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Cooling Flows in Clusters of Galaxies. A.C. Fabian ARAA 32, 277-318 (1994) and recent progress. The Origins of “ Cooling Flows ”. Clusters discovered to be extended X-ray sources (Gursky et al. 1971, UHURU; Kellogg et al. 1972; Forman et al. 1972)

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cooling flows in clusters of galaxies

Cooling Flows in Clusters of Galaxies

A.C. Fabian ARAA 32, 277-318 (1994)

and recent progress

the origins of cooling flows
The Origins of “Cooling Flows”
  • Clusters discovered to be extended X-ray sources (Gursky et al. 1971, UHURU; Kellogg et al. 1972; Forman et al. 1972)
  • Thermal emission was the natural interpretation (Felten et al. 1966; Lea et al 1973; Lea 1975) given the spectrum (Davidsen et al. 1975; Gorenstein et al. 1973.) and extent.
  • Intracluster Fe detected: hot gas (Mitchell et al. 1976, Ariel V; Serlemitsos et al. 1976 OSO8)
  • Cooling times in many clusters very short. (Cowie & Binney 1977; Fabian & Nulsen 1977; Mathews & Bregman 1978; Cowie, Fabian, & Nulsen 1980)
  • Unusual optical nebulae associated with cluster cooling flows. (Hu, Cowie & Wang 1985; Heckman et al 1989;Fabian, Nulsen & Canizares 1984)
some basic parameters of clusters
Some basic parameters of clusters
  • Cluster mass: 1014-1015 msolar

X-ray luminosity 1043-1045 erg/s

radius: 1-2 Mpc, core radius: 0.3-0.5 Mpc

  • Intracluster gas temperature: 107-108 K,

number density: 10-4-10-2cm-3

total gas mass 5*1013-5*1014 Msolar (in rich cluster)

chemical abundance: Fe ~0.3 times solar unit

  • Cluster galaxies velocity dispersion: 300-1200 km/s
  • Intracluster magnetic field: 1-40 μG

Perseus Cluster

WIYN (3.5m)


Chris, Conselice

filamentary structure are clearly seen

Image of the Perseus cluster from the Palomar Sky Survey. At the center is the cD (central dominant) galaxy Perseus A or NGC 1275
High resolution of the central regions of NGC 1275, where a smaller galaxy is being canabalised by the central galaxy. The central source and the filamentary structure are clearly seen. The large number of blue point sources are fresh clusters of stars forming from the shocked gas in the collision.

Zoomed in view of the very central regions of NGC 1275, showing the freshly formed clusters of stars and dusty regions which are probably a result of the collision.



X-ray image of Perseus A taken with the Chandra telescope, showing a central bright source, two “bubbles” above and below this source, and the shadow of an infalling galaxy (top right). The remaining emission is from the central part of a cooling flow (the inward flow of hot gas into the potential well of the elliptical) which is losing energy (i.e. cooling) via thermal bremsstrahlung emission in the X-ray region.

VLA radio image of Perseus A, overlayed on the X-ray image from the Chandra telescope. The central source is clearly seen, as well as radio lobes which apparently coincide with the two circum-nuclear “bubbles” in the X-ray image.
what is cooling flow

…but the total rate of energy change is not simple:

What is Cooling Flow?

T>3e7, thermal bremsstrahlung is the main radiation mechanism.



X-ray Luminosity is

heat loss

No heating


Extra assumptions: atomic physics determines L and T,

Locally Maxwellian, no absorption, metal distribution,

Exact prediction for mdot depends on grav. potential

evidence for cooling
Evidence for cooling

The central luminosity is extraordinary high.

radiative cooling times from chandra
Radiative cooling times from Chandra



Other clustersVoigt & Fabian 03

Perseus cluster

evidence for cooling1
Evidence for cooling

Allen et al 01

The central part is cooler due to high number density


X-ray spectrum: low ionization emission line

4 actual cooling flows

Mukai, Kinkhabwala, Peterson, Kahn, Paerels 2003

non x ray evidence for cool gas and young stars
Non X-ray evidence for cool gas and young stars

Optical: Crawford et al 99

UV: Oergerle et al 01

CO: Edge 02

Dust: Edge et al 99


More than 30-50% of the clusters have surface brightness tcool<H0-1 within central 100 kpc.

Cooling flow condition also occur in large, isolated elliptical galaxies.

evidence of a problem xmm rgs
Evidence of a problem: XMM/RGS

Sakelliou et al 02

Gas drops to Tmin~0.3Tvir

Chandra spectra consistent


Peterson et al 01,02

McNamara et al ; David et al ; Allen et al; Blanton et al ; Buote et al………

xmm spectroscopy
XMM spectroscopy
  • Peterson, et al. 2003
  • FeXVII and other lines from 1 keV gas not present.
  • Two-temperature or “truncated” cooling flow (at ~T/3 - T/2)
missing intermediate temperature x ray lines ovii fexvii
Missing intermediate temperature X-ray lines (OVII, FeXVII)
  • FeXVII in Perseus

then and now:

  • Einstein/FPCS 1 cm2
  • XMM/RGS 60 cm2
Result not explained by just heating gas at Tmin or just rearranging gas
  • Since and at 1keV

at constant P

  • The cooler the gas, the faster it cools
  • Why doesn’t it cool?
the cooling flow problem cfp
The cooling flow problem (CFP)

Why, and how, is the cooling of gas below Tvir/3 suppressed?


The stellar/gaseous parts of galaxies are due to the cooling of gas in their DM potential wells

Cooling in clusters and groups should be an observable example of this process.

The suppression of cooling in these objects may explain the upper mass cutoff of galaxies.

possible solutions of the cfp
Possible solutions of the CFP
  • Absorptionmissing soft X-ray
  • Mixingluminosity ~ LUV
  • Inhomogeneous metallicity
  • Heating ─ from outside

─ from centre (AGN)

─ non-steady

─ combinations


agn radio source
AGN/Radio source
  • Most CF have one, but no obvious correlation

 non-steady?

  • Energy Budget

seems inadequate for most systems (by 10x).

It maybe not the dominant heat source

Voigt & F 03

  • Extreme energies required for luminous clusters, eg A1835, RXJ1347
thermal conduction
Thermal Conduction
  • Big Problem: how is the conduction so “smart”.

conduction coefficient needs to be 19% of KSpitzer for one cluster, 41% for another, etc.

What isκ?

Voigt & Fabian 03


Possible Solution

add in the stellar mass loss from stars in cD

this gas is at 1 keV after themalization (provides Tmin)

the gas is flowing outward into the cluster

T will rise from Tmin to Tcluster in all cases,

but the shape varies with κSpitzer


not sensitive to κSpitzer as long as it is sufficiently large

produces a significant dT/dr that would be similar from

cluster to cluster

don’t get the intermediate temperature cooling lines

new clues from deep chandra observations of perseus a c fabian et al mnras 344 2003 l43
New clues from deep Chandra observations of PerseusA.C. Fabian et al., MNRAS. 344 (2003) L43

Three-colour image of the Perseus cluster from Fabian et al (2000). Red indicates regions of low-energy emission, blue indicates high-energy emission. Visible in dark green above the centre is an infalling dwarf galaxy, shown by absorption of high-energy X-rays.

Chandra 200 ks observation of the Perseus cluster.

the lowest note observed in the universe
The lowest note observed in the universe

Fluctuations in the Perseus cluster caused by sound waves generated from the central black hole.

about 57 octaves lower than middle-C.

Temperature map of the Perseus cluster.

ripples and weak shocks
Ripples and weak shocks
  • Bubbles make sound wave of long period (~107 years). Create weak shocks with some dissipation.
  • Further out, dissipation depends on viscosity. What is viscosity?

Weak shock to NE

K~108T 5/2n -1

Viscous heating=rad. cooling in inner 50 kpc of Perseus

collisional viscosity
Collisional viscosity
  • Has similar temperature dependence to conduction
  • But… MHD effects?
  • Viscous dissipation could be crucial
  • Turns sound energy from core and outer parts (refracted into core Pringle 89) into heat
  • Cooling flow problem is widespread and plays a significant role in massive galaxy formation
  • Many solutions proposed but all have drawbacks
  • Knowledge of transport processes in ICM may be crucial to the solution
interesting questions about clusters
Interesting questions about clusters
  • Why don't they cool as quickly as we expect?
  • What's the history of the gas in the cluster, and how did it get filled by the chemical elements we see today?
  • What is the cluster magnetic field role in the formation and evolution of clusters?
  • How does the cluster interact with the central black hole?

There seems to be an association between these events in a galaxy cluster:

CO emission

Hα emitting gas

star formation

AGN (radio lobe) activity

“classic” X-ray cooling flow structure

One interpretation of this is within the Cooling Flow paradigm (at reduced Mdot):

Cooling Flow → Cooled Gas (Hα + CO) → star formation (possibly triggered by AGN)

cluster magnetic field
Cluster magnetic field

Faraday rotation

Centaurus cluster

Chandra + VLA

cf b is correlated
CF/B is correlated!

A correlation is found between the cooling flow rate and the maximum Faraday rotation measures. Magnetic fields of strength 10–40 μG are found

to be common to the centres of clusters with strong cooling flows, and somewhat lower field

strengths of 2–10 μG are found in the non cooling-flow clusters.

G. B. Taylor et al. MNRAS (2002) 334,769