High Energy Astrophysics

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High Energy Astrophysics

• Clusters of Galaxies

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Introduction

• Clusters of Galaxies constitute the largest
  gravitationally collapsed structures in the
  universe
• Clusters are composed mainly of galaxies, hot
  gas and dark matter
• The first systematic study was carried out by
  Abell (1958) who compiled an extensive,
  statistically complete catalogue of rich clusters
  that has been one of the basics tools to study
  clusters.
• Clusters of galaxies were first detected in
  X-ray in the early 70s with the large sky area
  observations of the Uhuru X-ray satellite
  (Giaconni et al, 1972).

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Introduction First observed properties

• The X-ray luminosities are of the order of
  1043-45 erg/s
• The X-ray emission is spatially extended
• The X-ray emission did not vary in temporally in
  their brightness
• The X-ray spectra were consistent with a thermal
  bremsstrahlung spectrum from hot gas
• The X-ray spectra showed emission lines from Fe
  implying that the intracluster medium is enriched

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Optical properties

• Galaxy members 1000
• Size R 1.5 Mpc
• Mass 1013-15 Msolar
• baryon fraction 10-20
• Proportion of universe galaxes 10
• Galaxy types E, S0, S

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Optical properties
Catalogues

• The most extensive and more widely used
  catalogues of rich clusters are those of Abell
  (1958) and Abell et al 1989 and Zwicky et al
  1961-68.
• Selected by visual inspection of photographic
  plates
• Clusters are selected as overdensities compared
  to a background
• Abells criteria (1) at least 50 galaxies in
  the magnitude range m3 to m32, (2) contained in
  a radius RA1.7/z arcmin (3) estiamted cluster
  redshift 0.02 lt z lt 0.20

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Optical properties
Catalogues

• With the advent of high QE CCDs in the early
  90s, optical cluster catalogues revived.
• Automated searches
• Match Filter Palomar Distant Cluster Survey
  (Postman et al 1996) selection filtering the
  data with amodel of the spatial and luminosity
  distribution.
• Voronoid tesselation (Kim et al 2002)
• Color selection Red Cluster Sequence (Gladders
  Yee 2000)

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Optical properties
Richness

• Number of galaxies in the cluster
• Statistical measure depending on membership
  criteria

Luminosity function

• Gives the number distribution of the
  luminosities of the galaxies
• Schechter luminosity function
• parameters

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Optical properties
Morphological classification

• Based on different properties
• Zwicky compact, medium compact, open
• Bautz-Morgan Type I, II, III
• Rood-Sastry cD, B, L, C, F, I

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Optical properties
Velocity distribution

• Existence of morphological sequence from
  irregular to regular suggests that regular
  clusters may have undergone some sort of
  relaxation
• The redshift of a cluster is determined from the
  mean radial velocity of galaxies in a cluster
• Normally, the velocity distribution is
  characterized by the dispersion of radial
  velocities around the mean. The velocity
  dispersion completely characterizes the radial
  distribution function of velocities if it is
  Gaussian

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Optical properties
Velocity distribution

• Radial velocity distribution are found to be
  close to Gaussian suggesting that they are at
  least partly relaxed systems in which one can
  define a temperature
• sr (kT/m)1/2

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Optical properties
Spatial distribution

• The most regular clusters show a smooth galaxy
  distribution with a concentrated core
• Normally five parameters position, central
  projected density, core scale and maximum radial
  extent

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Optical properties
Cluster masses

• Assuming that cluster are bound,
  self-gravitating system one can calculate their
  masses
• If clusters were not bound they would disperse
  as their crossing times (tcr 109 years) are
  shorter than their ages
• Virial theorem
• M/L ratios
• Missing mass Dark Matter

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Optical properties
Galactic Content

• cD
• Proportion of E, S0, Sp

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Radio properties

• Emission from galaxies within the cluster
• Head-tail and wide-angle-tail radio sources
• Radio haloes

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Radio properties
Sunyaev-Zeldovich effect

• The free electrons in the ICM (inverse Compton)
  scatter low energy photons from the Cosmic
  Microwave Radiation

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Cluster X-ray Observations
Detections and identifications

• The first extragalactic object to be detected as
  an X-ray source was M81 in the Virgo cluster
  (1966)

UHURU

• clusters of galaxies are the most common bright
  extragalactic X-ray sources
• They are extremely luminous Lx 1043-45 erg/s
  and have a wide range of luminosities
• The X-ray sources associated with clusters are
  extended

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Cluster X-ray Observations
UHURU

• clusters of galaxies X-ray spectra show no
  strong sign of low energy photoabsorption
• The X-ray emission is not variable

HEAO-1 A2

• First X-ray spectrum thermal bremsstrahlung
• First all-sky survey first flux limited sample
  (Piccinotti et al 1982)

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Cluster X-ray Observations
EINSTEIN

• First X-ray imaging detectors X-ray
  morphologies
• EMSS X-ray LF evolution

ROSAT

• Highly improved sensitivity
• Soft X-rays
• RASS pointed observations

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Cluster X-ray Observations
ASCA

• Spectral resolution

BeppoSAX

• High energy response

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New Generation of X-Ray Observatories
Chandra

• Spatial resolution 1
• Instruments ACIS, spectrograph

XMM-Newton

• Large collecting area
• Good spectral resolution
• Instruments MOS, pn, spectrograph

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Cluster X-ray Observations
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Cluster X-ray Observations
X-ray luminosities luminosity function

• Observe count-rates gt flux gt luminosity
• They are extremely luminous Lx 1043-45 erg/s
• The luminosity function is the number of
  clusters per unit volume with X-ray luminosities
  in the range of Lx to LxdLx f(Lx)dLx
• The observed luminosity function is well-fit to
  a Schechter function
• f(Lx) A (L/L)a exp (-L/L)

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Cluster X-ray Observations
Spatial distribution of X-ray emission

• Extended emission
• The surface brightness is well fit in the
  majority of cases by the so-called beta model
  profile
• Sx Sxo 1 (r/rc)2-3ß1/2

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Cluster X-ray Observations
Morphology of the ICM

• Clusters present varied morphologies although
  they are mostly ellipsoids
• Forman Jones (1982) proposed a two-dimensional
  scheme for the X-ray morphology. First they are
  irregular (early) or regular (evolved).
  Secondly, the presence or absence of a dominant
  galaxy in the center X-ray dominant (XD) or non
  X-ray dominant (nXD).
• Evolutionary sequence
• Evolution of X-ray morphology gt Cosmology

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Cluster X-ray Observations
X-ray Spectra

• Clusters exhibit thermal bremsstrahlung spectra
  from their thin, high temperature, highly ionized
  intracluster medium
• Typical temperatures are of the order of a few
  keV
• Typical metallicities are of the order of 1/3
  solar
• Spectra show a-element enhancement
• In most (non-cooling flow) clusters there is
  negligible low energy absorption
• Cooling flows

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Hot Intracluster Medium Formation

• Clusters of Galaxies form from gravitational
  collapse of high density peaks
• Cluster collapse dominated by dark matter with
  baryons following the potential wells dominated
  by dark matter
• During collapse the baryons suffer adiabatic
  compression and heating by gravitationally
  induced shocks, resulting in the formation of a
  hot intracluster medium
• For typical cluster masses (1015 M?) the gas
  reaches temperatures of several 107 OK and
  becomes fully ionized.

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Hot Intracluster Medium Emission

• 10-15 cluster mass is hot gas trapped in the
  cluster potential well
• If gas dynamics corresponds to galaxy dynamics
  kBT µmpsv2 6 (sv
  /1000 kms-1)2 keV

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Hot Intracluster Medium Emission
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Hot Intracluster Medium Emission

• 10-15 cluster mass is hot gas trapped in the
  cluster potential well
• If gas dynamics corresponds to galaxy dynamics
  kBT µmpsv2 6 (sv
  /1000 kms-1)2 keV
• The hot fully ionized ICM emits thermal
  bremsstrahlung in X-rays
• The spectrum can be characterized by
  Raymond-Smith (1977) spectrum thermal
  bremsstrahlung, lines and edges. Further
  refinements Mekal (Mewe et al 1985, Kaastra 1992
  Liehdal 1995)

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Hot Intracluster Medium Emission
Thermal Bremsstralung Spectra

• Fellen et al (1966) first suggested that the
  X-ray emission from clusters was due to a diffuse
  intracluster gas at a temperature Tx108 K and an
  atomic density n10-3 cm-3
• At these T and n, the primary emission process
  for a gas composed mainly of hydrogen is thermal
  bremsstrahlung (free-free) emission
• The emissivity at a frequency ? of an ion of
  charge Z in a plasma with an electron temperature
  Tg is given by e?ff

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Hot Intracluster Medium Emission
Thermal Bremsstralung Spectra

• The emissivity is defined as the emitted energy
  per unit time, frequency and volume
• The Gaunt factor gff(Z, Tg, ?) corrects for
  quantum mechanical effects and for the effect of
  distant collisions and is a slowly varying
  function of frequency and temperature
• If the cluster gas is at a single temperature
  then the spectrum is close to an exponential
• If the cluster is relaxed and the gas and
  galaxies are both in equilibrium with the cluster
  potential then kBT µmpsv2 6 (sv /1000 kms-1)2
  keV

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Hot Intracluster Medium Emission
X-ray emission assumptions

• The time scale for elastic Coulomb collisions
  between particle in the plasma is much shorter
  than the age or cooling time of the plasma and
  thus the free particles will be assumed to have a
  Maxwell-Boltzmann distribution at a temperature
  Tg, the kinetic temperature of the electrons that
  determines the rates of all excitation and
  ionization processes

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Hot Intracluster Medium Emission
X-ray emission assumptions

• At the cluster low densities collisional
  excitation and de-excitation processes are much
  lower than radiative decays, and thus any
  ionization or excitation process will be assumed
  to be initiated from the ground state of an ion.
  Three-body collisional processes are ignored
• The radiation field is sufficiently dilute that
  stimulated radiative transitions are not
  important, and the effect of the radiation field
  on the gas is insignificant

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Hot Intracluster Medium Emission
X-ray emission assumptions

• At the cluster low densities, the gas is
  optically thin and the transport of the radiation
  field can be ignored
• Under these conditions, ionization and emission
  result primarily from collisions of ions with
  electrons
• The time scales for ionization and recombination
  are generally considerably less than the age of
  the cluster or any relevant hydrodynamic time
  scale and the plasma will be assumed to be in
  ionazation equilibrium

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Hot Intracluster Medium Emission
Ionization Equilibrium

• In equilibrium, the ionization state is
  determined by the balance between processes that
  produce or destroy each ion
• The collisional ionization rate is the sum of
  two processes direct collisional ionization and
  collisional excitation of inner shell electrons
  to autoionizing levels which decay to continuum.
  Recombination is also the sum of two processes,
  radiative and dielectronic recombination
• The electron density dependence cancels out and
  the equilibrium ionization state of a diffuse
  plasma depends only on the electron temperature

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Hot Intracluster Medium Emission
Ionization Equilibrium

• At cluster temperatures iron and metals are
  mainly in the full stripped, hydrogenic or
  helium-like stages

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Hot Intracluster Medium Emission
X-ray emission

• The X-ray continuum emission from a hot diffuse
  plasma is due primarily to three processes
  thermal bremsstrahlung (free-free emission),
  recombination (free-bound emission), and
  two-photon decay of metastable levels
• The radiative recombination continuum emissivity
  is usually calculated by applying the Milne
  relation for detailed balance to the
  photoionization cross sections
• The two-photon continuum comes from the
  metastable states of hydrogen or helium-like ions
• At clusters Ts thermal bremsstrahlung is
  dominant

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Hot Intracluster Medium Emission
X-ray emission

• Processes that contribute to the X-ray line
  emission from a diffuse plasma include
  collisional excitation of valence or inner shell
  electrons, radiative and dielectronic
  recombination, inner shell collisional
  ionization, and radiative cascades following any
  of these processes.
• The emissivity
• Lines and line ratios are particularly suited
  for determining the temperature, ionization
  state, and elemental abundances in the
  intracluster gas