1. IMPRS, April 8 Merging galaxy clusters: radio and X-ray studies hierarchical structure formation in the universe still ongoing at z = 0 X-ray substructure radio emission cluster weather cosmological shocks weather in cluster gas themes of a commencing graduate school...

2. clusters are powerful testbeds for cosmological models - cluster mass and abundance   M = 1 - gravitational lensing     0.7 - cluster abundance with z   M = 0.25 ± 0.1 Large-scale structure numerical simulations predict large-scale structure in the presence of (dominating) CDM, with ongoing collapse of the largest structures at z = 0

3. groups and clusters of galaxies = largest gravitationally bound and collapsed systems in the universe groups: 3 ···  30 galaxies; clusters: up to a few 1000 R ~ 2 Mpc M ~ 10 14 ··· 10 15 M  Local Group: ~ 35 members MW, M31, M33; all others dwarf galaxies Virgo Cluster: ~ 2100 members (Binggeli et al. 1985) Clusters of galaxies

4. Abell Catalogue (POSS + ESO SSS): 1682 clusters (Abell 1958) 4073 “ (Abell, Corwin & Olowin 1989) criterion:  50 galaxies with m 3  m  m 3 + 2 contained within ‘Abell Radius’  A = 1.5’/z, i.e. R A = 1.5  h -1 Mpc covers 0.028 < z < 0.20 cD - single dominant cD galaxy (A2029, A2199) B - dominant binary, like Coma F - flattened (IRAS 09104+4109) L - linear array of galaxies (Perseus) C - single core of galaxies I - irregular distribution (Hercules)  50 m1m1 m2m2 m3m3 m 3 +2

5. 3 mass components: visible galaxies, ICM, DM - galaxies: ~ 3%(optical, IR) - ICM: 10 ··· 15 %(X-rays) - DM: ~ 80%(  v, grav. lensing) Structures of galaxy clusters Coma A578 A1569 belief until ~ mid 80’s: “clusters are simple...” substructure however: ample evidence for substructure, rendered visible most convincingly in X-ray regime  ‘true-nature-images’ of clusters! radial variations of centroids twists in X-ray isophotes (e.g. Coma Cluster!) non-Gaussian skewed or even bimodal f(v)’s A3528 Böhringer (1996)

6. Optical techniques barely disclose gravitational potential in nearby clusters unless these are rich (too few test particles); distant ones: lensing... X-ray morphologies of clusters Fornax Cluster X-rays: continuous mapping of  in galaxy clusters systematic imaging: EINSTEIN, ROSAT hígh spatial rersolution: CHANDRA “ spectral “ : XMM mapping of T: ASCA Abell 2256

7. systematic X-ray survey of galaxy clusters: REFLEX (Böhringer et al. 1999) basically 1000s of clusters, mostly with but few (  100) photons... 452 clusters, 53% Abell (only!) for  m = 0.3  cluster mass contributes ~ 6% to total matter in the Universe

8. Radio emission from clusters of galaxies is the IGM/ICM magnetizied? how (and when) did it get magnetized? AGN (‘standard’) dwarf galaxies (Kronberg et al. 1999) evidence for B-fields: Dixit deus: “Fiat lux (campus magnetibusque)” Another diagnostic tool of cluster physics: radio emission: synchrotron radiation - radio halos & relics (e.g. Feretti 1999) - Faraday rotation   5  G (Clarke et al. 1999) - Inverse Compton emission   1  G (e.g Enßlin & Biermann 1998; Tsay et al. 2002) IC results not yet conclusive Clarke et al. (1999) Enßlin & Biermann (1998)

9. Thierbach et al. (2002) Feretti & Giovannini (1998) radio ‘halos’: central, diffuse, polarization < 5% Röttgering et al. (1999) radio ‘relics’: peripheral,  20% polarized no obvious particle/energy sources steep(ening) spectra at higher frequencies how frequemt? many if searched for with scrutiny!

10. 3C465 Perseus at 610 MHz ‘Weather stations’ in galaxy clusters ~ 10% of galaxies in clusters produce significant radio synchrotron emission (P ν  10 23 W Hz -1 at 20 cm) jets of radio plasma ejected from galaxy cores, forming lobes and tails  probe relative gas motions over 100’s of kpcs (NATs, WATs) however: ~90% of WATs & NATs in clusters with X-ray substructure; correlation between elongations in X-rays and bending of radio tails cluster mergers  bulk flow  ram pressure  bends of radio tails and distortion of X-ray surface brightness Perseus Cluster (Sijbring 1994): low-frequency kinks and bends suggest highly non-ballistic motions  caused by turbulent motions of the ICM plasma!  ’high winds’ b B eq B CMBsynchrotron ages from break frequency b (GHz), equipartition magnetic field B eq (  G), equivalent magnetic field of CMB B CMB (  G): R r j v j v g  j  ICMformer belief: tails simply trace ballistic motions of galaxies when radio plasma is exposed to ICM ram pressure (radius of curvature R, jet radius r j, jet velocity v j, galaxy velocity v g, density of jet  j, density of ICM  ICM density of ICM): Radio sources are - barometers to measure ICM pressure - anenometers to measure cluster winds (the only measure so far!)

11. classical cases of peculiar peripheral & extended radio sources: - A2256 (Röttgering et al. 1994; Röttgering et al. 1994) - 1253+275 in Coma (Giovannini et al. 1991) common properties: - peripheral - steep spectrum - linearly polarized  ordered B-field A 2256 opt. Radio relics: revived particle pools  loss  kinorigin of relic: several radio galaxies in the vicinity of 1253+275 (Giovannini et al. 1985);  loss <<  kin solved by large-scale accretion shocks (Enßlin et al. 1998); low galaxy density  turbulent reacceleration by galactic wakes ruled out. 16 clusters with known relics (compilation in Slee et al. 2001) only 4 clusters with relics have measured polarization (see Enßlin et al. 1998). A 2256 X- ray. Coma Cluster 327 MHz A 2256 1465 MHz degree p of polarization depends on compression ratio of shock, on particle spectrum, N(E) · dE ~ E -s · dE, and on the orientation of shock w.r.t. observer:

12. NGC315: a giant (~ 1.3 Mpc) radio galaxy (GRG) with odd radio lobe (Mack 1996; Mack et al. 1998). - morphology: precessing jets (Bridle et al. 1976), but western one with peculiar bend towards the host galaxy - unusually flat radio spectrum in western lobe: first steepens (as expected), then flattens to  high  0.7 (S ~ --  ). - strong linear polarization: p  30%. Cosmological shock waves at intersecting filaments of galaxies Enßlin et al. (2000): originally symmetric radio galaxy “falling” into an intergalactic shock wave, along with its environment. compression  reacceleration of particles  strong alignment of magnetic field & increased synchrotron emissivity origin of large-scale gas flow and shock wave?

13. from theory of shocks (Landau & Lifschitz 1966)  temperature jump T 1 /T 2  3.3 ···· 20 compression ratio R  2.9 ···· 3.8 pressure jump P 1 /P 2  9.6 ···· 75 O’Drury (1983):    0.54 ···· 0.79 expected  N(E) · dE ~ E -s · dES ~ -  NGC315 located within Pisces-Perseus Supercluster Enßlin et al. (2000) identify filaments of galaxies with rather different velocity dispersions (redshifts from CfA survey, Huchra et al. 1990, 1992, 1995): - filament I:  v  400 km s -1 - filaments II - V:  v  90 ···· 220 km s -1 if gas has comparable  v, this translates into k ·T I  280 eV k ·T II-V  15 ···· 85 eV gas in one of smaller filaments (II - IV) may get heated by shock wave when flowing into deeper gravitational potential of main filament (I). cosmological shockwave in NGC315 is putative; onfirmation requires - deep X-ray imaging to see heated gas - low-frequency search for relic-type, diffuse radio emission over entire shock region view from ‘above’ IIIIIIIVV NGC315

14. ‘Weather forecast’ head-tail (or other extended) radio sources must be studied, along with environment (X-ray studies) search for radio relics in cluster merger candidates at low frequencies, with scrutiny of spectral aging and linear polarization: essentially all cluster merger candidates should exhibit this.... new-generation X-ray telescopes with high spatial & spectral resolution  studies of gas motions to be compared with high-fidelity numerical simulations that take advantage from - new-generation supercomputers - adaptive mesh refinement - higher mass resolution - MHD Röttiger et al. 1998

17. Landau & Lifschitz (1966): pressure and temperature ratios between down- and upstream region (inside and outside cluster shock front) are:

18. GRGs: probes of tenuous IGM Clarke et al. Method (RM in clusters) Laing-Garrington ram pressure stripping (Virgo) how much mass in form of hot gas? importance of ghosts? primary/secondary/in situ