Dynamics and Star formation in Dynamics and Star formation in merging galaxy clustersmerging galaxy clusters

Sophie MaurogordatoCNRS

Laboratoire CASSIOPEE Observatoire de la Cote d’Azur,

Nice, France

Spectroscoy in Cosmology and Galaxy Evolution 2005-2015

Granada, 3-5 October 2007

Why studying merging clusters ?

1. Mechanism of cluster formation

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

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

2. Star formation in a peculiar environment

Universal SFR declines since z =1

SFR suppressed in dense environments (locally and at high redshift)

Is there a link between the decline in the universal SFR and environmental effects?

Which are the physical processes driving SF in clusters and how to they compete with each other at the different stages of the cluster formation?

MUSIC MUltiwavelength Sample of Interacting Clusters

The project: :

Coordinated optical/X-Ray/radio observations :10 low z [0.1-0.2] clusters at different stages of merging

• Wide-field multiband imaging BRH: ESO 2.2m WFI (50h) ,CFHT CFH12K (2n)

• Multi-object spectroscopy: - r< r200, 100 z per cluster, R ~ 500, ESO 3.6m EFOSC2(12n), CFHT MOS

(3n) - r200<r<4r200, 500 z per cluster, R ~ 700, 2dF and AAOMEGA, AAT

• X-Ray Spectro-imaging:XMM, Chandra

• Radio observations: VLA

MUSIC

The collaboration:

- OCA Nice: S.Maurogordato, C. Benoist, A. Bijaoui, G. Mars, E. Slezak (ESO)

- SAp-CEN Saclay: J.L. Sauvageot, M. Arnaud (XMM)

- Univ. Innsbruck: C. Ferrari (AAT, VLA)

- INAF, Oss. Bologna: A.Cappi

- MPA, Garching: G. Pratt

- IOA, Cambridge: E. Belsole

- Univ. Roma: H. Bourdin

MUSIC

The objectives:

Caracterize the merging scenario:

Comparison of density distributions (galaxies/gas/dark matter) Velocity distribution, mass ratios of the sub-clusters Signatures in the T maps of the gas

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) + radio

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

Disentangling merging processes in the multiple merger Abell 2163

• Optical/X-Ray coordinated observations• Collaboration: same as MUSIC + J. Brinchmann (Univ. Porto) & G.

Soucail (OMP, Toulouse)

• The data: Mosaic 5 XMM pointings (Arnaud et al.) Mosaic 5 WFI imaging (BVRIH) VIMOS spectroscopy HR Blue [4200-6200]A R~2500 : 512 z of which 330 cluster members, S/N > 10, z~0.0001

Maurogordato, Cappi, Ferrari et al. 2007, A&A, accepted

Density of galaxies/ density of gas

X-Ray peak in between the two galaxy clumps:Recent merger event in the central component

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

full: gas

Velocity distribution

Very high velocity dispersion: = 1400 km/s

Even without emission lines galaxies= 1400 km/s

Hints of bimodality

Velocity distribution

successful fitted by KMM:

3 velocity partitions

But, gaussian not excluded

Detection of a velocity gradient of 1500 km/s in the central component

all KMM1 KMM2

Spatial evolution of the mean velocity and of the velocity

dispersion

Before or after merging ?

The Northern component:

How can merging of sub-clusters affect SF in

clusters ? Induce starbursts:• Time-dependent gravitational field (Bekki 1999)• Combination of different effects (Gnedin 1999, Moore

1999)

Some 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

Some spectra

Lack of k+a galaxies

Relatively high fraction of EL galaxies at 0.5 r200: 30%

Peculiar distribution of EL galaxies

Summary

Signatures of merging on galaxy and gas distribution Irregularities in the density distribution (sub-clusters,

isophote twisting, centroid offsets)• Gas/galaxy segregation• Offset of brightest members (z, spatial)• Strong features in X-Ray T maps• Departure from gaussianity in the velocity distribution • Strong alignments effects

Can be used to constrain the merging history of the cluster

Link to the SF properties ? hints: in progress

My personal view…Spectroscopy is essential to study

cluster formation  good S/N ratio (>10) and spectral resolution R>2000

high precision redshifts >> fine dynamical effects SF in galaxy members

large number of redshifts (>500) >> reliable velocity dispersion of cluster and subclusters >> dynamics of the cluster

More powerful if combined with good optical imaging and multi

Infer the scenario and a good estimate of mass

Important: f(M,z) is used for cosmology and mergers are numerous at high z

Thank you

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

Evolution with time of the density and velocity distribution of galaxies during the merger event

Schindler and Bohringer 1993

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

Evolution of the density and temperature of the gas with time during the merging event

Takizawa 1999

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 ?

Abell 2163

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

Need a dedicated sample: MUSI C

Systematic X-Ray/ Optical observations: follow separately the distribution of gas and of galaxies.

Low redshift sample: break the degenaracy with redshift evolution z ≈ 0.1

Good spatial coverage: 30’ FOV (XMM, WFI) ≈ 2 h-1 Mpc High S/N for temperature maps and spectroscopy

Candidates sample different stages of the merging process (pre/mid/post) from gas/galaxy segregation

clusters selected from APM/ROSAT comparison (Kolokotronis et al. 2000)

10 clusters fully observed