Merging galaxy clusters: radio and X-ray studies

1. 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 IMPRS, April 8 themes of a commencing graduate school ...

2. Clusters of galaxies • 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 ~ 1014 ··· 1015 M  • Local Group: ~ 35 members • MW, M31, M33; all others dwarf galaxies • Virgo Cluster: ~ 2100 members (Binggeli et al. 1985)

3. m1 • Abell Catalogue (POSS + ESO SSS): 1682 clusters (Abell 1958) 4073 “ (Abell, Corwin & Olowin 1989) • criterion:  50 galaxies with m3 m  m3 + 2 • contained within ‘Abell Radius’ A = 1.5’/z, i.e. RA = 1.5  h-1 Mpc • covers 0.028 < z < 0.20 m2 m3 m3+2  50 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)

4. Böhringer (1996) Structures of galaxy clusters • 3 mass components: visible galaxies, ICM, DM • - galaxies : ~ 3% (optical, IR) • - ICM : 10 ··· 15 % (X-rays) • - DM : ~ 80% (v , grav. lensing) Coma • belief until ~ mid 80’s: “clusters are simple ...” • 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 A578 A1569 A3528

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

6. 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

7. Radio emission from clusters of galaxies Another diagnostic tool of cluster physics: radio emission: synchrotron radiation Clarke et al. (1999) • is the IGM/ICM magnetizied? • how (and when) did it get magnetized? AGN (‘standard’) dwarf galaxies (Kronberg et al. 1999) • evidence for B-fields: - radio halos & relics (e.g. Feretti 1999) Enßlin & Biermann (1998) - 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 Dixit deus: “Fiat lux (campus magnetibusque)”

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

9. ‘Weather stations’ in galaxy clusters • ~ 10% of galaxies in clusters produce significant radio synchrotron emission (Pν 1023 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) • former belief: tails simply trace ballistic motions of galaxies when radio plasma is exposed to ICM ram pressure (radius of curvature R, jet radius rj, jet velocity vj, galaxy velocityvg, density of jet j, density of ICM ICM density of ICM): • 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’ • synchrotron ages from break frequency b(GHz), equipartition magnetic field Beq(G), equivalent magnetic field of CMBBCMB(G): Perseus at 610 MHz 3C465 Radio sources are - barometers to measure ICM pressure - anenometers to measure cluster winds (the only measure so far!)

10. Radio relics: revived particle pools • 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 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: A 2256 X-ray. A 2256 opt. • origin 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). Coma Cluster 327 MHz

11. Cosmological shock waves at intersecting filaments of galaxies • 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%. • 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?

12. 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 ·TI  280 eV • k ·TII-V  15 ···· 85 eV NGC315 • from theory of shocks (Landau & Lifschitz 1966)  • temperature jump T1 /T2  3.3 ···· 20 • compression ratio R  2.9 ···· 3.8 • pressure jump P1 /P2  9.6 ···· 75 • O’Drury (1983):    0.54 ···· 0.79 expected  • N(E) · dE ~ E-s · dE S ~  - I II III IV V view from ‘above’ • 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

13. ‘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

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

17. 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