Dynamics and Star formation in merging galaxy clusters Sophie Maurogordato CNRS Laboratoire CASSIOPEE Observatoire de la Cote d’Azur, Nice, France Spectroscoy in Cosmology and Galaxy Evolution 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 [ ] clusters at different stages of merging Wide-field multiband imaging BRH  : ESO 2.2m WFI (50h),CFHT CFH12K (2n) Multi-object spectroscopy: - r< r 200, 100 z per cluster, R ~ 500, ESO 3.6m EFOSC2(12n), CFHT MOS (3n) - r 200 <r<4r 200, 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 indexes Multiple tests (Beers et al. 1990, Pinkney et al. )  Bi or multi-modality ? Partitioning ( KMM: McLachlan & Basford 1988) Dynamics of sub-clusters: Peculiar velocities, velocity dispersions Mass ratios, bound or not, incoming or outgoing solutions

III – Temperature maps of the gas Sauvageot et al 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 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 [ ]A R~2500 :  512 z of which 330 cluster members, S/N > 10,  z~ 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, 109,307 dotted: 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 allKMM1KMM2

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 Existence of a burst of SF before truncation ?

Poggianti et al Ferrari, Benoist et al 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) 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 1991 Weak: tidal effects (« harassment » Moore et al 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 H  emitting 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