feedback in galaxy clusters - tcan-mbhof stars – form 10 – 100 stars/year – have some (1010 m...
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Feedback in Galaxy Clusters
Brian Morsony
University of Maryland
1
Not talking about…
• Galaxy-scale feedback
• Local accretion disk feedback
2
Outline
• Galaxy cluster properties • Cooling flows – the need for feedback • Feedback candidates • AGN feedback • Conduction instabilities • AGN and conduction
3
Galaxy Cluster Properties
• Massive, 1014 – 1015 Solar masses total • Close to cosmological baryon fraction,
85-90% dark matter • 10-30% of baryons in stars • Most baryons (70-90%) in hot ICM gas • Gas is pressure supported
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Cluster Example - Perseus
5 Ken Crawford
Cluster Example - Perseus
• a
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Fabian et al. 2011
Cooling Flows
• For some clusters, cooling time of gas in center less than age of universe
• See X-ray temperature decreasing towards the cluster center
• Cool-core cluster
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Cool-core vs. non-cool core
• a
8 Fabian et al 2009
Cool-core vs. non-cool core
9 Sanderson et al. 2006
Cool-core vs. non-cool core
10 Cavagnolo et al. 2009
Cooling Flows
• Gas in cool-core cluster should continue to cool
• Pressure decreases, hot gas will flow in to replace it - Cooling flow
• Should be either – Lots of cold gas in cluster center – Lots of stars and star formation
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Need for feedback
• Cooling flows are not seen • Cluster have a large elliptical central galaxy
– 1012 MSun of stars – Form 10 – 100 stars/year – Have some (1010 MSun) cold gas
• Should have: – 1013 MSun of stars or gas – Form 1000+ stars/year
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Filaments in Perseus
13 C. Conselice
Filaments in Perseus
14 Fabian et al. 2008
Feedback
• What does feedback mean to me? • Need a heat source • Powerful enough to balance cooling • Able to maintain cool core • Knows how much cooling is going on and
adjust its self • Fairly stable on long time scales
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Example: Thermostat
• Metal contracts, triggers a switch • Heat source turns on, gets warm • Metal expands, turns heat off • Room cools, repeat • Heater needs to be powerful
enough, but not too powerful
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Feedback Candidates
• Supernova • Gravitational heating • Dynamical friction / sloshing • AGN • Conduction
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Supernova
• Gas cools -> gas forms stars -> stars make SN -> SN drive winds and heat gas
• Very important in galaxy-scale feedback
• Cluster are a very deep potential • Stars are a small fraction of baryons • Not enough energy
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Gravitational heating and Dynamical friction / sloshing
• Gravitational heating – As galaxies fall into cluster, gas is stripped – Gas has excess potential energy, converted to
heat • Dynamical friction / sloshing
– As galaxies or sub-clusters move through the cluster, they create tides
– Tidal energy dissipated as heat – Gas displaced from dark matter potential,
sloshing releases energy 19
Sloshing – Abell 2052
20 Blanton et al. 2011
Gravitational heating and Dynamical friction / sloshing
• These are sources of heat, not feedback
• Galaxy infall or cluster mergers don’t know about gas cooling rate
• Should see some cluster catastrophically cooling
21
AGN Jets
• Cool gas falls in, accretes onto SMBH • Accretion powers AGN jets • Kinetic energy of jets injected into cluster
core • Jets heat gas, shut off cooling • Accretion rate deceases, shuts of jets
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AGN Jets
• Perseus Cluster X-ray Image – Multiple X-ray cavities – “Sound waves” extending out from cluster
center 23
AGN Jets
• Inner cavities filled with radio emission – Radio bubbles
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AGN jet advantages
• They exists, ~all cool core clusters have X-ray cavities
• Have a clear feedback loop • Have (maybe) enough energy to balance
cooling
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AGN jet problems
• Jets are not isotropic – is energy well distributed?
• How is jet energy converted into heat? – Shocks? – Mixing? – Gravitational uplift? – Cosmic rays?
• Do they really produce enough energy to balance cooling?
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AGN Jets in hydrostatic cluster
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Morsony et al. 2010
AGN Jets in “realistic” cluster
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Morsony et al. 2010
Conduction
• Hot gas in outer cluster has lots of energy compared to cooling gas in core
• If you can tap into that, can stop cooling • Spitzer conduction time is short compared
to cooling time • But, clusters are (weakly) magnetized…
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Conduction
• Conduction is anisotropic, along field lines • Effectiveness depends on field structure
– For a tangled magnetic field, conduction suppressed by 100+, not effective
– For radial field, conduction not suppressed, effective
– For azimuthal field, conduction suppressed, not effective
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Magnetic field structure
• Thermal instabilities alter field structure • Clusters are stable to convection in absence
of magnetic fields • In outer cluster, magnetic fields lead to
magnetothermal instability (MTI), create turbulence
• In inner cluster, heat flux-driven buoyancy instability (HBI), creates stable azimuthal field
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Magnetic field structure • MTI
• HBI
32 McCourt et al. 2011
Conduction field structure
• From Karen Yang • Preliminary
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AGN Jets + Conduction
• Jets have magnetic fields – Partially aligned with jet – Jets are long, ~100 kpc – Could create connection for conduction to
happen • Jets generate turbulence
– Could disrupt HBI fields (more conduction) – Create tangled fields (less conduction)
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AGN Jets
• From Karen Yang • Preliminary
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AGN Jets + Conduction
• From Karen Yang • Preliminary
36
AGN vs. AGN + Conduction
• From Karen Yang
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Hot mode w/o conduction Hot mode with conduction
Summary
• Feedback is needed in galaxy clusters • Type of feedback uncertain • AGN accretion is an important part, either:
– Direct feedback from jet energy – Indirect feedback from impact on conduction – Quasar mode feedback?
• Other heating may also contribute
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