The RASS-SDSS Galaxy Cluster Survey

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, LX, 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 (LX)

✔SDSS data for optical (Lop

) 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 FunctionThe 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 LBCG

Lin & Mohr (2004) , Yang et al. (2005)

...at the faint endDGR=N(-18.5rr

●When measured within r

200

●or r500

, the DGR is constant

...at the faint endDGR=N(-18.5rr

●When measured within r

200

●or r500

, 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/0506201

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 M200

2.04 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 -0.10.3 dex

● AXU: mean deviation -1.00.4 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 LX-M

200 relation (mean

deviation -1.0 dex)

● AXU clusters do follow the Lop

-M200

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.