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Cooling flow

Cooling flow. Adriana Gargiulo Seminario Corso di astrofisica delle alte energie. …the fundamental parameter… . ICM energy . Cooling time:. Energy loss due to radiation in X rays. n H proton density L(T) value of cooling function at temperature T.

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Cooling flow

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  1. Cooling flow Adriana Gargiulo Seminario Corso di astrofisica delle alte energie

  2. …the fundamental parameter… ICM energy Cooling time: Energy loss due to radiation in X rays • nH proton density • L(T) value of cooling function at temperature T tcool < tHubble cooling happens tcool ≈ nH-1 !!! NOTE:

  3. Observational evidences for cooling flows - Imaging Surface brightness (SB) strongly peacked at the center. Since SB depends upon the square of gas density very short tcool Telescopes + Copernicus

  4. Observational evidences for cooling flows Rcool radius at whichtcool = tH. P(r ≥ rcool) weight of overlying gas where cooling is not important. P(r < rcool) cooling reduces the gas temperature: to maintain the pressure the gas density must rise Rcool The gas must FLOW inward

  5. Observational evidences for cooling flows In absence of a suitable fine – tuned heating source, the cooling and condensation of the gas in the central region is a straight-forward consequence of the basic energy equation of the hot gas. • Silk (1976) • Fabian & Nulsen (1977) • Cowie & Binney (1977) • Mathews & Bregman (1978) • Fabian et al. (1984) • Fabian (1994) 70 % - 80 % of clusters have a cooling flow: common and long - living

  6. Observational evidences for cooling flows - Spectra Independently: Strong support from spectroscopic observations The spectra show the existence of low temperature phases in addition to the hotter temperature gas. Einstein Observatory Solid State Spectrometer + Focal Plane Cristal Spectrometer White et al. 1994 Allen et al. 1994

  7. Observational evidences for cooling flows - Spectra Strong cooling flow found from images where not confirmed by spectra…BUT…A478 SSS data show strong X rays absorption. Allen et al. 1993

  8. Imaging vs spectra Two different kind of observation lead to the same result . Ms = mass deposition rate computed from spectra analysis MI = mass deposition rate computed from image analysis . White et al. 1991

  9. The “cooling flow problem” X vs Optical X : large cooling rates of the KeV gas in the centers of clusters (tens to hundreds of solar masses per year). Optical: small star formation rates observed in central cluster galaxies (few to several tens of solar masses per year). “Only a small fraction of the cooled gas can form stars with a normal IMF: most must remain dark” Fabian, 1994.

  10. Observational evidences against The surface brightness is not as peaked as would be expected if all the cooling gas were to reach the center: • Mass Dropout: a fraction of the gas cools out of the flow, at large radii, before reaching the center and some continues to flow inward most of the cooling gas never makes it to the center M(r) proportional to r. • The gas is heated in someway… . The gas must be inhomogeneous

  11. Inhomogeneous model (Nulsen ‘86) Each radial zone in the cooling flow region comprises different plasma phases covering a wide range of T,r. The gas comprising different temperature phases features an inflow in which all phases move with the same flow speed <v> << vs, forming a comoving flow There is no energy exchange between the different phases, between material at different radii, and no heating.

  12. XMM – Newton & Chandra To further test the cooling flow picture most detailed X-ray spectroscopic observations ASTRO E  launch accident

  13. XMM – Newton (2001) Unpreceded detailed spectroscopic diagnostics of the central regions of clusters Spectral signatures of different temperature phases range from the virial temperature Tvir to a limiting temperature Tlow (Tvir/3), which is still above the “drop out” temperature where the gas would cease to emit significant X-ray radiation Evidence of failure of inhomogeneous standard cooling flow model

  14. Fe L series The spectroscopic signatures sensitive in the temperature range of cooling flow are the emission lines from the complex of iron L series ions that have their ionization potentials in the temperature range near and below cluster virial temperature. Fractional abundance of a given ion plotted against temperature in KeV (Arnaud & Raymond, 1992) . The fractional abundance is multiplied by the abundance of that element relative to hydrogen in the solar neighborhood.

  15. Fe-L series The Fe-L line complex in X ray spectra as a function of the plasma temperature for a metallicity value of 0.7 solar. The energy change is caused by the fact that with decreasing temperature the degree of ionization of the Fe ions also decrease. H. Bohringer et al., 2002

  16. Spectra model Spectral prediction for an inhomogeneous flow based on: Peterson et al. 2003

  17. Spectra model Comparison between the model and the spectrum of Abell 1835. Notably absent in the data are the Fe XVII lines. The plasma appears to match the cooling flow model between 3 KeV and the maximum cluster temperature of 8 KeV but not below 3 KeV.

  18. Spectra model Model where the emission below 3 KeV is suppressed.

  19. …otherexamples

  20. The cooling paradox Does the gas cool? • The gas is radiatively cooling, but for some reason it evades detection. • The gas is being heated in some way so that very little gas cools.

  21. Cool cores What happens to the gas which should be cooling on very short timescales? Two classes of solutions have been proposed: The gas is prevented from cooling below a certain temperature by some form of heating. Different classes of mechanisms have been considered: Turbolence, shock, merging Heating from SN Conduction Heating from the central AGN The cooler gas is there but it is somehow hidden

  22. Properties of a successful heating model The heating source have to: • Provide sufficient heating to balance the cooling flow losses (1043 – 1044 erg s-1) • be fine-tuned: mass deposition triggers the heating process and the heating process reduce the mass deposition • Provide a global heating effect: local energy deposition would result in local heating while the mass deposition can still go on in the less well-heated regions.

  23. Heating from AGN The vast majority of cooling flow clusters contain powerful radio sources associated with central cD galaxies. Spectacular anti-correlation between decrements in the X-ray emission and extended radio emission. Chandra results: Holes in the X – ray surface brightness are seen to coincide with some radio lobes  bubbles of relativistic plasma blown by AGN

  24. Heating from AGN Radio lobes inflated by jets of central AGN appear to be making their way pushing aside the X –ray emitting plasma. The first cooling flow cluster with a central radio source observed by Chandra was Hydra A. Cooling time at center: 6 x 108 yr Diameter of cavities evacuated by the radio source: 25 kpc Radio source / X ray gas interaction (Mc Namara et al. 2000)

  25. X ray / Radio interaction Radio sources have a profound effect on the X – ray emitting ICM Is the energy deposition into the ICM from the radio sources sufficient to account for the lack of gas seen at very low temperatures in cooling flow clusters? TOTAL ENERGY OUTPUT OF A RADIO SOURCE Churazov et al. 2002 Internal energy of the bubble Work done to expand the bubble V = volume bubbles P = pressure of X ray bright shell surrounding the bubbles

  26. X ray / Radio interaction Compare energy input rate with luminosity of cooling gas Hidra A Erad = 2.7 x 1044 erg s-1 Lcool = 3 x 1044 erg s-1 In many systems the amount of energy is comparable to the amount required to offset cooling .

  27. Self – regulation mechanism The most simple physical situation would be given if simple Bondi type of accretion from the inner cooling core region would roughly provide the order of magnitude of power output that is observed and required Black hole mass = 3 x 109 Msol Mass accretion rate = 0.01 Msol yr-1 Energy output = 7 x 1043 erg s-1 Accretion radius (vkep = vs) = 50 pc Spherical accretion on to the black hole:

  28. How this energy is distributed on the right spatial scale ? About 40% of the energy is transferred by the PdV work done on ambient medium. Since, on average, the bubbles expand subsonically this energy will be converted into sound waves and in low amplitude shock waves. Ripples in the gas interpreted as due to sound waves generated by the cyclical bubbling of the central radio source (Fabian et al. 2003) The gas directly bounding the bubbles seems colder the energy is not deposited directly in the boundary of the bubbles, as it would be expected for supersonic expansion. Dissipation of sound waves, if ICM is viscous, may produce diffuse heating.

  29. …some problems • For Hydra A and Abell 2052 the radio source is depositing enough energy into the ICM to offset the cooling gas, but… • For Abell 262 the radio source power is more than an order of magnitude lower of that required to offset the cooling luminosity!!! • Dimension of cool cores vs accretion disk…??? Current efforts are concentrated on finding plausible heating sources to balance the cooling flow. …Grazie

  30. Bibliografia • Fabian “Cooling flows in clusters of galaxies” ARA&A 32, 277-318, 1994. • Bohringer et al. “The new emerging model for the structure of cooling cores in clusters of galaxies” A&A 382, 804-820, 2002 • Mathews & Brighenti “Hot gas in and around Elliptical galaxies” ARA&A 41, 191-239, 2003.

  31. Deprojections analysis of X ray imaging Starting point: surface brightness. Goal: deriving a temperature T appropriate to the count rate per unit volume (C) produced in the Einstein detector at the local pressure. Method: counts rate are accumulated in concentric annuli. Counts from the outer annulus were used as “background”. Counts from inner annuli are assumed to originate from spherical shells. Fabian et al. 1981

  32. Deprojections analysis of X ray imaging Q(E) = effective area of High Resolution Image (HRI) e(T,E) dE = emissivity of the gas in the band E – E + dE NH s(E) = optical depth D = distance to the cluster P = outer pression

  33. Estimate of mass deposition ratefrom imaging From the deprojection analysis  temperature profile T(r) estimate of mass deposition rate Lcool = T = temperature at rcool + PdV work = Radiation of thermal energy =

  34. Estimates of mass deposition rate from X spectra A volume V of gas at density n cooling at constant pressure from T to T – dT emits a luminosity : m mean molecular weight of the gas. The luminosity of the spectrum at each frequency is: en is the emissivity at frequency n.

  35. Estimates of mass from X spectra Integreting: where: en emissivity of cooling gas in a single spectral line (Canizares et al. 1988). Fit of the spectrum  mass deposition rate BACK

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