Transcript The BeppoSAX view of Galaxy Clusters:

CLUSTERS OF GALAXIES
The Physics of the IGM:
Cooling Flows
Cooling Flows

Observational evidences

The homogeneous model: one ρ and T at each radius

Observational evidence against homogeneous gas

The inhomogeneous model: Δρ and ΔT at each radius

The role of the magnetic fields in Cooling Flows

Estimates of dM/dt from imaging & spectral data

The fate of the cooling gas
Cooling in Clusters
LX  ngas2 Tg1/2 Volume
E ngasKTg Volume
tcool  E/LX Tg1/2 n-1
Cooling Flows
tcool ≈ Tg1/2 np-1

For large radii np is small

In the core np is large
tcool »tHubble
tcool ~ tHubble
The gas within rcool will cool
Cooling Flows
When the gas cools
The pressure becomes lower
The gas flows inwards,
The density increases in the center
The gas cools even faster
Observational Evidences for
Cooling Flows
X-Ray Imaging


Surface brightness strongly peaked at the center
Observational Evidences for
Cooling Flows
X-Ray Imaging


Surface brightness strongly peaked at the center
Peres et al. (1998)
Observational Evidences for
Cooling Flows
X-Ray Spectra


Low ionization lines in soft X-ray spectra
Canizares et al. (1984)
Observational Evidences for
Cooling Flows
X-Ray Spectra


Temperature gradients towards the center
rcool
De Grandi &
Molendi (2002)
Observational Evidences for
Cooling Flows
X-Ray Spectra


Low energy absorption features
T(r)
Allen et al. (1993)
NH(r)
Observational Evidences for
Cooling Flows
 X-Ray Spectra

Low ionization lines in soft X-ray spectra

Temperature gradients towards the center

Low energy absorption features

No direct evidence of the gas motion, resolution
of X-ray detectors is insufficent
Homogeneous Model
Hot gas – one Tg and ng at each r – radiation losses

P decreases
Gas will flow inwards under the pressure of
the overlaying gas

3 Dynamic regions:
1.
2.
r > rcool and tcool > tHubble Hydrostatic Equilibrium
rgal < r < rcool with rgal= radius at which the gas falls within
.
3.
r<rgal
potential well of the cD galaxy
Φ
1
2
3

Region 2






rcool
ΔΦ/Δr is small
radiation losses balanced by thermal energy + PV
vs>>vfree fall
The gas is in quasi-hydrostatic equilibrium
Region 3

rgal
ΔΦ/Δr ≠ 0
gravitational energy balances radiation losses
r
Hydrodynamic equilibrium for
Homogeneous Model
1.
Mass conservation
1 d 2
r ρv  0
2
r dr

2.

dM
 ρv 4 πr 2  const
dt
Momentum conservation
dP
d
 ρ
dr
dr
3.
Energy conservation
variation of H per
unit volume & time
H  U  PV 
5P
2 ρ
d
ρ
dt
5 P

  n2 ( T )
2 ρ
Entalphy=thermal E + work by P
energy radiated per
unit volume & time
.
M estimates for
Homogeneous model
Energy
loss rate
LX,cool
. 5
kT 

  M
 M

2
ρ
2
μ
m


H 

Mass flow rate
LX,cool  10% LX,bol
.
. 5 P 
Enthalpy
.
 M  30  300 M /yr
Peres et al. (1998)
Observational evidences against
.
M=const
The surface brightness is not as peaked as would be
expected if all the cooling gas were to reach the center
.
M≠const 
.
Mrα
with α≈1
A fraction of the gas drops
out the flow before
reaching the center
Most of the cooling gas
never makes it to the
center
Peres et al. (1998)
(Fabian, Nulsen &
Canizares 1984)
In-homogeneous Model

Different phases T,ρ coexist at every r

Phases are in Pressure equilibrium (ts<tcool)


(Nulsen ’86)
The phases comove with <v> « vs, B field ties the different
phases together
Heat conduction btw phases must be suppressed, again B fields
have been invoked
@ T≈106 K
tcoolts
The cool blobs decouple
from the flow and:
1.
Fall ballistically?
2.
Stay in place as cold gas?
3.
Stay in place and form stars?
Summary
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Gas that is already highly inhomogeneous cools and flows
inward under the pressure of the gas immediately on top.
The different phases are in pressure equilibrium and comove
(B field).
When a given phase cools below ≈106K it falls out of
pressure equilibrium while the other phases continue to flow
inwards
Cold gas deposition occurs on the whole CF region with
similar  for different clusters (dM(r)/dtrα)
The origin of the density inhomogeneity is unclear:
1.
fossil of the past stripping from galaxies (Soker et al. ’91)
2.
former mergers btw substructures with different T and ρ
.
M estimates for
in-homogeneous model
1.
From Imaging data

.
2.
.
within the context of the in-homogeneous model
ΔLj, luminosity in a given radial shell and Mj mass
flow rate in the same shell are related through a
linear formula  from this, values M can be
computed
From Spectral data
Tmax
.
5 k
 (T )
LX ,cool ( ) 
M 
dT
2 mH
(T )
0
stricly valid for homogeneous model, reasonable approx.
for inhomogeneous model (Wise & Sarazin ’93)
.
.
Comparison btw. Ms and MI
Allen (2000)
The role of B fields
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Tangled field inhibit thermal conduction by increasing
the particle mean free path
Once a blob has cooled down to ~ 106 K radiation
cooling becomes very fast
ρ ≈ constant, T decreases, Pgas decreases
repressurizing shocks are partially
suppressed by the PB
At T ~ 106 K trecon ≈ tcool  magnetic energy will be
converted into thermal energy thereby slowing down the
collapse of the blobs.
The fate of the
cooling gas

Cooling flow is a frequent phenomenon (~ 60%-70%)

Cooling flow is a persistent phenomenon
1012
Msun
.
[M/(100M
1.
Macc ≈
sun/yr)]
(Sarazin ’89)
2.
Macc « Mcluster ≈ 1014-1015 Msun
3.
Macc comparable to mass of the cD galaxy
The fate of the
cooling gas
(A) Ionized
Cold gas
(B) Neutral
(C) Molecular
(A)
Lines observed in optical and UV indicate that
ionized gas is present « Macc
The fate of the
cooling gas
(A) Ionized
Cold gas
(B) Neutral
(C) Molecular
(A)
(B)
Lines observed in optical and UV indicate that
ionized gas is present « Macc
21 cm observations in central galaxies give
MHI  109 Msun
The fate of the
cooling gas
(A) Ionized
Cold gas
(B) Neutral
(C) Molecular
(A)
(B)
(C)
Lines observed in optical and UV indicate that
ionized gas is present « Macc
21 cm observations in central galaxies give
MHI  109 Msun
Recent obs. (Edge 2002) have detected molecular
gas for the first time, again « Macc
?
SUMMARY

B fields play an important role in CF

A dominating fraction of galaxy clusters feature CF
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Analysis of X-ray images and spectra lead to consistent
determination of mass deposition rates.

From X-ray observations we find that CF deposit large quantities
of cold gas
At larger wavelenghts we do not find
evidence of such large masses
 the fate of the cooled gas is unknown
.
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It is somewhat disturbing that all
crucial evidences for cooling flows comes
from X-ray data
Even in the X-rays we do not have
direct observational evidence of:
1.
flowing gas
2.
multiphase gas at one radius