mass profiles of galaxy clusters
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Mass Profiles of Galaxy ClustersDrew Newman
Newman et al. 2009, “The Distribution of Dark Matter Over Three Decades in Radius in the Lensing Cluster Abell 611,” astro-ph/0909.3527, accepted to ApJ
• Purpose: Observational measures of cluster mass distribution (dark and baryonic) to make precise comparison with simulations (both N-body and those including baryons)
• Need data over wide range of scales to break degeneracies inherent to individual probes– Weak lensing (~100 kpc – 3 Mpc scales)– Strong lensing (~30 kpc – 100 kpc)– Stellar dynamics (~3 – 20 kpc)
Mass Model Motivation
• Focus on inner (log) density slope of dark matter
• Component #1: NFW / gNFW dark halo
• Component #2: Stars in the central galaxy
€
ρ(r) = ρ s(r /rs)
β (1+ r /rs)3−β
Weak Lensing – Abell 611Subaru/SuprimeCam
~10% ofarea shown
BVRI filtersfor photo-z’s
1 Mpc = 3.8’
Weak Lensing – Abell 611
Radial shear profile
2D mass reconstruction
Strong Lensing – Abell 611HST/ACS image
3 multiply images sources
2 of these with spectroscopic redshifts
Stellar VelocityDispersions – Abell 611
From Data to Mass Distributions• Draw sample models (MCMC) consisting of– Elliptical NFW or gNFW dark halo,– Stellar mass in cD galaxy,– Galaxies that may perturb image positions
• Compare to data:– Compute shear at locations of background galaxies,– Ray-tracing of multiply-imaged sources to other locations
in image plane,– Compute velocity dispersion profile (including
observational effects: seeing, binning)• Can we discriminate between NFW and gNFW DM
halos? If so, what is the inner slope allowed to be?
Results
• Definitely prefer a variable inner slope – Bayesian evidence larger
by factor (2.2 ± 1.0) x 104
• Logarithmic inner slope β < 0.3 (68%), i.e. quite shallow
Results
• Also, neither model reproduces the flat velocity dispersion profile
• How to match flat dispersion and lensing constraints at ~30-100 kpc?
DM-only simulations
• More modern cluster-scale simulations converge down to about 15 kpc/h
• Hints that slope become progressively more shallow
• But only on very small scales
Navarro et al 2004
Attempts to Include Baryons
• Adiabatic contraction– Cooling baryons contract
and “pinch” DM halo, steepening the cusp
– e.g. Gnedin et al. 2004, Gustafsson et al. 2007, Abadi et al. 2009, Pedrosa et al. 2009, etc.
– Cosmological N-body+gas dynamical simulations, with radiative cooling and attempts to include feedback processes
• Dynamical friction– Infalling baryon clumps
“heat” DM cusp, flattening it
– e.g. El-Zant et al. 2001, Romano-Diaz et al. 2008, Nipoti et al. 2004
– Frequent simplifications:• Infalling subhalos as purely
baryonic• Sometimes as unstrippable
point masses• Need to maintain clumps
over sufficient timescales without fragmenting, forming stars
Future
• Find density profile that is more observationally acceptable– Not a lot of theoretical motivation because
baryonic physics is not well enough understood• Extend to sample of ~10 clusters– All data collected for about half– For the rest, lack only velocity dispersions
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