1. The RASS-SDSS Galaxy Cluster Survey P. Popesso (ESO), A. Biviano (Osservatorio di Trieste), H. Böhringer (MPE), M. Romaniello (ESO).

2. The Survey: ● following the variations of the properties of cluster galaxies with the global properties of the systems (total mass, velocity dispersion, L X, L op ) ● understanding the selection effects introduced by different cluster detection and selection methods The method & dataset ● multiwavelenght approach: ✔ RASS data for the X-ray properties (L X ) ✔ SDSS data for optical (L op ) and dynamical (mass,  V ) cluster properties, spectrophotometric properties of cluster galaxies The cluster samples ● 130 X-ray selected clusters (the RASS-SDSS Cluster Sample) ● 137 optically selected (3D overdensity spectroscopically confirmed) Abell Clusters

3. Outlines: ● Part I – universality of the cluster Luminosity Function (LF) – the cluster dwarf galaxy population – morphology-density relation of the dwarf systems ● Part II – the Abell X-ray Underluminous Clusters (AXU Clusters) and their nature

4. The Composite Cluster Luminosity Function The cluster composite LF is obtained by stacking the individual LFs (background subtracted) within the virial radius. ● Best fit: double Schechter Function ● at the bright end:                at the faint end:                 in the r band     km/s/Mpc )

5. Is the cluster LF universal? The composite cluster LF, calculated within the virial radius, is a good representation for more than 90% of the clusters, as confirmed by the  2 test. When measured within the physical size of the systems (given by r 200 ), the Cluster LF is universal (Popesso et al. 2005)

6. ...at the bright end ● Composite cluster LF is able to locate the BCGs.

7. ...at the bright end Composite cluster LF is able to locate the BCGs (Popesso et al. 2005, A&A submitted). Solid line L BCG   Lin & Mohr (2004), Yang et al. (2005)

8. ...at the faint end DGR= N(-18.5  r  r  ● When measured within r 200 ● or r 500, the DGR is constant

9. ...at the faint end DGR= N(-18.5  r  r  ● When measured within r 200 ● or r 500, the DGR is constant

10. When the LFs is measured within a fixed metric aperture of 1 Mpc, it varies from cluster to cluster. ● the overall disagreement about the faint-end slope of the cluster LF in the literature is due to aperture effects

11. The Cluster LF by Galaxy Morphological type We use the color cut at u-r=2.22 of Strateva et al. (2001) to distinguish red early type galaxies from blue late type galaxies. Early type galaxies Late type galaxies Popesso et al. 2005c, astro-ph/0506201

12. The dependence on the environment

13. Conclusions Part I ● bimodal behaviour of the cluster LF (steepening at the faint end) ● the cluster LF is universal when measured with the virial radius ● DGR increasing with the clustercentric distance ● LF steepening due to dwarf early type galaxies which dominate the cluster core (< 0.6 r 200 ) ● are the dwarf early type galaxies the spheroidal remnants of stripped and faded spirals as predicted by harassment scenario (Moore et al. 1996, 1998)?

14. The Abell X-ray Underluminous Clusters ● Do different cluster detection and selection methods select the same cluster population? Lx  M 200 2.04  0.04 scatter ~60% (Popesso et al. 2005, A&A, 433, 431)

15. 137 optically selected Abell clusters (3D overdensity spectroscopically confirmed): ● 88 clusters have clear RASS X-ray counterpart ● 27 marginally significant ( < 3  ) detection ● 24 no X-ray detection (Popesso et al. 2005, A&A submitted) RASS is too shallow to detect such faint X-ray sources (see also Basilakos et al. 2004, Donahue et al. 2003)

16. ● normal X-ray emittting Abell clusters: mean deviation -0.1  0.3 dex ● AXU: mean deviation -1.0  0.4 dex ● Tail Index (Beers et al. 1991) and subclustering analysis confirm reliable estimate of the cluster mass

18. The nature of the AXU clusters

19. The velocity dispersion distribution AXU clusters: leptokurtic distribution in the outer region, typical of systems in accretion

20. Conclusions Part II ● Optical selection reveals a population of X-ray underluminous (AXU) systems ● AXU clusters do not follow the L X -M 200 relation (mean deviation -1.0 dex) ● AXU clusters do follow the L op -M 200 relation ● AXU systems show leptokurtic velocity distribution in the outer regions (systems in accretion?) ● systems in formation also at low z? ● multi-wavelength approach is needed for optimizing completeness ans reliability of cluster samples.