G.W. Pratt, Ringberg, 26/10/2005
Structure and scaling of nearby clusters of galaxies – in X-rays
Gabriel W. Pratt, MPE Garching, Germany
G.W. Pratt, Ringberg, 26/10/2005
Introduction
ΩM=1, ΩΛ=0, σ8=0.6 ΩM=0.3, ΩΛ=0.7, σ8=0.9
[Evrard et al. 2002]
G.W. Pratt, Ringberg, 26/10/2005
Rationale
• Cluster mass is most fundamental characteristic most useful for cosmology (whatever the cosmological test)
• We will never measure the mass of every cluster need mass-observable relations (e.g., M-T, LX-M) or proxies thereof (e.g., LX-T)
• We need to establish robust scaling relations (local and distant)
Detailed structural investigation only possible at low-z
astrophysics of the ICM & its evolution
G.W. Pratt, Ringberg, 26/10/2005
Introduction• Simplest model of structure formation is dark matter-driven hierarchical gravitational collapse
• Gas ‘follows’ DM
• Expect simple self-similar scaling of haloes with mass (& redshift) scaling laws, structural similarity
Bryan & Norman (1998); Navarro et al. (1995,1997)M T3/2
z=0z=0z=0.5z=1
G.W. Pratt, Ringberg, 26/10/2005
ROSAT X-ray EM profiles
(Arnaud et al. 2002; also Vikhlinin et al. 1999)
Real clusters are structurally similar, but the scaling laws are different
ASCA/Ginga LX-T relation LX T3
(Arnaud & Evrard 1999; also Markevitch 1998)
Non-gravitational effects influence gas properties?
Real clusters
LX T2
G.W. Pratt, Ringberg, 26/10/2005
Is our basic understanding of cluster formation correct?
• Are the dark matter properties consistent with predictions?
• e.g., NFW ρDM (r/rS)-1[1+ (r/rS)]-2 with c=R200/rs weakly dependent on mass
How good is our understanding of the gas physics?
• Structure and scaling of entropy
Key questions
G.W. Pratt, Ringberg, 26/10/2005
Converging observational support for dark matter predictions
G.W. Pratt, Ringberg, 26/10/2005
Universal profile
• Universal mass/density profile down to low mass
• NFW model good description
• < 15% dispersion in mass profiles at 0.1 R200
~2 keV
~8 keV
13 clusters 0.7—9 keV
[Vikhlinin et al. astro-ph/0507092; Chandra]
R/R500
ρ/ρ
c
[Pointecouteau et al. 2005; XMM]R/R200
M/M
200
10 clusters 2—9 keV
G.W. Pratt, Ringberg, 26/10/2005
M500M200
c 500
c 200
Concentration parameters
[Pointecouteau et al. 2005; XMM simulations by Dolag et al. 2004]
[Vikhlinin et al. astro-ph/0507092; Chandra]
<c500> = 3 (<c200> ~ 4.6)<c200> = 5
• Concentration parameters in range expected
Dark matter properties consistent with predictions
G.W. Pratt, Ringberg, 26/10/2005
The M—T relation: cosmological connection
G.W. Pratt, Ringberg, 26/10/2005
Context
[Pierpaoli, Scott & White 2001]
Value of cosmological parameters measurable with clusters using number count methods (σ8, ΩM) depends sensitively on the normalisation of the cluster M-T relation
In X-rays, we get M from ne and T
Need to know the gas physics in detail
M—T normalisation
σ8
G.W. Pratt, Ringberg, 26/10/2005
Mδ (
M)
kT (keV)
δ = 500δδ = = 25002500
M-T relationM
50
0 (
M)
kT/10 (keV)
[Arnaud et al. 2005; XMM]
• Slope under debate; observed normalisation no longer an issue
• ~35% too low wrt pure gravitational simulations pure gravitational simulations [Evrard et al. 1996][Evrard et al. 1996]
• Inclusion of non-gravitational physicsnon-gravitational physics [SN, radiative cooling; Borgani et al.
(2004] improves situation; observational treatment [cf Rasia]???
[Vikhlinin et al. astro-ph/0507092; Chandra]
M T1.7 M T1.5
G.W. Pratt, Ringberg, 26/10/2005
Non-gravitational processes and entropy
G.W. Pratt, Ringberg, 26/10/2005
• Gas entropy is generated in shocks and compression as the gas accretes into the dark matter potential well
• It preserves the gravitational accretion history and any subsequent modification by non-gravitational processes
• Useful X-ray observable S = kT ne-2/3
Why entropy?
• Radiative cooling reduces kT ne-2/3
• Heat input (pre-heating, AGN, SNe, mixing) raises kT ne-2/3
G.W. Pratt, Ringberg, 26/10/2005
[Pratt et al., astro-ph/0508234]
Entropy scaling
S T
If clusters are self similar,ρgas ρDM δc (0) = cst S T
• Find S T0.65 with slope stable to 0.5 R
200 [see also Ponman et al. 2003]
• S T0.65 LX T2.7
• Increased dispersion towards central regions
S T
S (0.1 R200) [Ponman et al, 2003]
G.W. Pratt, Ringberg, 26/10/2005
Entropy scaling: comparison with adiabatic simulations
• Hotter systems in relatively good agreement (slope & normalisation)
• Clear excess normalisation at all measured radii in poorer systems (x2.5 at 2 keV)
• Increased dispersion in central regions
• Need mechanism which increases normalisation ar large R and dispersion at small R [Pratt et al., astro-ph/0508234;
also Pratt & Arnaud 2005]
Adiabatic Adiabatic predictionprediction
(Voit 2005)(Voit 2005)
G.W. Pratt, Ringberg, 26/10/2005
Conclusions: dark matter• Universal mass/density profile in clusters, well described by standard NFW model, c in range expected from simulations
dark matter collapse understood
• Normalisation of M-T relation has converged, but is consistently lower than simulations
are simulations correctly reproducing the thermal structure in clusters?
how do the observational assumptions (particularly HE) affect final mass estimate?
G.W. Pratt, Ringberg, 26/10/2005
Conclusions: gas physics
• Slope of M—T relation is stable (universal mass profile), but steeper if lower mass objects (kT < 3 keV) are included in fit
• S—T relation is shallower than self-similar at all radii probed
• Entropy profiles are self-similar (~20% dispersion) outside ~0.2 R200 except for a normalisation factor
some non-gravitational processes boost entire entropy profile, preferentially in low mass systems (filamentary preheating?)
• Dispersion increases to >60% at < 0.05 R200
Cool core systems represent lower envelope [see also Voit & Donahue 2005]
AGN heating probably has an effect
G.W. Pratt, Ringberg, 26/10/2005
For more information:
Pratt, Arnaud & Pointecouteau, 2005, A&A, in press (astro-ph/0508234)
Arnaud, Pointecouteau & Pratt, 2005, A&A, 441, 893
Pointecouteau, Arnaud & Pratt, 2005, A&A, 435, 1
Thanks:
Monique Arnaud
Hans Böhringer
Judith Croston
Etienne Pointecouteau
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