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X-ray Spectra of Clusters of Galaxies. John Peterson Purdue University X-ray Gratings 2007 Boston, MA. Intracluster medium. Optical. X-ray. Heated due to large gravitational potentials Temperatures ~ 1-10 keV (10 7 to 10 8 K) Densities ~ 10 -5 to 10 -1 particles per cubic cm

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x ray spectra of clusters of galaxies

X-ray Spectra ofClusters of Galaxies

John Peterson

Purdue University

X-ray Gratings 2007

Boston, MA


Intracluster medium



Heated due to large gravitational potentials

Temperatures ~ 1-10 keV (107 to 108 K)

Densities ~ 10-5 to 10-1 particles per cubic cm

Sizes ~ 1 to 10 Mpc (1024 to 1025) cm


<=X-ray Spectra (prior to 2000)

X-ray Spectrum dominated by line emission and

Bremmstrahlung from collisionally ionized plasma

Plasma out of LTE

optically thin

At densities and temperatures (in core),

trecombination = 106 years (for Fe XVII at 1 keV)

tcool= (5/2 n k T)/(n2) = 108 to 109 years

tformation= 5 109 years

Collisional ionizations balanced by recombinations

Line emission dominated by collisional excitations+cascades,

Radiative recombination, and dielectronic recombination

Same model as stellar coronae

cooling flows
Long-standing prediction that cores of clusters should cool by emitting X-rays in less than a Gyr (Fabian & Nulsen 1977, Cowie & Binney 1977, Mathews & Bregman 1978)Cooling Flows

Density rises and

tcool is short

(e.g. Voigt et al. 2002)

from Images

Temperature Drops (e.g. Allen et al. 2001)

From CCD spectral fits


Cools unevenly=>Range of emperatures approximately at constant pressure

  • Differential Luminosity predicted to be:
  • dLx/dT=5/2 (Mass Deposition Rate) k/(mp)
  • Predicts a unique X-ray spectrum; Free parameters: Tmax, Abundances
  • The major assumption is that the emission of X-rays is the dominate heating or cooling term

Measuring a differential luminosity at keV temperatures

=> Need Fe L ions (temperature sensitive)

=> Need to resolve each ion separately (i.e. / ~ 100)

Very difficult to do in detail with CCD instrument

(ASCA, XMM-Newton EPIC, Chandra ACIS)

Works with XMM-Newton RGS (for subtle reasons)


RGS (dispersive spectrometer) :

High dispersion angles (3 degrees)

/ ~ 3 degrees / ang. size ~ 100

for arcminute size

Soft X-ray band from Si K to C K;

FOV: 5 arcminutes by 1 degree

Analysis not simple: dispersive, background, few counts


Detailed studies best done with full Monte Carlo


failure of the model
Failure of the Model

<= dL/dT= constant


8 keV  3 keV  ?

Peterson et al. 2001



into temperature

bins and set



Hot clusters

Peterson et al. 2003


Warm Clusters

Peterson et al. 2003


Cool clusters/groups

Peterson et al. 2003


Peterson et al. 2003

Differential Luminosity vs.

Fractional Temperature

Differential Luminosity vs. Temperature


Theoretical Intepretation: Essentially Three Fine-tuning Problems

1. Energetics: Need average heating or cooling power ~ Lx

Dynamics: Either need energy source to work at low temperatures or at t ~ tcool (before complete cooling would occur)

Cooling time ~ T2 / (cooling function)

If at 1/3 Tmax then why cool for 8/9 of the cooling time?

or why at low temperatures?

  • Get Energetic and Dynamics right at all spatial positions

Soft X-rays missing throughout entire cflow volume


Current Models

1. AGN reheating: relativistic flows inflate subsonic cosmic ray bubbles & cause ripples; dissipation efficiency? & feedback mechanism?

(Rosner & Tucker 1989; Binney & Tabor 1995; Tabor & Binney 1995; Churazov et al. 2001, Bruggen & Kaiser 2001; Quilis et al. 2001, David et al 2001; Nulsen 2002; Kaiser & Binney 2002; Ruszkowski & Begelman 2002; Soker & David 2003; Brighenti & Mathews 2003)

McNamara et al. 2000

Fabian et al. 2003


2. Heat transfer from the outside to the core: probably through conduction

Stability & is conduction coefficient realistic

(Tucker & Rosner 1983, Stewart et al. 1984, Zakamska & Narayan 2001; Voigt et al.2002; Fabian, Voigt, & Morris 2002; Soker 2003; Kim & Narayan 2003)

Voigt et al. 2003


3. Cooling through non radiative interactions with cold material:

Avoids producing soft X-rays?

(Begelman & Fabian 1990; Norman & Meiskin 1996; Fabian et al. 2001, 2002; Mathews & Brighenti 2003)

Fabian et al. 2002

4. Cluster Mergers (Markevitch et al. 2001)

5. Inhomogenous Metals (Fabian et al. 2001; Morris et al.200)

6. Differential Absorption (Peterson et al. 2001)

7. Cosmic Rays Interactions (Gitti et al. 2002)

8. Photoionization (Oh 2004)

9. Non-maxwellian particle ionization (Oh 2004)

Crawford et al. 2003


10. Dark Matter

(huge energy source):

Dark Matter-Baryon interactions (Qin & Wu 2001):

Requires high cross-section (/m ~ 10-25 cm2/GeV )

Dark Matter (Neutralino) Annihilations (Totani 2004):

Converts to relativistic particles

Requires a high central density for neutralino

Dark Matter-Baryon Interactions (Chuzhoy & Nusser 2004):

same cross-section but make

mass of dark matter ~ 1/3 of proton mass



Use hundreds of gaussian blobs

with own properties (e.g. temperature) instead of a

parameterized model


4 actual cooling flows:

Mukai et al. 2003


Long-standing problem of the origin of metals in the ICM:

Supernovae Ia (what fraction?)+ Type II (of what mass?) and of what metallicity (and therefore when)?+Stellar winds (for CNO)+ Hypernovae?

Zi = YieldIA(z)+  dM YieldII (z,M) dN/dM



Fe=>mostly Ia

O/Fe=0.7+/-0.2=>50% II

Ne/Fe=1.1+/0.3=>100% II

Mg/Fe=1.0+/0.3=>100% II

Si/Fe=2.3+/1=>100% II

Tamura et al. 2004

Matsushita et al. 2003

O/Fe 0.6 0.5 Mg/Fe 0.8 Si/Fe 1.4 1.2 S/Fe 1.1 1.1

Spatially resolved

Abundances much more


Peterson et al. 2003


Sersic 159-03, de Plaa et al. 2005

NGC 5044, Buote et al. 2005

Spatial Distribution of Abundances

Abundances depend on temperature model sensitively

Gradient in Metals ~ 100% per 100 kpc

Gradient in O/Fe or Si/Fe < 20% per 100 kpc


Evidence for a Low T (0.7 keV) diffuse

thermal component (WHIM) still unsettled

OVII emission, Kaastra et al. 2001

Absorption (3 sigma) behind

Coma, Takei et al. 2007


Large soft X-ray background from within the galaxy (McCammon et al. 2002)

Subtleties of particle

background in CCD fits, de Plaa et al 2005


Resonant Scattering

  • ~ ni i (cluster size) ~ few for some transitions

( Fe XXV He r, Fe XXIV 3d-2p, Fe XVIII 3d-2p, Fe XVII 3d-2p,

possibly some Ly alpha transitions)

But doppler velocities can lower this

(thermal width ~ 100 km/s, sound speed ~ 1000 km/s)

NGC 4636, Xu et al. 2002

Perseus, Churazov et al. 2004


Cooling flow model fails to reproduce X-ray spectrum; Several strong observational constraints (factor of 20!) Fails despite very simple theoretical arguments

Much more theoretical work needed for fine-tuning challenges

Much more observational work is needed to constrain the spatial distribution and to connect to other wavelengths

Abundances still need more study

Soft excess inconclusive

Resonant scattering inconclusive

Note: radiative cooling is supposed to form galaxies through

tiny cooling flows. Do we understand this now?