Galaxy Clusters and Implications for Dark Matter (Part II)

1
Galaxy Clusters and Implications for Dark Matter
(Part II)

• Presented by
• Kisha Delain and Sean ONeill
• 4/17/2003

2
Outline

• Methods of Dark Matter Analysis
• Is Hydrostatic Equilibrium a Valid Assumption?
• Cluster Mass Profiles from X-rays and Lensing
• Dark Matter Constraints
• Conclusions

3
Methods Used to Estimate the Properties of Dark
Matter in Clusters

• Cluster Dynamics
• virial theorem ? M3ltv2gtRcl/G
• Sunyaev-Zeldovich Effect
• Observations of CMB estimate integral of gas
  pressure along line of sight
• X-Ray Observations
• brehm thermo ? M(ltr) is function of T, r, and
  spatial gradients of ?, T
• Gravitational Lensing
• GR ? M(ltp) ?pc2/4G (? is deflection, p is
  impact parameter)

4
Methods Used to Estimate the Properties of Dark
Matter in Clusters

• Cluster Dynamics
• virial theorem ? M3ltv2gtRcl/G
• Sunyaev-Zeldovich Effect
• Observations of CMB estimate integral of gas
  pressure along line of sight
• X-Ray Observations
• brehm thermo ? M(ltr) is function of T, r, and
  spatial gradients of ?, T
• Gravitational Lensing
• GR ? M(ltp) ?pc2/4G (? is deflection, p is
  impact parameter)

5
Hydrostatic Equilibrium?

• As illustrated, the assumption of hydrostatic
  equilibrium must be examined before it can be
  used to derive mass estimates from X-ray
  observations.
• On a case-by-case basis, high-resolution X-ray
  observations and/or comparison with lensing can
  test the assumption.
• Features such as the presence of cooling flows
  and regular isophotes suggest that hydrostatic
  equilibrium may be valid.

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Chandra Observations of EMSS 13586245 (Arabadjis
et al, 2002)
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Mass Profiles of EMSS 1358 asDerived from X-ray
Observations (Arabadjis et al, 2002)
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Comparison of Lensing and X-ray Results for EMSS
13586245 (Arabadjis et al, 2002)
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NFW Profile

• Simulations done by Navarro, Frenk, and White
  (1997) suggest that equilibrium CDM density
  profiles in clusters all have similar shape,
  independent of halo mass, density fluctuations,
  or cosmology.
• Density profile
• ? ? 1/(r/rs)(1r/rs)2
• rs is scale radius

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Chandra Observations of Abell 2029 (Lewis et al,
2000)
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Chandra Observations of Abell 2029 (Lewis et al,
2000)
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Possible Types of Dark Matter

• As seen in lecture, baryonic DM and hot relics
  are insufficient to explain cluster dynamics.
• Assuming some sort of cold dark matter, we can
  examine whether self-interacting or collisionless
  CDM is favored by simulations and observations.

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Simulations of Dark Matter Clusters by Yoshida et
al (2000)
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Simulations of Dark Matter Clusters by Yoshida et
al (2000)
Collisionless
Self-interacting
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What Can Chandra Say About the Nature of Dark
Matter?

• High-resolution X-ray observations have the
  advantage of being able to partially probe the
  cores of clusters.
• Central density profiles (along with external
  astrophysical constraints) can exclude possible
  DM interaction cross-sections.
• When one also considers the core mass profiles of
  dwarf galaxies, the DM cross-sections are
  constrained to be velocity-dependent.

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Summary of Data on Self-Interacting Dark Matter
Cross-Sections (Arabadjis et al, 2002)
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Conclusions

• Dark Matter Profiles
• Dark matter can be constrained through a variety
  of methods, including lensing and X-ray studies.
• The DM is cold, and the observations currently
  favor collisionless or low interaction
  probability DM.
• Deeper Chandra X-ray observations of relaxed
  clusters will provide more information on typical
  mass profiles.

• Hydrostatic Equilibrium
• Sometimes valid, but not always!
• Specific clusters must be examined for absence of
  mergers, regular spacing of isophotes, presence
  of cooling flows, or other signs of relaxation.
• The determination of hydrostatic equilibrium
  allows X-ray data to supply mass profiles.