Cosmic ray feedback Diversity of cool cores AGN feedback: mechanical versus cosmic-ray heating Christoph Pfrommer in collaboration with Svenja Jacob Heidelberg Institute for Theoretical Studies, Germany June 16, 2015 / ICM physics and modelling, MPA Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Cosmic ray feedback Diversity of cool cores Outline Cosmic ray feedback 1 Observations of M87 Cosmic rays Heating Diversity of cool cores 2 Cool core sample Bimodality Conclusions Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Radio mode feedback by AGN: open questions energy source: release of non-gravitational accretion energy of a black hole jet-ICM interaction and rising bubbles: 1.) magnetic draping → amplification 2.) CR confinement vs. release 3.) excitation of turbulence heating mechanism: 1.) self-regulated to avoid overcooling 2.) thermally stable to explain T floor 3.) low energy coupling efficiency Perseus cluster (NRAO/VLA/G. Taylor) cosmic ray heating: 1.) are CRs efficiently mixed into the ICM? 2.) is the CR heating rate sufficient to balance cooling? 3.) how universal is this heating mechanism in cool cores? Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Messier 87 at radio wavelengths ν = 1 . 4 GHz (Owen+ 2000) ν = 140 MHz (LOFAR/de Gasperin+ 2012) high- ν : freshly accelerated CR electrons low- ν : fossil CR electrons → time-integrated AGN feedback! LOFAR: halo confined to same region at all frequencies and no low- ν spectral steepening → puzzle of “missing fossil electrons” Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Solutions to the “missing fossil electrons” problem solutions: special time: M87 turned on 10 5 B = 10 µ G Coulomb ∼ 40 Myr ago after long synch. + IC 10 4 E [Myr] silence electron loss timescales, τ = E / ˙ B = 20 µ G ⇔ conflicts order unity duty 10 3 cycle inferred from stat. AGN feedback studies (Birzan+ 2012) 10 2 Coulomb: n e [cm − 3 ] = 10 − 2 Coulomb cooling removes 10 − 1 total loss fossil electrons 10 1 → efficient mixing of CR 10 0 electrons and protons with 10 0 10 1 10 2 10 3 10 4 p = ˜ p / m e c dense cluster gas C.P . (2013) → predicts γ rays from CRp-p interactions: p + p → π 0 + . . . → 2 γ + . . . Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating The gamma-ray picture of M87 high state is time variable → jet emission low state: (1) steady flux (2) γ -ray spectral index (2.2) = CRp index = CRe injection index as probed by LOFAR (3) spatial extension is under Rieger & Aharonian (2012) investigation (?) → confirming this triad would be smoking gun for first γ -ray signal from a galaxy cluster! Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Estimating the cosmic-ray pressure in M87 hypothesis: low state of γ -ray emission traces π 0 decay in ICM: X-ray data → n and T profiles assume steady-state CR streaming: P cr ∝ ρ γ cr / 2 ∝ P th � F γ ∝ d V P cr n enables to estimate X cr = P cr / P th = 0 . 31 (allowing for Coulomb cooling with τ Coul = 40 Myr) Rieger & Aharonian (2012) → in agreement with non-thermal pressure constraints from dynamical potential estimates (Churazov+ 2010) Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Interactions of cosmic rays and magnetic fields CRs scatter on magnetic fields → isotropization of CR momenta CR streaming instability: Kulsrud & Pearce 1969 if v cr > v A , CR current provides steady driving force, which amplifies an Alfvén wave field in resonance with the gyroradii of CRs scattering off of this wave field limits the (GeV) CRs’ bulk speed ∼ v A wave damping: transfer of CR energy and momentum to the thermal gas → CRs exert a pressure on the thermal gas by means of scattering off of Alfvén waves Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Cosmic-ray transport total CR velocity v cr = v + v st + v di (where v ≡ v gas ) CRs are advected with the flux-frozen B field in the gas CRs stream adiabatically down their own pressure gradient relative to the gas: � v st = − v A b b · ∇ P cr B 2 | b · ∇ P cr | with b = B | B | and v A = 4 πρ CRs diffuse in the wave frame due to pitch angle scattering by MHD waves: v di = − κ di b b · ∇ P cr , P cr Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Cosmic-ray heating vs. radiative cooling (1) CR Alfvén-wave heating: (Loewenstein, Zweibel, Begelman 1991, Guo & Oh 2008, Enßlin+ 2011) � X cr ∇ r � P th � Ω + δ P cr � H cr = − v A · ∇ P cr = − v A δ l Alfvén velocity v A = B / √ 4 πρ with B ∼ B eq from LOFAR and ρ from X-ray data X cr inferred from γ rays P th from X-ray data pressure fluctuations δ P cr /δ l (e.g., due to weak shocks of M ≃ 1 . 1) radiative cooling: C rad = n e n i Λ cool ( T , Z ) cooling function Λ cool with Z ≃ Z ⊙ , all quantities determined from X-ray data Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Cosmic-ray heating vs. radiative cooling (2) Global thermal equilibrium on all scales in M87 10 -24 radial extent of radio halo: 10 -25 C rad , H CR [ergs cm − 3 s − 1 ] 10 -26 10 -27 H CR , P smooth + δ P H CR , P smooth 10 -28 C rad (0 . 7 Z ⊙ � Z � 1 . 3 Z ⊙ ) 1 10 100 radius [kpc] C.P . (2013) Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Local stability analysis (1) heating cooling kT isobaric perturbations to global thermal equilibrium CRs are adiabatically trapped by perturbations Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Local stability analysis (1) unstable FP heating cooling kT isobaric perturbations to global thermal equilibrium CRs are adiabatically trapped by perturbations Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Local stability analysis (1) unstable FP heating stable FP cooling kT isobaric perturbations to global thermal equilibrium CRs are adiabatically trapped by perturbations Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Local stability analysis (1) separatrix unstable FP heating stable FP cooling region of stability region of instability kT isobaric perturbations to global thermal equilibrium CRs are adiabatically trapped by perturbations Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Local stability analysis (2) Theory predicts observed temperature floor at kT ≃ 1 keV X CR = 0 . 31 X CR = 0 . 031 5 instability criterion, arsinh( D ) “islands of stability” 0 “ocean of instability” -5 10 5 10 6 10 7 10 8 temperature T [K] C.P . (2013) Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Virgo cluster cooling flow: temperature profile X-ray observations confirm temperature floor at kT ≃ 1 keV Matsushita+ (2002) Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Emerging picture of CR feedback by AGNs (1) during buoyant rise of bubbles: CRs diffuse and stream outward → CR Alfvén-wave heating CR streaming and diffusion (2) if bubbles are disrupted, CRs are injected into the ICM and caught in a turbulent downdraft that is excited by turbulent advection: adiabatic compression the rising bubbles and CR energization → CR advection with flux-frozen field → adiabatic CR compression and energizing: P cr / P cr , 0 = δ 4 / 3 ∼ 20 for compression factor δ = 10 CR injection (3) CR escape and outward stream- by bubble disruption ing → CR Alfvén-wave heating Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
Observations of M87 Cosmic ray feedback Cosmic rays Diversity of cool cores Heating Prediction: flattening of high- ν radio spectrum 10000 1000 flux density [Jy] 100 10 radio data continuous inj. continuous inj., switch o ff 1 hadronically induced emission 10 1 10 2 10 3 10 4 10 5 frequency ν [MHz] C.P . (2013) Christoph Pfrommer AGN feedback: mechanical versus cosmic-ray heating
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