Feedback in Galaxy Clusters

Brian Morsony

University of Maryland

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Not talking about…

•  Galaxy-scale feedback

•  Local accretion disk feedback

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Outline

•  Galaxy cluster properties •  Cooling flows – the need for feedback •  Feedback candidates •  AGN feedback •  Conduction instabilities •  AGN and conduction

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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

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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

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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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