2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/15488We use the ROSAT Deep Cluster Survey (RDCS) to trace the evolution of the cluster abundance out to $z\simeq 0.8$ and constrain cosmological models. We resort to a phenomenological prescription to convert masses into $X$-ray fluxes and apply a maximum-likelihood approach to the RDCS redshift- and luminosity-distribution. We find that, even changing the shape and the evolution on the $L_{bol}$-$T_X$ relation within the observational uncertainties, a critical density Universe is always excluded at more than $3σ$ level. By assuming a non-evolving $X$-ray luminosity-temperature relation with shape $L_{bol}\propto T_X^3$, it is $Ω_m=0.35^{+0.35}_{-0.25}$ and $σ_8=0.76^{+0.38}_{-0.14}$ for flat models, with uncertainties corresponding to $3σ$ confidence levels.4 pages including 2 figures. To appear in ``Large Scale Structure in the X-ray Universe'', Santorini, Greece, 20-22 September 1999Astrophysicstext