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