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 (wfi@2.2m), CFHT (CfH12K)

Multi-object spectroscopy R~600ESO(EFOSC2@3.6m)CFHT (MOS@3.6m)

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