Structure and scaling of nearby clusters of galaxies

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 massobservable 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 z=0. 5 z=1 M T 3/2 G. W. Pratt, Ringberg, 26/10/2005 Bryan & Norman (1998); Navarro et al. (1995, 1997)

Real clusters are structurally similar, but the scaling laws are different LX T 2 ROSAT X-ray EM profiles (Arnaud et al. 2002; also Vikhlinin et al. 1999) ASCA/Ginga LX-T relation LX T 3 (Arnaud & Evrard 1999; also Markevitch 1998) Non-gravitational effects influence gas properties? G. W. Pratt, Ringberg, 26/10/2005

Key questions Is our basic understanding of cluster formation correct? • Are the dark matter properties consistent with predictions? • e. g. , NFW ρDM (r/r. S)-1[1+ (r/r. S)]-2 with c=R 200/rs weakly dependent on mass How good is our understanding of the gas physics? • Structure and scaling of entropy G. W. Pratt, Ringberg, 26/10/2005

Converging observational support for dark matter predictions G. W. Pratt, Ringberg, 26/10/2005

Universal profile 13 clusters 0. 7— 9 ke. V ~2 ke. V ρ/ρc M/M 200 10 clusters 2— 9 ke. V ~8 ke. V R/R 200 [Pointecouteau et al. 2005; XMM] R/R 500 [Vikhlinin et al. astro-ph/0507092; Chandra] • Universal mass/density profile down to low mass • NFW model good description • < 15% dispersion in mass profiles at 0. 1 R 200 G. W. Pratt, Ringberg, 26/10/2005

Concentration parameters <c 500> = 3 (<c 200> ~ 4. 6) c 500 c 200 <c 200> = 5 M 500 M 200 [Pointecouteau et al. 2005; XMM simulations by Dolag et al. 2004] [Vikhlinin et al. astro-ph/0507092; Chandra] • 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 In X-rays, we get M from ne and T σ8 Value of cosmological parameters measurable with clusters using number count methods (σ8, ΩM) depends sensitively on the normalisation of the cluster M-T relation Need to know the gas physics in detail M—T normalisation [Pierpaoli, Scott & White 2001] G. W. Pratt, Ringberg, 26/10/2005

M-T relation Mδ (M ) M 500 (M ) δ = 500 δ = 2500 M T 1. 7 M T 1. 5 k. T/10 (ke. V) [Arnaud et al. 2005; XMM] k. T (ke. V) [Vikhlinin et al. astro-ph/0507092; Chandra] • Slope under debate; observed normalisation no longer an issue • ~35% too low wrt pure gravitational simulations [Evrard et al. 1996] • Inclusion of non-gravitational physics [SN, radiative cooling; Borgani et al. (2004] improves situation; observational treatment [cf Rasia]? ? ? G. W. Pratt, Ringberg, 26/10/2005

Non-gravitational processes and entropy G. W. Pratt, Ringberg, 26/10/2005

Why entropy? • 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 = k. T ne-2/3 • Radiative cooling reduces k. T ne-2/3 • Heat input (pre-heating, AGN, SNe, mixing) raises k. T ne-2/3 G. W. Pratt, Ringberg, 26/10/2005

Entropy scaling If clusters are self similar, ρgas ρDM δc (0) = cst S T S (0. 1 R 200) [Ponman et al, 2003] S T • Find S T 0. 65 with slope stable to 0. 5 R 200 [see also Ponman et al. 2003] • S T 0. 65 LX T 2. 7 S T • Increased dispersion towards central regions [Pratt et al. , astro-ph/0508234] 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 (x 2. 5 at 2 ke. V) Adiabatic prediction (Voit 2005) • 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] 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 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 (k. T < 3 ke. V) 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 R 200 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 R 200 Cool core systems represent lower envelope [see also Voit & Donahue 2005] AGN heating probably has an effect G. W. Pratt, Ringberg, 26/10/2005

Thanks: Monique Arnaud Hans Böhringer Judith Croston Etienne Pointecouteau 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 G. W. Pratt, Ringberg, 26/10/2005