The Observed ConcentrationMass Relation for Galaxy Clusters

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The Observed Concentration-Mass Relation for
Galaxy Clusters

• Julie Comerford (UC Berkeley)
• with Priya Natarajan
• astro-ph/0703126 (MNRAS, in press)
• CINC Workshop, 5.11.07

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Clusters Probe Structure Formation
As the most recent structures to assemble in a
hierarchical ?CDM universe, galaxy clusters are
an important link to understanding the growth of
structure Numerical simulations suggest, and
several observations confirm, that clusters are
well-fit by the NFW density profile (Navarro,
Frenk, White 1997) rs scale radius
?scharacteristic density
Abell 1689
r
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Galaxy Cluster Concentrations
Concentration
Reflects central density of halo, which depends
on halos assembly history and time of
formation Simulations suggest haloconcentrations
decreasegradually with virial mass,and evolve
as (1z)-1 What is the c(M) trend in
observational data?
Hennawi et al. 2007
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New Cluster Concentrations
New concentrations for 10 strong lensing
clusters Best-fit NFW profiles to 10 strong
lensing clusters (Comerford et al. 2006) Use
zcluster and zarc, as well as best-fit ?s and
rs, to determine cluster c and Mvir
MS2137
Doubles the existing number of cluster (c, Mvir)
measurements from strong lensing
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The Observational Sample
Combine our 10 new concentration measurements
with published (c, Mvir) cluster
measurements Variety of observational methods
used in the literatureX-ray, weak lensing,
strong lensing, etc. Total sample 62 clusters
Abell 2218 Image from KIPAC website
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Observed Concentration-Mass Relation
Distribution of concentrations is broad, so bin
data into 4 mass bins Best-fit power
law Compare to simulations
Hennawi et al. 2007 Bullock et al. 2001
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X-ray vs. Lensing Concentrations
Lensing concentrations are systematically higher
than X-ray concentrations Expect lensing
clusters to be biased toward higher
concentrations because higher concentration halos
are more effective lenses
Lensing X-ray 1s scatterBest-fit-1s
scatter
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Discrepancy with Simulations
Strong lensing clusters have on average higher
concentrations than the total cluster
population Simulations 34 higher (Hennawi et
al.) Observations 55 higher Possible causes of
the discrepancy 1. X-ray clusters not in
hydrostaticequilibrium (Loeb Mao 1994) 2. Halo
triaxiality in lensing clusters(Gavazzi 2005) 3.
Substructure along the line of sightto the
lensing cluster (King Corless 2007)
GADGET simulation
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Concentrations for Fixed Mass
Numerical simulations suggest concentrations for
fixed halo mass are log-normally distributed With
the exception of a few outliers, we find the same
effect First time this has been confirmed in
observations
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Conclusions
Observed cluster concentrations and virial
masses are best fit by a power law cvir
Mvir-0.15, with a similar slope but slightly
higher normalization than from simulations
Lensing clusters have systematically higher
concentrations than X-ray clusters, in part
because lensing clusters are naturally biased
toward higher concentration, but also because
observational modeling may be skewed by
assumptions of hydrostatic equilibrium or neglect
of halo triaxiality or substructure For fixed
mass, the majority of observed clusters are
distributed log-normally in concentration, in
agreement with simulations Millennium
simulation will offer best comparison set to our
observed concentration-mass relation for galaxy
clusters