“Studio della relazione tra presenza di aloni radio e assenza
di cool cores in un campione completo di ammassi di galassie”
INTRODUZIONE A:
•Aloni radio
•Ammassi di galassie con e senza cool core
SCOPO DELLA TESINA
METODI:
•Il campione di ammassi in radio
•Il sottocampione osservato in X
COOL CORE vs NON COOL CORE CLUSTERS
•CC have more peaked surface brightness (density) distribution
Hydra A
A3376
EPIC flux images (erg cm-2 s-1) scaled by the maximum value, same
scale, same contours levels
COOL CORE vs NON COOL CORE CLUSTERS
•CC have more peaked surface brightness (density) distribution
COOL CORE vs NON COOL CORE CLUSTERS
•CC have more peaked surface brightness (density) distribution
COOL CORE vs NON COOL CORE CLUSTERS
•CC have more peaked surface brightness (density) distribution
1e-03
3e-05
1.8e-05
4e-05
5e-02
6e-03
4e-03
5e-03
COOL CORE vs NON COOL CORE CLUSTERS
•CC have DECREASING temperature profiles in the inner regions
A2199
CC
COOL CORE vs NON COOL CORE CLUSTERS
•CC have DECREASING temperature profiles in the inner regions
A3562
NCC
COOL CORE vs NON COOL CORE CLUSTERS
•CC have DECREASING temperature profiles in the inner regions
COOL CORE vs NON COOL CORE CLUSTERS
•CC have more peaked surface brightness (density) distribution
•CC have DECREASING temperature profiles in the inner regions
A SIMPLE MODEL: COOLING FLOW!
Cluster=sphere of gas in hydrostatic equilibrium
Radiation losses cool the gas, more efficiently in the high density
regions ( ε~n2). In order to keep hydrostatic pressure, the gas has to
increase its density, recalling mass from the outskirts to the center
(cooling flow)
… BUT THE COOLING FLOW MODEL IS WRONG!!
Lack of cool gas below a certain temperature value. Something
prevents the gas from cooling (AGN feedback)
COOL CORE vs NON COOL CORE CLUSTERS
•CC have more peaked surface brightness (density) distribution
•CC have DECREASING temperature profiles in the inner regions
•CC have short cooling time
u = energy density ~ nkT
tcool 
u

 n p1Tg1/ 2
ε = bremms emissivity ~n2T1/2


tcool  8.5 1010 yr  3 3 
 10 cm 
np
1
1/ 2
 Tg 
 8 
 10 K 
tcool < Hubble time (13.7 Gyr) only in the cores of CC clusters
COOL CORE vs NON COOL CORE CLUSTERS
•CC have short cooling time
tcool
 np

10
 8.5 10 yr  3 3 
 10 cm 
1
1/ 2
 Tg 
 8 
 10 K 
COOL CORE vs NON COOL CORE CLUSTERS
•CC have lower ENTROPY profiles
Specific entropy per particle s=T/n2/3 (keV cm2)
Entropy profiles in CC are
steeper and lower in the
inner regions
Pratt et al., 2009
Cool core
Non cool core
COOL CORE vs NON COOL CORE CLUSTERS
•CC have central peaks in Metal Abundance distribution
CC
NCC
The metal abundance central excess is consistent with enrichment from
the large elliptical central galaxy (BCG=Brightest Central Galaxy)
invariably found in those systems
COOL CORE vs NON COOL CORE CLUSTERS
•CC have more peaked surface brightness (density)
distribution
•CC have DECREASING temperature profiles in the inner
regions
•CC have short cooling time
•CC have lower ENTROPY profiles
•CC have central peaks in Metal Abundance distribution
Use these observational features to define indicators of the CC state
Good indicators should be effective and easy to calculate
COOL CORE vs NON COOL CORE CLUSTERS
•CC have more peaked surface brightness (density)
distribution
Slope of the brightness or density profile at a given radius
•CC have DECREASING temperature profiles in the inner
regions
Temperature drop in the inner region
•CC have short cooling time
Cooling time at a given radius/central cooling time
•CC have lower ENTROPY profiles
Central entropy (k0), entropy ratio
•CC have central peaks in Metal Abundance distribution
CLUSTER RADIO EMISSION
•Radio galaxies
•Extended emission (~Mpc)
from the ICM: Halos
Relics
(observed only in merging
clusters)
Synchrotron emission from the
ICM
Presence of RELATIVISTIC
PARTICLES and MAGNETIC
FIELDS in the ICM
The particle acceleration
mechanisms are likely related
to mergers
Synchrotron Radiation
dE
 1.6  10 15  2 H 2
dt
(erg s-1 if H in G)
 s  4.2 2 H
(MHz if H in G)
RADIO :
H = 10-6 G
  1000
OPTICAL :
H=1G
  104
X-RAY :
H = 10 G
  105
ENSEMBLE OF ELECTRONS
Synchrotron emissivity:
    N0 H (1 ) / 2 
Spectral index
N ( E )  N 0 E 

 1
2
AGEING:
only e- with
E < E* survive
spectral break
*  H-3 t
Original spectrum
Aged spectrum
-2
CLUSTER RADIO EMISSION
Radio halos:
COMA CLUSTER
•Cluster wide diffuse emission
•Located at the cluster center
•Low surface brightness
(μJy arcsec-2 @1.4 Ghz)
•No polarization
•Steep radio spectrum
Coma Cluster: first
cluster where a
radio halo was
detected
ROSAT PSPC
(White et al. 1993)
α=1 +
exponential
cutoff
HALO
Radio 90 cm
RELIC
(Feretti et al. 1998)
Thierbach et al.
2003
CLUSTER RADIO EMISSION
Radio relics:
Similar to radio halos but
•Located in cluster outskirts
•Elongated in shape
•Highly polarized
A3667
α=1
HALO
Radio 90 c
Röttgering et al. 1997
Jonhston-Hollit, 2001
Coma relic
Thierbach et al.
2003
CLUSTER RADIO EMISSION
Conditions in radio halos and relics
Radio power:
˜
1024 – 1025 W Hz-1
(@1.4 GHz)
Energy density:  10-14-10-13 erg cm-3
lower than the thermal one (10-11-10-12 erg cm-3)
Magnetic field:
˜ 0.1
- 1 μG
Lorentz factor: γ > 1000
Particle Lifetime:
8 yr
10
˜
Diffusion velocity:
100 km/s
The diffusion velocity of electrons in
the ICM is not sufficient to cover Mpc
scale distances during their lifetime
RELATIVISTIC ELECTRONS NEED
TO BE RE-ACCELERATED
CLUSTER RADIO EMISSION
How common is extended radio emission in clusters?
The presence of extended radio emission is NOT a common property in
galaxy clusters.
Radio halos and relics detected in:
•~10%of a complete X-ray
flux limited sample
•~35% of clusters with
Lx>1045 ergs s-1
(Giovannini et al 2000,
but possible evolution with z
suggested, Cassano et al.2007)
Feretti et al. 2000
CLUSTER RADIO EMISSION
ALL cluster containing a radio halo or relic show some indication of
recent dynamical activity. We are not presently aware of any radio halo
or relic in a cluster where a merger has been clearly excluded
Extended radio emission is probably related to cluster mergers
CAVEAT: not all merging clusters
host a radio halo or relic!!
Buote 2001
CLUSTER RADIO EMISSION
Extended radio emission is probably related to cluster mergers
Cluster mergers have enough energy to accelerate particles, but what
are the acceleration mechanisms?
•Shock acceleration (First order
Fermi acceleration)
•Stochastic acceleration by
turbulence following a merger
•Secondary Electron production (but
not obviously related to merger)
Still an open question: no
clear correlation between
merger shocks and radio
halos, unknown turbulence
of the ICM
CLUSTER FORMATION
We now know that the
Universe shows a large
scale structure, which can
be well explained by the
hierarchical scenario of
structure formation
In this scenario, small
structures form first,
while larger objects are
“built” later by the
accretion of smaller
subunits.
SIMULATION OF
THE
FORMATION OF
A GALAXY
CLUSTERS
Dark Matter only,
i.e. Gravity only
http://www-theorie.physik.unizh.ch/~moore/movies/expand_wrbb.mpg
CLUSTER FORMATION
Galaxy clusters are the
largest objects in the
Universe.
Snapshot from a
cosmological simulation
In the hierarchical scenario,
they form the youngest
population: the present is
the epoch of cluster
formation!
Cluster form through the
accretion of smaller subunits
and the interactions between
nearly equal size objects:
CLUSTER MERGERS
CLUSTER FORMATION
What is the energy involved during a cluster merger?
The velocity can be derived assuming a simple model,
conserving energy and angular momentum. It depends on the
mass of the objects and on the impact parameter. For typical
values:
v~2000-3000 km/s
Cluster mergers are the most energetic events in the Universe
since the Big Bang and they can release up to 1064 erg
Sarazin 2001, astro-ph/0105418
CLUSTER MERGERS and CC-NCC DISTRIBUTION
Cluster mergers drive shock waves and turbulence in the ICM:
•They alter the gas distribution and smooth out density
(brightness) gradients
•They heat the ICM
•They mix the gas modifying entropy and metal abundance
gradients
They have been suggested as the dominant mechanism to explain
the CC-NCC distribution
In this scenario, the CC state is the natural relaxed state to which
galaxy clusters evolve. Clusters remain in this state unless
disturbed by a merger
CC=relaxed object, NCC=interacting object
CLUSTER MERGERS and CC-NCC DISTRIBUTION
However, other models have been suggested to explain the CCNCC distribution, because of
1. Presence of intermediate peculiar objects
2. Difficulties in reproducing the observed distribution with
numerical simulations
Independent models: primordial division into the two classes
(McCarthy et al. 2004, Poole et al 2008, O’Hara et al 2006)
The question is still debated (Sanderson et al. 2009; Leccardi,
Rossetti & Molendi, 2009; Rossetti & Molendi, 2009)
“Studio della relazione tra presenza di aloni radio e assenza
di cool cores in un campione completo di ammassi di galassie”
SCOPO DELLA TESINA
I dati osservativi e i modelli ci indicano i radio aloni sono legati ai
mergers tra ammassi di galassie
I mergers sono anche indicati come responsabili della distribuzione di
ammassi in CC-NCC in uno dei modelli principali (ma non l’unico!!!)
Vogliamo verificare e mettere insieme questi modelli.
Gli ammassi con radio alone hanno un cool core?
Gli ammassi che non hanno un alone radio (ma che sono abbastanza
luminosi in X da poter essere osservabili in radio), come sono
distribuiti tra CC e NCC?