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Merging clusters of galaxies: Merging clusters of galaxies:
an optical viewan optical view
Sophie MaurogordatoCNRS
Laboratoire CASSIOPEE Observatoire de la Cote d’Azur,
Nice
In collaboration with:
M. Arnaud, J.L. Sauvageot (SAP, CEA-CEN Saclay)C.Benoist, A. Bijaoui, G.Mars,E. Slezak (Cassiopee, Nice)G. Soucail (OMP, Toulouse)A.Cappi, L. Feretti (Obs. & CNR Bologna)C.Ferrari, S. Schindler (Univ. Innsbruck)H. Bourdin (Univ. Roma)E. Belsole (IAO, Cambridge)G. Pratt (Max Planck Institute, Garching)B. Vandame (ESO, Garching)M. Plionis (IOA, Athens)R. Hunstead (Univ. Sydney)
Clusters in the process of merging
In the hierarchical model, galaxy clusters form by merging of smaller mass units
Irregular, morphologically complex clusters are seen in
the process of formation .
Key issue on the mass assembly of the universe at the scale of several Mpc.
Combined X-Ray/ Optical analysis allows to follow separately the distribution of gas and of galaxies.
Evolution with time of the density and velocity distribution of galaxies during the merger event
Schindler and Bohringer 1993
Evolution of the density and temperature of the gas with time during the merging event
Takizawa 1999
MUltiwavelength Sample of Interacting Clusters
Scientific goals:
Caracterize the merging scenario:
Comparison of density distributions (galaxies/gas/dark matter) Velocity distribution, mass ratios of the sub-clusters Signatures in the TX maps
optical + X-Ray observations + Numerical simulations axis and date of collision
Test for the impact of the merging process on galaxy properties:
Star formation ? Luminosity functions?
SFR properties: optical (colors+ H+ spectra) + IR + radio
MUltiwavelength Sample of Interacting Clusters
Optical:S.Maurogordato, C.Benoist, G.Mars, E. Slezak
CASSIOPEE/OCA
C.Ferrari, Univ. Innsbruck, A
A. Cappi, Oss. Bologna, I
M.Plionis, Athens, Gr
X:
J.L. Sauvageot, M. Arnaud, SAp, CEA-Saclay
E. Belsole, Univ. Bristol, UK.
H. Bourdin, Univ. Roma, I
G.Pratt, Max Planck Institute, Garching
Radio:
C.Ferrari, S. Schinder, Univ. Innsbruck,
L. Feretti, IRA Bologna
R.Hunstead, Univ. Sydney
IRC. Ferrari, Univ. Innsbruck,A
P.A. Duc, D.Elbaz, Sap/CEA Saclay
S.Maurogordato, C.Benoist, CASSIOPEE
Optical
BRI H imaging: ESO ([email protected]), CFHT (CfH12K)
Multi-object spectroscopy R~600ESO([email protected])CFHT ([email protected])
2dF(AAT)
X-Ray:
Spectro-imaging: XMM, Chandra
Radio:
VLA, ATCA
IR
Proposal Spitzer submitted
MUltiwavelength Sample of Interacting Clusters
Optical:S.Maurogordato, C.Benoist, A. Bijaoui,G.Mars,
E. Slezak,CASSIOPEE/OCA
C.Ferrari, Univ. Innsbruck, A
A. Cappi, Oss. Bologna, I
B. Vandame, ESO Garching
M.Plionis, Athens, Gr
X:
J.L. Sauvageot, M. Arnaud, SAp, CEA-Saclay
E. Belsole, IAO, Cambridgel, UK.
H. Bourdin, Univ. Roma, I
G.Pratt, Max Planck Institute, Garching
Radio:
C.Ferrari, S. Schindler, Univ. Innsbruck,
L. Feretti, IRA Bologna
R.Hunstead, Univ. Sydney
IRC. Ferrari, Univ. Innsbruck,A
P.A. Duc, D.Elbaz, Sap/CEA Saclay
S.Maurogordato, C.Benoist, CASSIOPEE
MUSIC: Sample Selection
• Small sample: 10 clusters merger candidates with systematic X-Ray/Optical observations
• X-Ray bright clusters: First targets from XMM GT program Sauvageot et al.
• Low redshift: z ≈ 0.08-0.1 Spatial coverage: 30’ FOV (XMM, WFI) ≈ 2 h-1 Mpc Spectral resolution (R ≈ 600) on a 3.6m telescope High S/N Temperature maps
• Sample different stages of the merging process (pre/mid/post) from gas/galaxy segregation (Kolokotronis et al. 2000)
6 clusters fully observed: All are mergers !
A 2933
A 2440
A 1750
A 3921
A 2384A 2142
A 2065A 4038
How to characterize the mergers ?
• Optical: Analysis of the galaxy distribution
• Density distribution (2D)
Mapping: Dressler 1980, adaptative kernel (Kriessler and Beers 1997), multiscale analysis (Slezak et al. 1990, Escalera et al 1994)
Detection of sub-clusters & significance
Partitioning: best fit and reconstruction
! Projection effects : decontamination of background/foreground Red Sequence, Photometric redshifts
Multiscale analysis of A2163
Reconstruction of the Density field with different wavelets planes
• Velocity distribution (1D)
Departure from gaussianity:
skewness, kurtosis, tail and asymetry indexes
Multiple tests (Beers et al. 1990, Pinkney et al. )
Partitioning ( KMM: McLachlan & Basford 1988)
Systems with high velocity dispersion:requires good sampling for testing if real !
CBI= 74018 ± 125 km/s
SBI = 1325 ± 145 km/s (125 members)
-strong departure from gaussianity
-well fitted by KMM 3-partition
Central: CBI = 74249 ± 90 km/s , SBI= 879 ± 55 km/s
Ferrari et al., 2003
CBI = 60000 ± 100 km/s
SBI = 1451 ± 120 km/s (205 members)
- Not significantly deviant from gaussianity
- slight bimodality , just after the collision ?
- Very massive system
S. Maurogordato,. A. Cappi, C. Ferrari, G. Soucail et al., 2006
Abell 2163 Abell 521
Velocity offset of the sub-clusters
A 1750: sub-clusters distinct in velocity space
Pre-merger
A2933: the subclusters are less clearly separated: the interaction has begun
Abell 2163:
bimodality in the core region:
West and East sub-clusters mixed
Undergoing merger
Maurogordato,. Cappi, Ferrari, Soucail et al., 2006
Subclustering in both density/velocity space
Dressler and Schectman 1988, Girardi et al. 1997, KMM 3D, h-tree (Serna & Gerbal 1996), DEDICA (Pisani et al. 1998), Tomography (Ferrari et al. 2003)
Comparison galaxies/gas density field
A2933: a pre-merger
2 sub-clusters in X-Ray and optical, spatially coincident
Abell 521: a multiple merger
Ferrari et al. 2003, Arnaud et al. 2000, Maurogordato et al. 2000
A2163: a pre and post-merger
Main component: segregation gas/galaxies post-merger
Northern component: gas/galaxies coincident pre-merger
Arnaud, Sauvageot, Pratt, Bourdin et al. 2006
Properties of BCGs
- No unicity (as in relaxed clusters)- Spatial offset from the centroid of the cluster- Often spatially associated to sub-clusters- Alignment of the PA with that of the cluster (or
subcluster)- Frequent multi-nuclei structure - Velocity offset from the mean velocity of the
cluster (and of the sub-clusters)- Offsets : signature of the dynamical stage
A 521 A 2933 N
A2933 SA2163
Alignment effects
PA gas density distribution PA galaxy density distribution PA of dominant galaxies sub-clumps/ general axis
Plionis et al. 2003
Strong signatures in the Tx maps
Sauvageot et al. 2005
Belsole et al. 2003, 2004
Bourdin et al. 2004
Evolution in galaxy clusters
Observational evidences
• SF lower in clusters/field-lower percentage of star-forming objects// field -HI deficiency in clusters
• SF in clusters depends on:Density (MD relation) redshift (Butcher-Oemler effect)Mass (downsizing effect)dynamical state ?
Which is (are) the culprit(s)? • Infall of galaxy in the IGM > gas stripping (Gunn & Gott 1972)Ram pressure: High IGM density + relative velocity
• galaxy-galaxy interations :Strong: galaxy mergers (low relative velocities) Herquist & Barnes 1991Weak: tidal effects (« harassment » Moore et al. 1998
• Strangulation (gas halo removed, Bower & Balogh 2004)…
Probably a mix of different mechanisms
+ increase of SFR in field galaxies and of infall rate of galaxies on clusters with z
How does merging affect SF in clusters ?
Induce starbursts:• Time-dependent gravitational field (Bekki 1999)• Combination of previous effects (Gnedin 1999, Moore
1999)
Observational evidence• Distribution of SB, PSB galaxies in Coma Caldwell et al. 1993, Poggianti et al. 2004, in A521 and
A3921 Ferrari et al. 2005
Existence of a burst of SF before truncation
Poggianti et al. 2004
Ferrari, Benoist et al. 2005
Ferrari,Maurogordato et al. 2003
Reconstructing the SF history in A2163
Coll. J. Brinchmann (Porto Univ. ) & S. Charlot (IAP)
in progress
High spectral resolution R=2500 , S/N > 10, VIMOS/VLT spectra (300/205 cluster members) 400-520 nm
Lick indexes, Dn(4000), EW ([OII], [OIII], Balmer lines)
Test for recent (< 2 Gyr) star formation bursts
Detection of k, k+a, e(a), e(b), e(c) population and localisation as respect to the signature of merging events
Summary Generic properties of merging clusters as predicted by numerical simulations Irregularities in the density distribution (sub-clusters, isophote twisting, ventroid
offsets)• Gas/galaxy segregation• Offset of brightest members (z, spatial)• Strong signatures in X-Ray T maps• Departure from gaussianity in the velocity distribution • Strong alignments effects
Careful comparison to simulations to modelize the merger history X-Ray + optical data
Is star formation affected by the merging process and how? Higher fraction of SB/PSB galaxies/ regular low z clusters• Cases of spatial correlation with the merger, to be extended to larger sample
Multiwavelength data ( cf talk Chiara Ferrari: Abell 3921) Numerical simulations with SF