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?