THE PROPERTIES OF X-RAY BRIGHT GALAXY GROUPS F. GASTALDELLO Università di Bologna e California Irvine NGC 5044 Buote et al. 2002

THE PROPERTIES OF X-RAY THE PROPERTIES OF X-RAY BRIGHT GALAXY GROUPSBRIGHT GALAXY GROUPS

F. GASTALDELLO

Università di Bologna e California Irvine

NGC 5044Buote et al. 2002

• 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)

OUTLINE OF THE LESSONOUTLINE OF THE LESSON

Properties of groupsProperties 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)


I will not treat spiral only groups, like the Local Group or groups with very faint X-ray 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.

• Structure formation: galaxies groups clusters • Problems in the optical (small statistic) overcomed by the discovery

of X-ray 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


ROSAT X-RAY CONTOURS ON DSS IMAGESMULCHAEY 2003

Properties of groups and clustersProperties of groups and clusters

BHACALL 1999

CLUSTERS GROUPS/POOR CLUSTERS

LX (erg/s) 1043 - 1045 1041.5 - 1043

kTX (keV) 2 – 15 ≤ 2

N gal 100-1000 5 – 100

σv (km/s) 500-1200 (median 750) 200 – 500

Mtot (< 1.5 Mpc) 1014 – 5 x 1015 1012.5 - 2 x 1014

Number Density 10-5 – 10-6 Mpc-3 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

• 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

NGC 5044 CORE XMM

Compare apples with apples …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 profilesSurface Brightness profiles

HELSDON & PONMAN 2000

• Central excess over the frequently adopted model, as in cool core, relaxed clusters

T profilesT profiles

BUOTE 2000

T profilesT profiles

BUOTE 2000

• Already with ROSAT data, evidence of a characteristic temperature profile

A SPECIAL ERA IN X-RAY ASTRONOMY

Chandra XMM-Newton

•1 arcsec resolution •High sensitivity due to high effective area, i.e. more photons

GASTALDELLO ET AL. 2007

RESULTS FOR MASSRESULTS FOR MASS•After accounting for the mass of the hot gas, NFW + stars is the best fit model

MKW 4

NGC 533

RESULTS FOR MASSRESULTS FOR MASS•No detection of stellar mass due to poor sampling in the inner 20 kpc or localized AGN disturbance

NGC 5044

Buote et al. 2002

• NFW a good fit to the mass profile

Pointecouteau et al. 2005

Clusters X-ray resultsClusters X-ray results

MASS SUMMARYMASS 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 Breaking of self-similarity and entropy “floor”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 relationThe L-T relation

Mulchaey 2000

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)

X-ray surface brightnessX-ray surface brightness

Ponman, Cannon & Navarro 1999

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.

Entropy in the IGMEntropy in the IGM

Entropy floor

Self-similar scaling

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 .


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 self-similar scaling of KT.

KT

PROPOSED EXPLANATIONSPROPOSED EXPLANATIONS1. 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

Voit, Kay & Bryan 2004

Non-radiative simulations produce clusters with self-similar entropy profiles

K(r)=aT (r/r200)1.1


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


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 COMPARISON WITH MASSIVE CLUSTERS AND GRAVITATIONAL SIMULATIONSGRAVITATIONAL SIMULATIONS

PRATT ET AL. 2006

ENTROPY PROFILESENTROPY PROFILES


GASTALDELLO ET AL. 2008, IN PREP.







GAS FRACTIONSGAS FRACTIONS

GASTALDELLO ET AL. 2007

ENTROPY SUMMARYENTROPY 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

• 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) ?

Properties of groups: Properties of groups: AbundancesAbundances

RENZINI 2000

•Chandra inner regions

XMM outer regions

NGC 533

DATA ANALYSYSDATA ANALYSYS

The Fe BiasThe 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 single-phase gas or represent real multi-phase gas

Strongest evidence from Xmm observation of M87 (Molendi & Gastaldello 2001, Molendi 2002)

DATA ANALYSISDATA 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 ClustersRelaxed and Not Relaxed Clusters

Central abundance gradient, Flat profile further out similar to unrelaxed clusters

• CC (relaxed CC (relaxed clusters)clusters)

• NCC (NCC (not not relaxedrelaxed clusters)clusters)

●CCoNCC

De Grandi & Molendi (2001)

Abundance Gradients in GroupsAbundance 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 GroupsAbundance Gradients in Groups

FOSSIL GROUPSFOSSIL 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 OFFSETNGC 5044 OFFSET

•Fe abundance nearly constant beyond 150 kpc at an extremely low value of 0.15 solar. If this offset region is azimuthally representative, then MFe/LB = 0.007. But the baryon mass fraction is fb~ 0.14, only slightly 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 !

BUOTE ET AL. 2004

•We can quantify the iron enrichment from dwarfs using an on-the spot approximation. This falls short by a factor of 3-4 and can seriously affect our understanding of enrichment by galactic winds.


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


dE galaxies and gas enter the group via cosmic accretion filaments

N S E W

Number 16 8 20 17

LB (1010sol.) 0.80 0.045 1.45 1.64

If the Fe abundance is significantly higher in the western offset, this would demonstrate that metals can be very inhomogeneous and strong evidence that matter enter groups along filaments

AGN FEEDBACKAGN 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 …



CAON ET AL. 2000

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THE PROPERTIES OF X-RAY BRIGHT GALAXY GROUPS F. GASTALDELLO Università di Bologna e California Irvine NGC 5044 Buote et al. 2002