The intergalactic medium

The intergalactic medium

Trevor Ponman

University of Birmingham

Thanks to: Stefano Borgani, Alexis Finoguenov, Steve Helsdon, Christine Jones, Scott Kay, John Osmond, Ewan O’Sullivan, Alastair Sanderson, Mark Voit

- mostly in galaxy systems

Flow chart - evolution & feedbackCollapse &

hierarchical growth

IGM Galaxies

Stripping & strangulation

Feedback (energy & metals)

The IGM

• The baryon contribution to is constrained (e.g. by WMAP and Big Bang nucleosynthesis studies) to be b0.045 h70

-2.

• This corresponds to mean density b2.5x10-7 amu cm-3.

• Gas with density <10-5 cm-3 can currently only be studied in absorption.

The IGM at low redshift

At z~2-4 most of the baryons appear to reside in the Lyman- absorption systems, with T~30000 K.

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

Simulations suggest that the Lyman forest “clouds” are filamentary structures, photoionized by UV flux from stars and AGN. Colder gas is concentrated into galaxies.


The density of forest clouds drops sharply over the range z=3 to 1.5

Simulations predict that at low z, most of the IGM has been driven to higher temperatures, by shock heating in collapsing filaments.


Most of the IGM is left in the “Warm-Hot Intergalactic Medium”, with T~105-106 K.

Detection of this is very challenging! (cf talk by Fabrizio Nicastro)

Dave et al 2001

The IGM in collapsed systemsVirialised systems have overdensities />100, allowing emission from hot baryons to be detected.

Compression & shocks during collapse and virialisation heats most baryons in groups and clusters of galaxies to T>106 K.

X-ray emissionXMM mosaic of MKW4, with optical contours - see O’Sullivan et al 2003

The IGM in collapsed systems

Even some very poor groups, with low velocity dispersions, have detectable intergalactic X-ray emission.

E.g. Chandra study of the NGC 1587 group, which has only ≈100 km s-1.

Helsdon et al 2004

The IGM in collapsed systemsCosmological 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: ‹›~M/R3 is same for all systems

T~M/R ~ R2 ~ M2/3 from V.T.

r200~T1/2

LX~ 2·V·(T) ~ 2·T3/2·(T)

where (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 LT2 slope expected for self-similar systems.

In practice, LT3 for clusters, with possible further steepening to LT4 in group regime (notwithstanding recent result of slope 2.5 inGEMS group sample by Osmond & Ponman 2004!).

What is causing this effect?

X-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 intracluster medium

It is helpful to consider the entropy of the ICM:

• Gas will always rearrange itself such that entropy increases outward

• Entropy is conserved in any adiabatic rearrangement of gas

Entropy map of NGC 4325 group - Finoguenov

Entropy in the intracluster medium

It is helpful to consider the entropy of the ICM:

• Gas will always rearrange itself such that entropy increases outward

• Entropy is conserved in any adiabatic rearrangement of gas

Define “entropy” as K=T/n2/3

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

Voit, Kay & Bryan 2004

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

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

Entropy in the ICM

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 .

Entropy in the ICM

A larger study, of 66 systems by Ponman, Sanderson & Finoguenov (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

Entropy in the ICM

Recent work by Pratt & Arnaud, Sun et al and Finoguenov confirms the lack of isentropic cores with higher quality data from XMM and Chandra.

The KT2/3 scaling is also confirmed, but there is more scatter in entropy for cooler (<2 keV) systems.

Sun et al 2004

Feedback and entropy

Voit & Bryan (2001) pointed out that for T<2 keV, contours of constant entropy correspond approximately to lines of constant cooling time.

Entropy floor

Self-similar scaling


Voit & Bryan (2001) pointed out that for T<2 keV, contours of constant entropy correspond approximately to lines of constant cooling time.

Hence low K gas should be removed from clusters either by cooling, or by being heated due to nearby feedback processes fueled by neighbouring cooling regions.

Voit & Bryan 2001


The cooling threshold approach can also reproduce the observed KT2/3 behaviour in cluster cores reasonably well.

Voit & Ponman 2004


The entropy threshold approach can also reproduce the observed KT2/3 behaviour in cluster cores reasonably well.

And the L:T relation is quite well modelled, at least above T~1 keV.

Voit et al 2002

Feedback simulations

Cosmological simulations including feedback have been carried out by Borgani & coworkers, and by Kay.

It is difficult to reproduce the observed properties (L:T, S:T, M:T, S(r), fgas, fit etc.) in a single scheme.

Borgani et al 2004

Feedback simulations

Perhaps the most intriguing result so far from this work is the strong feedback model of Kay (2004), which manages to reproduce the S(T) slope at both 0.1r200 and at r500 remarkably well, with a feedback scheme which involves heating gas around “star particles” to 1000 keV cm2!

The net energy input is still reasonable for supernova feedback, however, at ~1 keV per particle. Kay 2004

0.1r200

r500

Metal abundances in the ICM

• Metallicity in clusters often shows a central enhancement, outside which it drops to 0.2-0.3 solar.

• Recent XMM results (e.g. Pratt & Arnaud) confirm these features.

• The central peak may be plausibly explained by ejecta from the central galaxy - with predominantly SNIa origin (lower O/Fe).

Molendi 2004

Beppo-SAX abundances for CF and non-CF clusters

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).

Cooling of the ICM

Cooling appears to be taking place in the cores of many groups and clusters.

Cooling of the ICM

Cooling appears to be taking place in the cores of many groups and clusters.

Cooling times in most cluster cores are «tH.

Fabian 2004

Cooling of the ICM

However, gas cooling to T≈0 produces a characteristic set of emission lines.

Peterson et al 2003

Cooling of the ICM

Peterson et al 2003

…and high resolution spectroscopy with XMM and Chandra has failed to find lines corresponding to very cool gas.

Cooling of the ICM

The conclusion seems to be that gas cools by a factor ~3 from its peak temperature, but little or no gas cools below this point.

If this is the case, then the heating rates required to counteract cooling are substantial.

Fabian 2004

Heating of the ICM

So - we appear to have two “heating problems:

1. Heating required to break self-similarity in baryonic components of groups & clusters

2. Heating needed to counteract cooling within cluster cores.

What heat sources are available?

Heating of the ICM - mergersVery large amounts of energy (>1063 erg) can be released in

cluster mergers, but:

a) Only a minority of clusters are strongly affected at a given time.

b) Merger shocks would not break self-similarity.

Govoni et al 2004

T (keV)

Heating of the ICM - supernovae

Star formation, and the resulting supernova explosions, potentially provides up to ~1 keV per particle of energy, given the observed Si abundance in the ICM (Finoguenov et al 2001). This might be enough to account for the similarity breaking.

However, there is little active star formation in cluster cores today, so this process cannot solve the cooling flow problem.

Deep Chandra observation of the Antennae - Fabbiano et al 2004

Heating of the ICM - AGN

Chandra has uncovered complex structures within the cores of many groups & clusters

Chandra observation of the NGC 4636 group

In many cases this seems to be associated with activity in a central active galaxy.

VLA 1.4 GHz contours


In M87, a 137 ksec Chandra exposure, reveals arms and shock-like arcs.

Christine Jones - see also Forman et al 2004


In M87, a 137 ksec Chandra exposure, reveals arms and shock-like arcs.

VLA image showing structures on 3 different scales Owen et al 2000

Energy input from uplifted gas and shocks - totalling >1059 erg.

Heating of the ICM - AGNThe Perseus cluster shows a series of AGN-generated cavities.

Core of the Perseus cluster - Fabian et al 2003

Simulations of viscous dissipation of sound waves - Ruszkowski et al 2004Unsharp masked image

Ripples in the surrounding ICM appear to be sound waves, which may provide distributed heating.

Effects of the ICM on galaxies

Most galaxies reside within groups, and there is clearly a relationship between galaxy and IGM properties.

Mulchaey 2000

X-ray bright groups

For example, almost all groups with a detectable hot IGM have an early-type central galaxy.


And galaxies within groups show a morphology-density relation much like those in clusters - but stronger.

Dressler clusters

Groups

Groups scaled to 3D density

Groups scaled for interaction rate

These environmental effects could result from galaxy-gas or galaxy-galaxy interactions.


Clear examples of ram pressure stripping of galactic gas by hydrodynamical interaction with the ICM are not common.

A recent example is the active disk-like galaxy C153, moving at v>1500 km s-1 through the complex merging cluster A2125 (Wang et al 2004).

Chandra emission (blue/green) superimposed on V band image (red).

C153


Hydrodynamical simulations of ram pressure stripping can reproduce such effects (e.g. Stevens et al 1999), but the rarity of X-ray bright examples suggests that these may arise from galaxies experiencing a first infall into a cluster (Acreman et al 2003).

See talk by Irini Sakelliou.

The impact of group collapse

The collapse and virialisation of galaxy groups produces a violent environment which can affect group galaxies.

E.g. intergalactic shocks in Stephan’s Quintet (Trinchieri et al 2003) – due to collision of infalling galaxy with existing stripped HI?


The collapse and virialisation of galaxy groups produces a violent environment which can affect group galaxies.

XMM study of the unusual X-ray bright group around NGC 5171 (Osmond et al 2003).

Highest LX system in GEMS study of 60 groups in which X-rayemission is not centred on adominant early-type galaxy.


Metal rich central filament

Hot shocked region to W

It appears that gas is being stripped from the central galaxies as two subgroups merge.

NGC 5171

Summary Feedback has affected the IGM in groups and

clusters. Energy input from Sne and AGN are likely to

be important. Neither is yet well understood or modelled. Collapse and merging of groups may impact

significantly on their galaxies. There may be emerging evidence that

integrated abundances are lower in groups than in clusters - what is this telling us?

- The End -

Outline of talk The IGM at large The IGM in collapsed systems Cooling in the IGM Heating of the IGM Effects of the IGM on galaxies Some questions

L:T relation – does it steepen?

Osmond & Ponman 2004 slope=2.50

From the GEMS sample of 35 groups selected to have intergalactic hot gas, and with LX extrapolated to r500, from ROSAT data, Osmond & Ponman obtained an L:T slope of 2.50.4 – no steeper than for clusters.


Orthogonal regression slope=3.53

Further investigation (Helsdon & Ponman) shows that this result was misleading. With highly scattered data:

a) Don’t use a bisector approach!

b) Beware of the effects of flux limits.

Simulated data


Osmond & Ponman 2004 slope=2.50

Orthogonal regression slope=3.53

Orthogonal regression with binning slope=3.87 0.50

So – L:T probably does steepen, from 3 to 4.

Entropy in the ICM

It was also found that this KT2/3 scaling applied at larger radii (e.g. r500), and that K(r) profiles rise approximately linearly with r, with no large isentropic core, for systems right across the range T1-13 keV.

KT

Ponman, Sanderson & Finoguenov 2003

Models – Cooling

Radiative cooling of gas raises the entropy of the ICM, since low entropy gas is removed at the centre of halos.

However, cooling-only models suffer from serious overcooling, and result in galaxy mass fractions well above the 5-10% observed.

Muanwong et al 2002

Hot gas

Total

Galaxies

Models – Preheating

Preheating the IGM to a high adiabat at an epoch before cluster formation, via galaxy winds or mechanical injection from AGN, is energetically attractive (since K/E is larger when gas density is lower). Many models of this type have been investigated.

Suitable choice of Kinit can produce a reasonable match to the observed L:T relation.

log10 LX [ergs s-1]

kT [k

eV

]

Babul et al. 2001

Models – Preheating

However:

a) A generic feature of preheating models is the generation of an isentropic core in groups and cool clusters.

b) This results in strongly rising central temperatures.

c) Unless preheating is localised in cluster precursors, it is difficult to avoid destroying the Ly clouds (Benson & Madau 2003).

0.01 0.1 1.0

r/Rv

K(r)

T(r)

Tozzi & Norman 2001

log Mhalo

Models – Feedback

Ponman et al 2003

ST

However, the cooling threshold has only secondary effects in the outer regions of the cluster, where departure from self-similarity is also seen. How to generate extra entropy in these outer regions?

Models – Smoothing

However, the cooling threshold has only secondary effects in the outer regions of the cluster, where departure from self-similarity is also seen. How to generate extra entropy in these outer regions?

A clue is available from the fact that smooth accretion can generate entropies as high as those observed, whilst accretion in unheated cosmological simulations does not.

Voit & Ponman 2004

Models – SmoothingA clue is available from the fact that smooth accretion can generate entropies as high as those observed, whilst accretion in unheated cosmological simulations does not.

If feedback within filaments feeding clusters leads to smoother accretion, then the pre-shock entropy of accreting gas will be raised. The accretion shock then raises this value by a factor – shock multiplier effect.

Voit & Ponman 2004

NGC 507 deprojected abunds

Deprojection of XMM data for NGC 507 (O’Sullivan) gives a similar result.


• Metallicity in clusters often shows a central enhancement, outside which it drops to 0.2-0.3 solar.

Buote et al 2004

r200/3

In contrast, Buote et al find ~0.15 solar abundance outside the core of the NGC5044 group.


• ASCA studies of integrated abundances in a large sample of clusters shows that S/Si is lower in clusters than in Milky Way stars, or in galaxy groups. Either standard SNII yields are incorrect, or there is some additional source of Si (e.g. Population III - Loewenstein 2004)

• The inferred Fe contribution from SNIa in groups is high, but (using Si/Fe) is much lower in rich clusters.

T(r) profiles

• Temperature profiles in groups vs clusters

XMM profiles from 14 clusters

Molendi 2004

Beppo-SAX profiles for CF and non-CF clusters

XMM cluster profiles appear fairly flat out to ~0.5r200.

Do group T(r) profiles fall more steeply?Sun et al 2004

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The intergalactic medium