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Giovanni Covone » 12.Introduction to the study of the intracluster medium


Introduction: the ICM

The space among galaxies is not as empty.

Astrophysical evidences of the Intracluster Medium (ICM) and the Intergalactic Medium (IGM).

ICM: Intense X-ray diffuse emission.

Emission lines from ionized iron.

Chemical abundances in the ICM.

Temperature of the hot gas in the galaxy clusters halos.

Importance of studying the ICM

In rich clusters the hot intracluster medium is the dominant, luminous component of the mass.

Studies of the ICM yield information on a rich variety of phenomena.

  • Evolution of clusters.
  • Mapping the structure and mass distribution of clusters.
  • Role of the central galaxy.
  • Trace large scale structure.

Gas mass in galaxy clusters

ICM mass to the stellar mass is plotted against the temperature of the gas. Credit: David et al. (1988).

ICM mass to the stellar mass is plotted against the temperature of the gas. Credit: David et al. (1988).


Cluster merger: spatial distribution of the ICM

Distribution of hot gas (blue) and cool gas (red) in massive galaxy cluster MACSJ0717. Credit: NASA/Chandra.

Distribution of hot gas (blue) and cool gas (red) in massive galaxy cluster MACSJ0717. Credit: NASA/Chandra.


X-ray luminosities

The ICM behaves as a fully ionized plasma at temperature of about 7 keV.

Emissivity dominated by thermal bremsstrahlung.

The emissivity at frequency \nu:

\epsilon_{\nu}  \propto n_e n_i \, g(\nu, T) \, T^{-1/2} \, {\rm exp} \left(-\frac{h \nu}{k_B T} \right)

n_e and n_i: number density of electrons and ions.

g(\nu, T ) \propto {\rm ln}(kBT/h\nu): Gaunt factor.

Approximation for T  > 3 keV clusters.

By integrating, the X-ray luminosity:

L_ X \sim  10^{43} - 10^{45} \, {\rm erg/s}

Application to X-ray surveys.

Scaling relation for galaxy clusters

By assuming that the ICM gas has the same dynamics as galaxies, the typical ICM temperature

k_B \, T \simeq \mu \, m_p\, \sigma_v^2 \simeq 6 \, \left( \frac{\sigma_v}{10^3 \, {\rm km/s}} \right)^2 \, {\rm keV} \, ,

m_p: proton mass

\mu : the mean molecular weight

\mu = 0.6 (for a primordial composition)

Scaling relation for nearby and distant clusters and outliers.

The scaling relation between ICM temperature and velocity dispersion in galaxy clusters. Credit: Rosati et al. (2002).

The scaling relation between ICM temperature and velocity dispersion in galaxy clusters. Credit: Rosati et al. (2002).


Mass of galaxy clusters from X-ray data

Galaxy cluster mass estimate from X-ray data.

Assume: hydrostatic equilibrium, spherical symmetry.

Local gas pressure: \text{p}

Local gas density: \rho_{\rm gas}

\frac{{\rm d} p}{{\rm d} R} \, = \, - \frac{G M(<R) \, \rho_{\rm gas} (R)}{R^2}

M(<R): total gravitating mass within R.

By inserting the equation of state for a perfect gas:

M\left(\frac{{\rm d \, log} \rho_{\rm gas}}{{\rm d \, log} \, R} \, + \, \frac{{\rm d \, log} T}{{\rm d \, log}} \right) .

Application: measure the total gravitating cluster mass.
Comparison with theoretical models for cosmic structure formation.

Interaction between galaxies and ICM

Interaction of galaxies with the local cosmic environment.

Galaxy interaction with the hot ICM and a galaxy cluster gravitational field.
Observational evidence:

  • morphology–density relation;
  • in local clusters (z\sim0.03)  spiral galaxies are deficient in neutral hydrogen;
  • lower star formation activity in clusters.

Proposed physical mechanisms:

  • dynamical interactions of cluster galaxies with ICM (ram pressure);
  • gravitational interactions with nearby companions.

Difficult observations of the on-going transformation.

The “comet galaxy” in Abell 2667

The rich galaxy cluster Abell 2667 and the “comet galaxy” observed by the Hubble Space Telescope. Credit: NASA/ESA.

The rich galaxy cluster Abell 2667 and the "comet galaxy" observed by the Hubble Space Telescope. Credit: NASA/ESA.


The dynamical model

Tidal interaction

a_{\rm rad} \, = \, G \, M_{\rm cluster} \,  \left[ \frac{1}{r^2}  - \frac{1}{(r+R)^2}  \right] \, . versus a_{\rm gal} \, = \, \frac{G M_{\rm gal}}{R^2} \, .

Ram pressure stripping (ICM)

ICM pressure P_{\rm ram} \, = \, \rho_{\rm ICM} \, v^2 > 2 \, \pi \Sigma_{\rm star} \, \Sigma_{\rm gas} \, ,

\rho_{\rm ICM}: ICM density ;

\Sigma_{\rm star} \, \Sigma_{\rm gas} \,: galaxy stellar and gas density

v \sim 1500 \, {\rm km/s}

\Sigma_i (r) = \frac{M_i }{2 \pi R_{0,i}} \, {\rm exp} \left( - \frac{r}{R_{0,i}} \right)\, ,

where R_0 is the scalelength of the exponential profile.

R_{\rm strip} \sim  0.64 R_0 \, {\rm ln} \left[ \frac{G (L_H / L_{\odot})^2}{2 \pi \rho_{\rm ICM} v^2 \, R_{0, {\rm star}}^2} \right]

M_{\rm strip} \, = \, \frac{L_H}{L_{\odot}} \,\left( \frac{R_{\rm strip}}{R_0} + 1 \right) \, {\rm exp} \left( -\frac{R_{\rm strip}}{R_0} \right) \,.

Ratio of radial tidal acceleration to the internal galaxy acceleration versus R. Credit: Cortese et al. (2007).

Ratio of radial tidal acceleration to the internal galaxy acceleration versus R. Credit: Cortese et al. (2007).

Variation of the HI deficiency as a function of the cluster-centric distance. Credit: Cortese et al. (2007).

Variation of the HI deficiency as a function of the cluster-centric distance. Credit: Cortese et al. (2007).


Evolution of the “comet galaxy”

Animation showing the evolution of the in-falling galaxy from 200 million years ago and 800 million years into the future. Credit: NASA/ESA.

Animation showing the evolution of the in-falling galaxy from 200 million years ago and 800 million years into the future. Credit: NASA/ESA.


I materiali di supporto della lezione

P. Rosati et al. (2002): “Evolution of X-rays Clusters of Galaxies”, ARAA vol. 40, p. 539

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