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

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

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

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 )

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)

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

...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)

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

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

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

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/

The dependence on the environment

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)?

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

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)

● 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

The nature of the AXU clusters

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

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.