composed of multiple groups each, either at the same or at different redshifts. In total, we characterized nine groups. We measure flat surface brightness profiles with slope parameter β < 0.6, i.e, less than the canonical β = 2/3. For the two main central groups, we even measure β < 0.4. When the fluxes for the three observations are split up across the nine identified groups, none of them exceeds the typical flux limit adopted in previous RASS cluster catalogs, ≈3 × 10^–12 erg s^–1 cm^–2 in the 0.1 - 2.4 keV energy band.
Conclusions. The observations reveal that groups with flat surface brightness profiles exist. Determining whether they form a new, separate population requires additional follow-up observations of further systems from our previous RASS sample, given the complexity we have discovered. Such extended low-surface-brightness systems, as well as multiple systems and projection effects, need to be taken into account when determining the selection functions of group and cluster samples.