Trevor Ponman University of Birmingham · Intergalactic gas in groups Virialised systems have overdensities δρ/ρ>100, allowing emission from hot baryons to be detected. Compression

The intragroup medium

Trevor PonmanUniversity of Birmingham

Preview

A zeroth-order model for group formation

Complications, complications…

The intergalactic medium at large

Intergalactic gas in groups

Some observed properties & their implications

Conclusions

Zeroth order model

Galaxies form bybaryon coolingwithin dark halos.

These then clusterinto groups.

Zeroth order model

Rv

Accretionshock

Galaxies form bybaryon coolingwithin dark halos.

These then clusterinto groups.

Galaxy dark halosmerge.

Infalling gas iscompressed andshocked at Rv.

Zeroth order model

Gas is stopped and shock heated to ~Tvir at radius Rv.

HI outside galaxies destroyed.

Knight &Ponman 1997

Zeroth ordermodel

Shock then expandswith Rv as the groupgrows self-similarly.

Knight &Ponman 1997

time

Complications

Geometry - non-spherical

Hierarchical growth

Galaxy interactions

Cooling

Feedback - SNe andAGN

Non-spherical geometry

Gas in the Milleniumsimulation - courtesy ofFrazer Pearce

Complications


Hierarchical growth

Galaxy interactions

Cooling


Hierarchical structure formation

Time

Complications


Hierarchical growth

Galaxy interactions

Cooling


Galaxy interactions

Interacting galaxies inStephan’s Quintet(Chandra+optical)- Trinchieri, Sulentic et al

Flow chart - evolution & feedbackCollapse &

hierarchical growth

IGM Galaxies

Stripping &strangulation

Feedback (energy& metals)

Complications


Hierarchical growth

Galaxy interactions

Cooling


Cooling

Courtesy ofAlastair Sanderson

Groups

Clusters

Complications


Hierarchical growth

Galaxy interactions

Cooling


Feedback - energy injection

Deep Chandra observation of theAntennae - Fabbiano et al 2004


• The baryon contribution to Ω isconstrained (e.g. by WMAP andBig Bang nucleosynthesis studies)to be Ωb≈0.045 h70

-2.

• This corresponds to mean densityρb≈2.5x10-7 amu cm-3.

• Gas with density <10-5 cm-3 cancurrently only be studied inabsorption.


At z~2-4 most of the baryonsappear to reside in the Lyman-α absorption systems, withT~30000 K.

These also contain metals,with abundances up to ~0.1solar.

Simulations suggest that theLyman forest “clouds” arefilamentary structures,photoionized by UV flux fromstars and AGN. Colder gas isconcentrated into galaxies.


The density of forestclouds drops sharplyover the range z=3 to1.5

Simulations predictthat at low z, most ofthe IGM has beendriven to highertemperatures, byshock heating incollapsing filaments.


The density of forestclouds drops sharplyover the range z=3 to1.5

Simulations predictthat at low z, most ofthe IGM has beendriven to highertemperatures, byshock heating incollapsing filaments.


Most of the IGM is leftin the “Warm-HotIntergalactic Medium”,with T~105-106 K.

Dave et al 2001

Chandra LETG spectrum

The intergalactic medium at largeThis “warm” gas hasbeen detected inabsorption againstbackground AGN in thefar UV and X-ray.

Observed incidenceseems to agree withsimulations.

WHIM features at z=0may be associated withthe Local Group, ormay be Galactic.

Nicastro et al 2005

Fang et al 2002

Chandra LETG spectrum

The intergalactic medium at largeThis “warm” gas hasbeen detected inabsorption againstbackground AGN in thefar UV and X-ray.

Observed incidenceseems to agree withsimulations.

WHIM features at z=0may be associated withthe Local Group, ormay be Galactic.

Nicastro et al 2005


Virialised systems haveoverdensities δρ/ρ>100,allowing emission fromhot baryons to bedetected.

Compression & shocksduring collapse andvirialisation should heatmost baryons in groupsand clusters of galaxiesto T>106 K.

→ X-ray emissionXMM mosaic of MKW4, with opticalcontours - O’Sullivan et al 2003


Even some very poorgroups, with low velocitydispersions, havedetectable intergalactic X-ray emission, though thismay be very irregular.

E.g. Chandra study of theNGC 1587 group, whichhas only σ≈100 km s-1.

Helsdon et al 2004

Intergalacticgas in groups

Cold gas (HI) is alsofound outsidegalaxies in somegroups.

However, aggregateHI mass is typically~109-1011 M,whereas total gascontent of a 1013 M

group should be~ 1012 M. Verdes-Montenegro

et al 2001

Intergalacticgas in groups

However, aggregateHI mass is typically~109-1011 M,whereas total gascontent of a 1013 M

group should be~ 1012 M.

Verdes-Montenegroet al 2001

Chandra imageof HCG40

Some interesting gas properties

Scaling properties of hot gas - entropy AGN heating and entropy Shock amplification

Metals in the group gas Gas stripping in groups The evolutionary status of optically

selected groups Fossil groups

Scaling properties

Cosmological simulations including gravity and simple gas physics produce dark halos which are almost self-similar, when scaled to a radius enclosing fixed overdensity (e.g. r200).

Also, gas tracks dark matter within these halos. This behaviour would generate clusters with well-defined X-ray scaling relations. For fixed z: ‹ρ›~M/R3 is same for all systems

T~M/R ~ R2 ~ M2/3 from V.T. ∴ r200~T1/2

LX~ ρ2·V·Λ(T) ~ ρ2·T3/2·Λ(T) where Λ(T) ~ T1/2 for bremss,

→ LX~T2 (Navarro et al 1995)

The L:T relation

Mulchaey 2000

It has been clear for many years that the cluster L:T relation does not follow the L∝T2 slope expected for self-similar systems.

In practice, L∝T3 for clusters, with possible further steepening to L∝T4 in group regime (notwithstanding slope of 2.5 in GEMS group sample derived by Osmond & Ponman 2004!).

What is causing this effect?

Entropy in the intragroup medium

It is helpful to considerthe entropy of the IGM:

• Gas will always rearrangeitself such that entropyincreases outward

• Entropy is conserved in anyadiabatic rearrangement ofgas

Define “entropy” as K=T/n2/3

(so true thermodynamicentropy is s=k ln K + s0 .)

Voit, Kay & Bryan 2004

Non-radiativesimulations produceclusters with self-similar entropy profiles

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


Study, of 66 systemsby Ponman,Sanderson &Finoguenov (2003),showed that K(0.1r200)scales asK∝T2/3, rather than theself-similar scaling ofK∝T.

K∝T

Systems grouped into 8 temperature bins


Extra baryon physics is required to account forobserved entropy behaviour in groups/clusters

Cooling can raise the entropy, but to matchobserved properties requires ~50% of thebaryons to cool in group-sized halos

Hence we are seeing evidence for extra energyinjection - i.e. feedback

Two potential sources: nuclear energy (SNe),and gravitational energy (AGN)

Can observations give clues as to when andhow the feedback is provided?

AGN heating?

Perseus Cluster & 3C 84 Sound Waves in Perseus

10 kpc

The most promising solution to the cooling flow problem inclusters seems to be feedback from a central AGN.

AGN heating?

Chandra has uncovered complexstructures within the cores ofmany groups & clusters

Chandra observation of theNGC 4636 group

In many cases this seems to beassociated with activity in acentral active galaxy.

VLA 1.4 GHzcontours

However, observedentropy profiles do notshow isentropic cores.

Alastair Sanderson - 20clusters with T>2 keV

AGN heating?

Try cutting profiles on radio properties:

Jetha, Ponman & Sakelliou

AGN heating?

log P1.4>21.5log P1.4<21.5

Entr

opy

/ T

2/3

No apparent difference in entropy profiles

Or cut on LK(BGG) as proxy for black hole mass:

Jetha, Ponman & Sakelliou

AGN heating?

Still no difference…

Low LK(BGG)

Entr

opy

/ T

2/3

High LK(BGG)

Entropy at large radii

PSF03 also found thatthe S∝T2/3 scaling seenat 0.1r200 applied atlarger radii (e.g. r500).

Ponman, Sanderson& Finoguenov 2003

S∝T



S∝T


S∝T



This involves excessentropies of severalhundred keV cm2, whichsounds energeticallyvery challenging.

S∝T


Entropy amplification

How to generate suchlarge amounts of extraentropy in these outerregions?

A clue is available fromthe fact that smoothaccretion can generateentropies as high as thoseobserved, whilst accretionin unheated cosmologicalsimulations does not.

Voit & Ponman 2004


If feedback withinfilaments feeding clustersleads to smootheraccretion, then the pre-shock entropy ofaccreting gas will beraised. The accretionshock then raises thisvalue by a factor – shockmultiplier effect.

Voit & Ponman 2004


Borgani et al tested this ideaby comparing entropy profilesin clusters derived from thesame cosmological simulationswith and without preheatingand feedback.

In the absence of radiativecooling, preheating to anentropy floor of 100 keV cm2

(at z=3) results in strongentropy amplification.

However the entropy profilesfor groups are much too flat.

Borgani et al 2005


When radiative cooling andfeedback from galaxies wereincluded, the amplificationachieved was minimal, unlesspreheating was included inaddition to the winds.

A combined wind+preheatingmodel was moderatelysuccessful, but producedentropy profiles in clustersflatter than those observed.

Borgani et al 2005


Speculation: AGN heating within groups & clusters may

counteract cooling, and hence prevent largeamounts of mass deposition and central galaxygrowth

However, it does not have a major effect on thesurrounding IGM

The similarity breaking is largely the product ofactivity (AGN and/or SNe) within precursorfilaments

Metal abundances in the ICM

• Metallicity in clusters oftenshows a centralenhancement, outside whichit drops to 0.2-0.3 solar.

• XMM results (e.g. Pratt &Arnaud) confirm thesefeatures.

• The central peak may beplausibly explained by ejectafrom the central galaxy -with predominantly SNIaorigin (lower O/Fe).

Molendi 2004

Beppo-SAXabundances for CFand non-CF clusters

Are abundances in groups lower?A montage of group abundance profiles from Chandra (Helsdon) suggeststhat they drop to ~0.1 solar outside the core region (cf Buote et al 2004study of NGC5044).

Are abundances in groups lower?Analysis by JesperRasmussen (morelater) of Chandradata for 15 groups,confirms that ironabundance does droprapidly to ~0.1 solar,by r~0.2r200.


Where did the metals go?


Where did the metals go?

Another speculation: If entropy is raised in precursor filaments via

injection of SNII-enriched galactic winds, thenthis higher entropy gas could expand out offilaments and be hard for a modest group tocapture.

Gas stripping from group galaxiesNGC2276 is a starformingspiral with peculiar HImorphology located in theX-ray bright NGC2300group.

New Chandra data show ahead-tail morphology, witha probable bow shock.

It appears that gas ispumped up by starformation, and thenremoved by stripping, at arate of ~5 M /yr

I.e. stripping can occur ingroups after all.

Rasmussenet al 2006

X-ray bright groups

Properties of FoF-selected groups - theXI project

Birmingham-Carnegieproject using XMM andIMACS to study optically-selected groups.

Sample of 25 groups atz~0.06 extracted byMerchan & Zandivarez(2002) from a FoFanalysis of the 2dFGRS.

X-ray bright groups




XMM observations ofthe first 4 systems showweak/irregular or no hotIGM - very different fromX-ray selected groups

X-ray bright groups

Rasmussen et al 2006




XMM observations ofthe first 4 systems showweak/irregular or no hotIGM - very different fromX-ray selected groups

These groups all fall atthe bottom of the L-sigma relation.

X-ray bright groups

Rasmussen et al 2006

Fossil groupsChandra observations of the volume limited sample of Jones et al (2003)

Groups scaledto 3D density

Groupsscaled forinteractionrate

J1256.0+2556 z=0.232 J1340.0+4023 z=0.171 J1416.4+2315 z=0.137

J1552.2+2013 z=0.135 J1331.5+1108 z=0.081 NGC 6482 z=0.0131

Fossil groupsChandra observations of the volume limited sample of Jones et al (2003)

Confirms the high LX/LR of fossils

However, L-T relation is normal

Conspiracy? T raised as well as LX, due to high concentration?

Groups scaledto 3D density

Groupsscaled forinteractionrate

Khosroshahi et al 2006

Conclusions Similarity breaking in the hot IGM in groups might be due

largely to energy injection in precursor structures. Bothentropy and metallicity behaviour may be telling us this.

Gas can apparently be stripped from group galaxies, if it isroughed up first. Analagous to Chris Mihos’ IC lightgeneration.

Optically-selected groups appear to have very different X-rayproperties to most of the systems which X-ray astronomershave been studying. Could much of their gas be in the warm(WHIM) phase?

Fossil groups are emerging as the best candidates for oldundisturbed supergalactic structures, and warrant a lot morestudy.

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Trevor Ponman University of Birmingham · Intergalactic gas in groups Virialised systems have overdensities δρ/ρ>100, allowing emission from hot baryons to be detected. Compression