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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 ...
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)
• Abell Catalogue (POSS + ESO SSS):
m1
m2
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
cD
B
F
L
C
I
- single dominant cD galaxy (A2029, A2199)
- dominant binary, like Coma
- flattened (IRAS 09104+4109)
- linear array of galaxies (Perseus)
- single core of galaxies
- irregular distribution (Hercules)
m3
m3+2
 50
Structures of galaxy clusters
Böhringer (1996)
• 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
A3528
A578
A1569
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 ...
M 
G
3 
  r2 
r
1
1
X-rays: continuous mapping of  in galaxy clusters
• systematic imaging
: EINSTEIN, ROSAT
• hígh spatial rersolution : CHANDRA
• “ spectral “
: XMM
• mapping of T
: ASCA
M ( r ) 
G    mH
k T  r 2


dr
dr


 d (log  ) d (log T ) 
Fornax
Cluster
Abell 2256
• 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
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)
Isyn
 N0 B
1


- Faraday rotation   5 G (Clarke et al. 1999)
  RM  2 ,
RM  0.81 
Enßlin & Biermann (1998)
 ( cm3 )  ( G )  ( kpc)
ne
B
ds
  1 G
(e.g Enßlin & Biermann 1998; Tsay et al. 2002)
IC results not yet conclusive
- Inverse Compton emission
I  I 
IC
syn
B
 (1 )
 ( kT )
3
 E IC


B
 (1 )

Isyn
IIC
Dixit deus: “Fiat lux (campus magnetibusque)”
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)
‘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 velocity vg , density of jet j , density of ICM
ICM density of ICM):
R
rj

 j V j
 ICM  Vg
2
2
• 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 CMB BCMB (G):
  108  b1/ 2 
Beq
 BCMB
2
2
Beq
yr
Perseus
at 610 MHz
3C465
Radio sources are
- barometers to measure ICM pressure
- anenometers to measure cluster winds
(the only measure so far!)
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:
p
s7 3
 f ( R,  )
s 1
• 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).
A 2256 Xray.
A 2256
opt.
Coma Cluster 327
MHz
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?
• 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
• 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
S ~  -
R2
R 1
• 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
NGC315
I
II
III
IV
V
view from
‘above’
‘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
Landau & Lifschitz (1966): pressure and temperature
ratios between down- and upstream region (inside and
outside cluster shock front) are:
P2 4  R  1

P1
4R
T2
P1 1


T1 P2 R
• 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
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