Merging clusters of galaxiesMerging clusters of galaxies

Bernard’s Cosmic StoriesBernard’s Cosmic StoriesValencia 2006Valencia 2006

Sophie MaurogordatoCNRS

Laboratoire CASSIOPEE Observatoire de la Cote d’Azur,

Nice

Clusters in the process of merging

In the hierarchical paradigm, galaxy clusters form by merging of smaller mass units

Studying merging clusters:

Key issue on the mass assembly of the universe at the scale of several Mpc

Cosmological issue (z evolution is model dependent)

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

Optical

BR H Wide field imaging: ESO (wfi@2.2m), CFHT (CfH12K)

High spectral resolutionMulti-object spectroscopy

ESO(EFOSC2@3.6m,VIMOS@UT3)CFHT (MOS@3.6m)

2dF(AAT)

X-Ray:

Spectro-imaging: XMM, Chandra

Radio:

VLA, ATCA

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. 2000)

• Low redshift: z ≈ 0.1 good compromise Good spatial coverage: 30’ FOV (XMM, WFI) ≈ 2 h-1 Mpc High S/N for temperature maps and spectroscopy Not yet z evolution

• Candidates sample different stages of the merging process (pre/mid/post) from gas/galaxy segregation from APM/ROSAT(Kolokotronis et al. 2000)

6 clusters fully observed: All are mergers !

Roettiger et al. 1998, ApJS, 109,307dotted: dark matter

full: gas

A 2933

A 2440

A 1750

A 3921

A 2384A 2142

A 2065A 4038

How to characterize the mergers ?The pieces of the puzzle

I - Density distribution (2D) of galaxies and gas

Mapping: Dressler 1980, adaptative kernel (Kriessler and Beers 1997), multiscale analysis with wavelets(Slezak et al. 1996, Escalera et al 1994)

Departure from regularity (centroid offset), power ratios…

Detection of sub-clusters & significance

Test for segregation between gas and galaxies

! Projection effects : decontamination of background/foreground CM diagram:Red Sequence, Photometric redshifts

II - Velocity distribution (1D)

• Departure from gaussianity:

skewness, kurtosis, tail and asymetry indexesMultiple tests (Beers et al. 1990, Pinkney et al. ) Bi or multi-modality ?Partitioning ( KMM: McLachlan & Basford 1988)

• Dynamics of sub-clusters:

Peculiar velocities, velocity dispersionsMass ratios, bound or not, incoming or outgoing solutions

III – Temperature maps of the gas

Sauvageot et al. 2005

Belsole et al. 2003, 2004

Bourdin et al. 2004

Aim: Recover a scenario for the merging process for each cluster

Comparison to simulations: (Roettiger et al. 1998, Ricker & Sarazin 2001, Schindler,

Kapferer et al. 2006, and now dedicated simulations Sauvageot et al. in progress)

Need to reproduce:

gas & galaxies density distribution velocity field of galaxies temperature maps of the gas

Abell 3921Abell 3921

Witnessing the central phase of the collision (0.0 0.3 Gyr)

Offset merger

in the plane of the sky

Mass ratio 1:3

A3921-A

A3921-B

Belsole, Sauvageot et al., 2005, A&A, 430, 385 Ferrari, Benoist et al., 2005, A&A, 430, 19

z=0.09

Abell 2933

A merger at the beginning of the interaction with a large impact parameter

Abell 2163

A recent merger in the core + an infalling sub-cluster in the North

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 can merging of sub-clusters 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

Summary Generic properties of merging clusters as predicted by numerical

simulations

Irregularities in the density distribution (sub-clusters, isophote twisting, centroid 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?...

Some evidences

• Higher fraction of SB/PSB galaxies/ regular low z clusters• in some cases, spatial correlation with the merger

work under progress

• Properties of the CM relations (dispersion, tilt ?) and distribution and frequency of « blue galaxies » (BO)

• Distribution and frequency of Hemitting galaxies• Test for recent (< 2 Gyr) star formation bursts from high R (2500) spectroscopy: Detection of k, k+a, e(a), e(b), e(c) population and localisation as respect to the

signature of merging events

to be extended to larger samples, SDSS & CFHTLS clusters

Thank you