Merging clusters of galaxiesBernard’s Cosmic StoriesValencia 2006 Sophie Maurogordato CNRS 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 Sampleof InteractingClusters 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+ Ha+ spectra) + IR + radio
MUltiwavelength Sampleof InteractingClusters • Optical: • S.Maurogordato, C.Benoist, G.Mars, E. Slezak • CASSIOPEE/OCA • C.Ferrari, Univ. Innsbruck, A • Cappi, Oss. Bologna, I • M.Plionis, Athens, Gr Optical BR Ha Wide field imaging: ESO (email@example.com), CFHT (CfH12K) High spectral resolution Multi-object spectroscopy ESO(EFOSC2@3.6m,VIMOS@UT3) CFHT (MOS@3.6m) 2dF(AAT) X-Ray: Spectro-imaging: XMM, Chandra 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: VLA, ATCA Radio: C.Ferrari, S. Schinder, Univ. Innsbruck, L. Feretti, IRA Bologna R.Hunstead, Univ. Sydney
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 !
dotted: dark matter full: gas Roettiger et al. 1998, ApJS, 109,307
A 2933 A 1750 A 2440 A 3921 A 2384 A 2142 A 2065 A 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 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. 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 3921 A3921-B A3921-A • Witnessing the central phase of the collision (0.0 0.3 Gyr) • Offset merger • in the plane of the sky • Mass ratio 1:3 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 1991 Weak: 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
Ferrari,Maurogordato et al. 2003 Poggianti et al. 2004 Ferrari, Benoist et al. 2005
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 Ha 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