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THE PROPERTIES OF X-RAY BRIGHT GALAXY GROUPS F. GASTALDELLO NGC 5044 Buote et al. 2002 Università di Bologna e California Irvine OUTLINE OF THE LESSON • Focus on observations of X-ray bright groups: the high mass end of the distribution, collapsed and evolved • Mass properties • Entropy profiles and non gravitational heating • Metal abundances in groups and metal enrichment • AGN feedback (briefly) Properties of groups Constitutes the most common galaxy association, at least 50% of all galaxies at the present day are in groups (e.g Tully 1987) Properties of groups I will not treat spiral only groups, like the Local Group or groups with very faint Xray emission. Thet are still important though and there is active search to look for diffuse gas (through X-ray/UV absortion) in the Local Group, for example. Properties of groups MULCHAEY 2003 • • • ROSAT X-RAY CONTOURS ON DSS IMAGES Structure formation: galaxies groups clusters Problems in the optical (small statistic) overcomed by the discovery of Xray emission (hints from Einstein, main leap with ROSAT and ASCA): ~50% of all nearby groups have an hot X-ray emitting IGM Extended, usually centered on the brightest elliptical. Similar in many respects to the cool core clusters Properties of groups and clusters CLUSTERS GROUPS/POOR CLUSTERS LX (erg/s) 1043 - 1045 1041.5 - 1043 kTX (keV) 2 – 15 N gal 100-1000 σv (km/s) Mtot (< 1.5 Mpc) Number Density 500-1200 (median 750) 1014 – 5 x 1015 10-5 – 10-6 Mpc-3 ≤2 5 – 100 200 – 500 1012.5 - 2 x 1014 10-3 – 10-5 Mpc-3 Groups and poor clusters provide a natural and continuous extension to lower mass, size, luminosity and richness of rich, massive and rare clusters BHACALL 1999 Properties of groups NGC 5044 CORE XMM • • Wealth of emission lines: O, Fe, Si, S allows investigation of supernova enrichment But groups are not scaled down versions of clusters 1. Different galaxy evolution: galaxy-galaxy interaction rather than ram-pressure stripping, because of lower velocity dispersions 2. Not closed box: non-gravitational processes, given the small potential well, have a bigger impact Compare apples with apples … MULCHAEY 2003 • X-ray groups are fainter and they can be observed only to smaller radii compared to clusters: something to bear in mind when doing comparisons Surface Brightness profiles • Central excess over the frequently adopted model, as in cool core, relaxed clusters HELSDON & PONMAN 2000 T profiles BUOTE 2000 T profiles • Already with ROSAT data, evidence of a characteristic temperature profile BUOTE 2000 A SPECIAL ERA IN X-RAY ASTRONOMY Chandra •1 arcsec resolution XMM-Newton •High sensitivity due to high effective area, i.e. more photons GASTALDELLO ET AL. 2007 RESULTS FOR MASS •After accounting for the mass of the hot gas, NFW + stars is the best fit model MKW 4 NGC 533 RESULTS FOR MASS •No detection of stellar mass due to poor sampling in the inner 20 kpc or localized AGN disturbance Buote et al. 2002 NGC 5044 Clusters X-ray results Pointecouteau et al. 2005 • NFW a good fit to the mass profile MASS SUMMARY •NFW is a good fit also for massive groups •DM collapse seems to be understood also at these scales, less massive than rich clusters Breaking of self-similarity and entropy “floor” In the widely accepted hierarchical cosmic structure formation predicted by cold dark matter models and in the absence of radiative cooling and supernova/AGN heating, the thermodynamic properties of the hot gas are determined only by gravitational processes, such adiabatic compression during collapse and shock heating by supersonic gas accretion (Kaiser 1986) clusters and group of galaxies should follow similar scaling relations, for example if emission is bremsstrahlung and gas is in hydrostatic equilibrium L T2 and if we define as “entropy” K = T/n2/3, then K T (so S=k lnK + s0, it’s also called adiabat because P = K ργ) The L-T relation It has been clear for many years that the cluster L-T relation does not follow the LT2 slope expected for self-similar systems. In practice, LT3 for clusters (Edge & Stewart 1991), with possible further steepening to LT4 in group regime (Helsdon & Ponman 2000) Mulchaey 2000 X-ray surface brightness Overlay of scaled X-ray surface brightness profiles shows that emissivity (hence gas) is suppressed and flattened in cool (T<4 keV) systems, relative to hot ones. Ponman, Cannon & Navarro 1999 Entropy in the IGM Ponman et al. (1999) & Lloyd-Davies et al (2000) studied ROSAT and ASCA data for a sample of clusters core entropy appeared to show a “floor” at ~100-150 keV cm2 at r=0.1 r200 . Entropy floor Self-similar scaling Entropy in the IGM A larger study, of 66 systems by Ponman et al. (2003), now indicates that there is not a “floor” but a “ramp”, with K(0.1r200) scaling as KT2/3, rather than the selfsimilar scaling of KT. KT PROPOSED EXPLANATIONS 1. EXTERNAL PREHEATING MODELS: the IGM was heated prior to the formation of groups and clusters (e.g. Tozzi & Norman 2001) results in isoentropic cores 2. INTERNAL HEATING MODELS: the gas is heated inside the bound system by supernovae or AGN (e.g. Loewenstein 2000) 3. COOLING MODELS: low entropy gas removed from the system, producing an effect similar to heating (e.g. Voit & Bryan 2001) All three models can reproduce the L-T relation and excess entropy but with some problems: 1 requires too large amount of energy at high redshift 2 requires 100% efficiency from supernovae or fine tuning for AGN 3 overpredicts the amount of stars in groups and clusters More realistic scenarios with both heating and cooling are required (e.g. Borgani et al. 2002) External preheating models with different levels of heating. Large isoentropic cores are produced Internal heating with rising entropy profiles BRIGHENTI & MATHEWS 2001 Entropy in the intracluster medium Non-radiative simulations produce clusters with selfsimilar entropy profiles K(r)=aT (r/r200)1.1 Voit, Kay & Bryan 2004 Entropy in the IGM Higher quality data from XMM and Chandra shows the lack of isentropic cores (e.g. Pratt & Arnaud 2002, Sun et al. 2004). The KT2/3 scaling is confirmed, but there is more scatter in entropy for groups. Sun et al 2004 Entropy in the IGM This scatter is shown in this small sample by Mushotzky et al. 2003. Reflects the relative history of the object, when and where the heat was produced relative to the collapse epoch of the object ? Mushotzky et al. 2003 COMPARISON WITH MASSIVE CLUSTERS AND GRAVITATIONAL SIMULATIONS PRATT ET AL. 2006 ENTROPY PROFILES ENTROPY PROFILES GASTALDELLO ET AL. 2008, IN PREP. ENTROPY PROFILES GASTALDELLO ET AL. 2008, IN PREP. COMPARISON WITH MASSIVE CLUSTERS AND GRAVITATIONAL SIMULATIONS GASTALDELLO ET AL. 2008, IN PREP. COMPARISON WITH MASSIVE CLUSTERS AND GRAVITATIONAL SIMULATIONS GASTALDELLO ET AL. 2008, IN PREP. GAS FRACTIONS GASTALDELLO ET AL. 2007 ENTROPY SUMMARY BROKEN POWER LAW ENTROPY PROFILES FOR GROUPS WITH STEEPER INNER SLOPES AND FLATTER OUTER SLOPES SEEM TO POINT TO HIGHER IMPORTANCE OF FEEDBACK PROCESSES WITH RESPECT TO MASSIVE CLUSTERS LOWER GAS FRACTIONS ARE ANOTHER EVIDENCE OF THIS FACT Properties of groups: Abundances RENZINI 2000 • • Iron abundance in the ICM is nearly the same for all massive clusters, ~ 0.3-0.4 solar (De Grandi et al. 2003, Tozzi et al., 2004) and the MFe/LB ~ 0.015 (Loewenstein 2004) uniform enrichment everywhere Groups are different: you can not reproduce the same results of clusters with the same IMF and supernovae yields (e.g Brighenti & Mathews 1999). MFe/LB much lower in groups: loss of metal rich gas expelled by supernova driven winds when most of the galactic stars formed. Or star formation less efficient (Springel & Hernquist 2003) ? DATA ANALYSYS •Chandra inner regions XMM outer regions NGC 533 The Fe Bias Fitting multi T spectrum with single temperature models give underestimated abundances (“Fe bias” Buote 2000) Multiple components may arise from a radially varying singlephase gas or represent real multi-phase gas Strongest evidence from Xmm observation of M87 (Molendi & Gastaldello 2001, Molendi 2002) DATA ANALYSIS Chandra is crucial in the inner region where a steep temperature gradient is present When data are available, we use Chandra in the core and XMM in the outer regions Relaxed and Not Relaxed Clusters ●CC oNCC • CC (relaxed clusters) • NCC (not relaxed clusters) De Grandi & Molendi (2001) Central abundance gradient, further out similar to unrelaxed clusters Flat profile Abundance Gradients in Groups Central abundance gradient, similar to relaxed clusters RASMUSSEN & PONMAN 2007 Are abundances in groups lower? A montage of group abundance profiles from Chandra (Helsdon) suggests that they drop to ~0.1 solar outside the core region (cf Buote et al 2004 study of NGC5044). Abundance Gradients in Groups FOSSIL GROUPS • Merger timescales for the brightest members in densest groups much less than an Hubble time (Ponman 1993) • Fossil groups can form: a single giant elliptical surrounded by dwarf galaxies and with a group-size X-ray halo • They have been found in deep X-ray surveys with ROSAT (e.g. Ponman et al. 1994, Vikhlinin et al. 1999) PONMAN ET AL. 1994 Fossil groups are excellent venues to study supernova enrichment: the undisturbed X-ray gas preserves in its radial distribution information about supernovae events from the earliest times, something lost in rich clusters NGC 5044 OFFSET BUOTE ET AL. 2004 •Fe nearly beyond 150 at anusing extremely low value •Weabundance can quantify theconstant iron enrichment fromkpc dwarfs an on-the spot of 0.15 solar. IfThis this falls offset region azimuthally then approximation. short by aisfactor of 3-4representative, and can seriously M 0.007. But the baryon mass fraction is fb~ 0.14, only slightly Fe/LB =our affect understanding of enrichment by galactic winds. less than the WMAP value of 0.16 (Spergel et al. 2003). Some inaccuracies can derive by extrapolation from the observed 327 kpc to the virial radius of 870 kpc. Nevertheless, 15% of the baryons have been ejected containing half of the iron ! NGC 5044 OFFSET POSSIBLE EXPLANATIONS: •Stars in NGC 5044 does not produce iron with the same efficiency as in clusters, i.e. SNIa in dwarfs are not at the expected rate or fail to enrich the gas •Iron selectively ejected from the group •High entropy gas enriched and heated by early SNII and SNIa may not have penetrated deeply inside because of its buoyancy •The southern offset observation is not representative NGC 5044 OFFSET dE galaxies and gas enter the group via cosmic accretion filaments If the Fe abundance isNsignificantlyShigher in theEwestern offset, W this would Number demonstrate that and 16 metals can 8be very inhomogeneous 20 17strong evidence that matter enter groups along filaments 10 LB (10 sol.) 0.80 0.045 1.45 1.64 AGN FEEDBACK THE “OLD” MASS SINK PROBLEM IS NOW THE “FEEDBACK PROBLEM” AGN FEEDBACK, PUT ON A FIRMER GROUND BY THE CHANDRA IMAGES, HAS BROADER ASTROPHYSICAL IMPLICATIONS FOR GALAXY FORMATION AND EVOLUTION STILL POORLY INVESTIGATED AT THE GROUP SCALE Fabian et al. 2003 Fabian et al. 2003 NGC 5044 AGAIN … NGC 5044 AGAIN … NGC 5044 AGAIN … CAON ET AL. 2000