the build-up of the colour-magnitude relation in low-z galaxy clusters

THE BUILD-UP OF THE COLOUR-MAGNITUDE RELATION IN LOW-Z

GALAXY CLUSTERS

Bertinoro School 23th-29th May 2009

Diego Capozzi1,2, Chris A. Collins1, John P. Stott1

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(1) Astrophysics Research Institute, Liverpool John Moores University - (2) Dep. Of Physical Sciences, Università Federico II di Napoli

OVERVIEW

• Colour-Magnitude Relation (CMR)

• Recent issues about CMR

• Our work

• Results

• Future

CMR

• Interpreted as a correlation between galaxy M and mean stellar Z– Alternatively as due to a change of the mean age– age-metallicity degeneracy

• Its origin can be explained through:– concurrent formation of Es in CCs in high-z

monolithic collapse (Kodama & Arimoto 1997)– hierarchical formation of Es via merging over

cosmic time (Kauffmann & Charlot 1998)

CMR

• Does its morphology evolve with z?– Slope: measures differential properties of Es as

a function of L– Zero-point: for high-z (z >0.6) clusters it’s a

measure of the formation epoch of the dominant stellar population of Es

Recently

• Analysis of CMR build-up and its relation with environment has provided:– clues to Es downsizing as

• function of redshift• function of environment

De Lucia et al. (2005)

What do we want to do?• To study the evolution of early-type

galaxies in galaxy clusters• Low-z (0.05<z<0.26)• Optically (~500 maxBCG+HMF) and X-

Ray (97 XCS) selected clusters• Using SDSS DR6 data

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CMR MorphologyGalaxies with R<1 Mpc

Iterative biweight fit

Bootstrap for determining errors on slope and zero-point

Some differences among cluster samples seen

Interpretation: work in progress

g-r vs. Mr

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Giant/Dwarf Ratio

Mean BackGiants: Mr<-20.52

Dwarfs: -20.52< Mr<-18.32

Local Background: 2<R<3 Mpc

Mean Background: Average among 17 control regions

Best fit line: only statistical samples used

Our results consistent with De Lucia et al. (2007) values for SDSS clusters

G/D ratios vs LX

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No trends of g/d ratios as a function of LX have been found

Results & Discussion

• We analysed clusters galaxy evolution at low-z

• Optically and X-Ray selected samples used

• Some differences on the CMR morphology among the samples

• Our g/d ratios are in accordance with De Lucia et al. (2007) values for SDSS clusters

• No dependence of g/d ratios on LX

• Are non BCG clusters diverse? Are BCG and X-Ray clusters similar?

Downsizing Or is it?