Weak Lensing by Triaxial Dark Matter Halos

1
Weak Lensing by Triaxial Dark Matter Halos

• Consequences for parameter estimation at galaxy
  cluster scales

Virginia L. Corless Institute of
Astronomy Supervisor Dr. Lindsay
King INAF-COSMOCT School on Gravitational
Lensing November 1, 2006
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Outline

• A conflict with ?CDM?
• A fully triaxial NFW model
• Lensing by a triaxial halo
• Parametric profile reconstructions
• Conclusions

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Highly concentrated massive clusters?

• NFW and other simulations predict very massive
  halos (M1015 M?) to have low concentrations
  (C4)
• Abell 1689, MS2137-23 observed to have much
  higher concentrations, C11-14
• Oguri, 2005 and Gavazzi, 2005
• investigate effectiveness of triaxiality
• for resolving specific cases

Broadhurst, T., et al., 2005. ApJ 621,
53-88. Oguri, M., Takada, M., Umetsu, K.,
Broadhurst, T., 2005. ApJ 632, 841-846 Gavazzi,
R., 2005. AA 443, 739-804
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• Unable to fit triaxial models directly because of
  significant degeneracies along the line of sight.
• Systematically investigate effects of expected
  levels of triaxiality (e.g. Shaw et al., 2006)
  on standard methods of parameter estimation.
• Determine systematic errors in mass and
  concentration estimates.

Shaw, L., Weller, J., Ostriker, J., Bode, P.,
2006. ApJ 646, 815-833
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Triaxial NFW
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Prolate
Oblate
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Lensing Simulations
Prolate

• Convergence ?, shear ?, calculated numerically
  via method of Oguri et al.
• Catalogues of background galaxies lensed by
  triaxial NFW
• Use maximum-likelihood method to fit spherical
  NFW models to catalogues

Spherical
Oblate
Oguri, M., Lee, J., Suto, Y., 2003. ApJ 599,
7-23
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Line of Sight

• Oblate halos
• Mass underestimated by 40
• Concentration underestimated by factor of 2

• Prolate halos
• Mass overestimated by 40
• Concentration overestimated by factor of 2

Prolate
Oblate
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Plane of the sky

• Oblate halos
• Mass overestimated by 5
• Concentration overestimated by factor of 25

• Prolate halos
• Mass underestimated by 40
• Concentration underestimated by 25

Prolate
Oblate
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Halos averaged over orientation
Spherical halo
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Halos averaged over orientation
Prolate halo ab0.4
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Halos averaged over orientation
Oblate halo a0.4, b1.0
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Lensing Efficiency
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Conclusions

• Triaxiality can cause significant errors in
  parameter estimation
• Averaged over orientation, parameter
  distributions skewed to high values for prolate
  halos and low values for oblate halos
• Halos that are measured to have high
  concentrations and masses are also the best
  lenses --gt good lenses are more likely strange
  halos!
• Ongoing work effects on model discrimination,
  weighting parameter distributions by lensing
  efficiency, similar systematic effects of other
  expected deviations from spherical NFW models,
  breaking the degeneracies.

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Halos averaged over orientation
Oblate halo a0.4, b1.0
Most likely halo a0.65, b0.8
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Halos averaged over orientation
Prolate halo ab0.4
Oblate halo a0.4, b1.0
Most likely halo a0.65, b0.8
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Halos averaged over orientation and axis ratios