CLUSTERS OF GALAXIES The Physics of the IGM: Cooling Flows

Observational evidences Observational evidences The homogeneous model: one ρ and T at each radius Observational evidence against homogeneous gas Observational evidence against homogeneous gas The inhomogeneous model: Δρ and ΔT at each radius The role of the magnetic fields in Cooling Flows The role of the magnetic fields in Cooling Flows Estimates of dM/dt from imaging & spectral data The fate of the cooling gas The fate of the cooling gas

Cooling in Clusters L X  n gas 2 T g 1/2 Volume E  n gas KT g Volume t cool  E/L X  T g 1/2 n -1

Cooling Flows Cooling Flows t cool ≈ T g 1/2 n p -1 For large radii n p is small t cool »t Hubble In the core n p is large t cool ~ t Hubble The gas within r cool will cool

Cooling Flows When the gas cools The pressure becomes lower The gas flows inwards, The gas flows inwards, The density increases in the center The gas cools even faster

Observational Evidences for Cooling Flows X-Ray Imaging X-Ray Imaging Surface brightness strongly peaked at the center Surface brightness strongly peaked at the center

Observational Evidences for Cooling Flows X-Ray Imaging X-Ray Imaging Surface brightness strongly peaked at the center Surface brightness strongly peaked at the center Peres et al. (1998)

Observational Evidences for Cooling Flows X-Ray Spectra X-Ray Spectra Low ionization lines in soft X-ray spectra Low ionization lines in soft X-ray spectra Canizares et al. (1984)

Observational Evidences for Cooling Flows X-Ray Spectra X-Ray Spectra Temperature gradients towards the center Temperature gradients towards the center r cool De Grandi & Molendi (2002)

Observational Evidences for Cooling Flows X-Ray Spectra X-Ray Spectra Low energy absorption features Low energy absorption features Allen et al. (1993) T(r) N H (r)

Observational Evidences for Cooling Flows X-Ray Spectra X-Ray Spectra Low ionization lines in soft X-ray spectra Low ionization lines in soft X-ray spectra Temperature gradients towards the center Temperature gradients towards the center Low energy absorption features Low energy absorption features No direct evidence of the gas motion, resolution of X-ray detectors is insufficent No direct evidence of the gas motion, resolution of X-ray detectors is insufficent

3 Dynamic regions: 3 Dynamic regions: 1. r > r cool and t cool > t Hubble Hydrostatic Equilibrium 2. r gal < r < r cool with r gal = radius at which the gas falls within. potential well of the cD galaxy 3. r<r gal Homogeneous Model Hot gas – one T g and n g at each r – radiation losses P decreases Gas will flow inwards under the pressure of the overlaying gas Gas will flow inwards under the pressure of the overlaying gas

Region 2 Region 2 ΔΦ/Δr is small radiation losses balanced by thermal energy + PV v s >>v free fall The gas is in quasi-hydrostatic equilibrium Region 3 Region 3 ΔΦ/Δr ≠ 0 gravitational energy balances radiation losses r Φ r cool r gal 2 1 3

Hydrodynamic equilibrium for Homogeneous Model 1. Mass conservation 2. Momentum conservation 3. Energy conservation variation of H per unit volume & time energy radiated per energy radiated per unit volume & time Entalphy=thermal E + work by P

M estimates for Homogeneous model Energy loss rate. Mass flow rate Enthalpy.. Peres et al. (1998)..

A fraction of the gas drops out the flow before reaching the center A fraction of the gas drops out the flow before reaching the center Most of the cooling gas never makes it to the center Observational evidences against M=const The surface brightness is not as peaked as would be expected if all the cooling gas were to reach the center M≠const  M  r α with α≈1 M≠const  M  r α with α≈1... Peres et al. (1998) (Fabian, Nulsen & Canizares 1984)

@ T≈10 6 K t cool  t s The cool blobs T≈10 6 K t cool  t s The cool blobs decouple from the flow and: from the flow and: 1. Fall ballistically? 2. Stay in place as cold gas? 3. Stay in place and form stars? In-homogeneous Model (Nulsen ’86) Different phases T,ρ coexist at every r Different phases T,ρ coexist at every r Phases are in Pressure equilibrium (t s <t cool ) The phases comove with « v s, B field ties the different phases together The phases comove with « v s, B field ties the different phases together Heat conduction btw phases must be suppressed, again B fields have been invoked

Summary Gas that is already highly inhomogeneous cools and flows inward under the pressure of the gas immediately on top. The different phases are in pressure equilibrium and comove (B field). The different phases are in pressure equilibrium and comove (B field). When a given phase cools below ≈10 6 K it falls out of pressure equilibrium while the other phases continue to flow inwards Cold gas deposition occurs on the whole CF region with similar  for different clusters (dM(r)/dt  r α ) Cold gas deposition occurs on the whole CF region with similar  for different clusters (dM(r)/dt  r α ) The origin of the density inhomogeneity is unclear: The origin of the density inhomogeneity is unclear: 1. fossil of the past stripping from galaxies (Soker et al. ’91) 2. former mergers btw substructures with different T and ρ

1. From Imaging data within the context of the in-homogeneous model ΔL j, luminosity in a given radial shell and M j mass flow rate in the same shell are related through a linear formula  from this, values M can be computed within the context of the in-homogeneous model ΔL j, luminosity in a given radial shell and M j mass flow rate in the same shell are related through a linear formula  from this, values M can be computed 2. From Spectral data stricly valid for homogeneous model, reasonable approx. for inhomogeneous model (Wise & Sarazin ’93) stricly valid for homogeneous model, reasonable approx. for inhomogeneous model (Wise & Sarazin ’93) M estimates for in-homogeneous model....

Comparison btw. M s and M I Allen (2000)..

Peres et al. (1998)

The role of B fields on large scales In ICM |B|≈1μG P B ≈1% P gas does not influence the dynamics of the gas. For decreasing r compression P B increases For decreasing r compression P B increases ΔΦ/Δr ≈ 0 ≈ ΔP gas /Δr P gas ~ constant r ≈ 10 kpc P B ≈ P T Magnetic reconection of field lines becomes effective E≈ erg/s (Soker & Sarazin ’88) Displacement of CF gas by radio lobes Displacement of CF gas by radio lobes

The role of B fields Tangled field inhibit thermal conduction by increasing the particle mean free path Once a blob has cooled down to ~ 10 6 K radiation cooling becomes very fast Once a blob has cooled down to ~ 10 6 K radiation cooling becomes very fast ρ ≈ constant, T decreases, P gas decreases repressurizing shocks are partially suppressed by the P B suppressed by the P B At T ~ 10 6 K t recon ≈ t cool  magnetic energy will be converted into thermal energy thereby slowing down the collapse of the blobs.

The fate of the cooling gas Cooling flow is a frequent phenomenon (~ 60%-70%) Cooling flow is a frequent phenomenon (~ 60%-70%) Cooling flow is a persistent phenomenon Cooling flow is a persistent phenomenon 1. M acc ≈ M sun [M/(100M sun /yr)] (Sarazin ’89) 2. M acc « M cluster ≈ M sun 3. M acc comparable to mass of the cD galaxy.

The fate of the cooling gas (A) Ionized Cold gas Cold gas (B) Neutral (C) Molecular (A) Lines observed in optical and UV indicate that ionized gas is present « M acc

Kent & Sargent (1979)

The fate of the cooling gas (A) Ionized Cold gas Cold gas (B) Neutral (C) Molecular (A) Lines observed in optical and UV indicate that ionized gas is present « M acc (B) 21 cm observations in central galaxies give M HI  10 9 M sun

Peres et al. (1998)

The fate of the cooling gas (A) Ionized Cold gas Cold gas (B) Neutral (C) Molecular (A) Lines observed in optical and UV indicate that ionized gas is present « M acc (B) 21 cm observations in central galaxies give M HI  10 9 M sun (C) Recent obs. (Edge 2002) have detected molecular gas for the first time, again « M acc

Molecular gas  Star Formation Current star formation is probably small Current star formation is probably small t cool(HI) « t Hubble  stars may have already formed t cool(HI) « t Hubble  stars may have already formed HOWEVER: HOWEVER: star formation must have favored small mass stars M<M sun, otherwise: 1. SNII would have balanced cooling and stopped CF 2. the cD would be bluer and more luminous Theoretical arguments favor formation of small mass stars currently we have no exhaustive explanation for the cooling gas

?

SUMMARY B fields play an important role in CF B fields play an important role in CF A dominating fraction of galaxy clusters feature CF A dominating fraction of galaxy clusters feature CF Analysis of X-ray images and spectra lead to consistent determination of mass deposition rates. Analysis of X-ray images and spectra lead to consistent determination of mass deposition rates. From X-ray observations we find that CF deposit large quantities of cold gas From X-ray observations we find that CF deposit large quantities of cold gas At larger wavelenghts we do not find. evidence of such large masses At larger wavelenghts we do not find. evidence of such large masses  the fate of the cooled gas is unknown

It is somewhat disturbing that all crucial evidences for cooling flows comes from X-ray data It is somewhat disturbing that all crucial evidences for cooling flows comes from X-ray data Even in the X-rays we do not have direct observational evidence of: Even in the X-rays we do not have direct observational evidence of: 1. flowing gas 2. multiphase gas at one radius