Decoding X-Ray Observatories: Clusters of Galaxies

1. CLUSTERS OF GALAXIES From Cooling-Flows to Cool Cores

2. Forward About 7 years ago 2 new X-ray satellites have been launched

3. X-ray Observatories • After the rocket experiments during the 1960s, the first X-ray Earth-orbiting explorers were launched in the 1970s: Uhuru, SAS 3, Ariel5 • followed in late 1970s early 1980s by larger missions: HEAO-1, Einstein, EXOSAT, and Ginga.

4. X-ray Observatories • In the 1990s the ROSAT survey detected more than 100,000 X-ray objects • the ASCA mission made the first sensitive measurements of the X-ray spectra from these objects • BEPPOSAX contributed along this line

5. Current X-Ray Missions Chandra XMM-Newton

6. The X-ray Telescope Chandra

7. Chandra detectors

8. PSF

9.  DISPERSIVE SPECTROMETERS • All convert  into dispersion angle and hence into focal plane position in an X-ray imaging detector • BRAGG CRYSTAL SECTROMETERS (EINSTEIN, SPECTRUM X-GAMMA): Resolving power up to 2700 but disadvantages of multiplicity of cristals, low throughput, no spatially resolved spectroscopy • n x  = 2d x sin • TRANSMISSION GRATINGS (EINSTEIN, EXOSAT, CHANDRA) • m x  = p x sin • where m is the order of diffraction and p the grating period • REFLECTION GRATINGS (XMM) • m x  = p (cos - cos) • The resolving power for gratings is given by , assuming a focal lenght f and a position X relative to the optical axis in the focal plane • X = f tan  fsin X = f  so •  is constant

10. Rosat Good Spatial resolution Low or no Spectral resolution ASCA Low Spatial resolution Good Spectral resolution Chandra Versus Previous Generation X-ray Satellites Previous X-ray telescopes had either good spatial resolution or spectral resolution Chandra got both

11. Chandra view of “the Creation” of Michelangelo Rosat view of “the creation” of Michelangelo ASCA view of “the creation” of Michelangelo Chandra Versus Previous Generation X-ray Satellites An Imaginary Test

12. The RGS Result A1795 Tamura et al. (2001a); A1835 Peterson et al. (2001); AS1101 Kaastra et al. (2001); A496 Tamura et al. (2001b); sample of 14 objects Peterson et al. (2003) There is a remarkable lack of emission lines expected from gas cooling below 1-2 keV. The most straightforward interpretation is that gas is cooling down to 2-3 keV but not further. Peterson et al. (2001) Standard CF model predicts gas with T down to at least 0.1 keV!

13. The EPIC Result EPIC has a spectral resolution ~ 10 times worse than RGS. It cannot resolve individual lines. However it can discriminate btwn. models with and without a minimum temperature The major discriminant is the Fe L Shell blend profile

14. Comparison btwn. multi-temperature models Model spectra degraded to the EPIC resolution Spectra above ~1.3 keV are similar. Below we observe a prominent line-like feature: Fe-L shell line complex. In the spectrum with Tmin=0.1 keV we see a shoulder down to ~ 0.8, this is due to low ionization lines from gas colder than 0.9 keV. In the spectrum with Tmin=0.9 keV the shoulder is absent because the low ionization lines are missing Tmin=0.9 keV Tmin=0.1 keV Molendi & Pizzolato (2001)

15. EPIC vs. RGS Tmin EPIC minimum temperatures are in good agreement with RGS minimum temperatures. The result on Tmin is a solid one! All clusters observed so far show a Tmin Values range between ~1 and ~3 keV

16. Way out: hiding the flow An attempt to save CF models (Fabian et al. 2001) How can we have gas cooling below ~1 keV without observing low ionization lines?

17. Way out: Hiding the flow Gas Mixing For r< 20 kpc gas with T~103 K is present. Mixing of hot, T ~3•107 K with cold gas may rapidly cool the hot gas to T ~ 3 • 105 K Gas at temperatures of T~103 K is seen in the innermost ~ 20-30 kpc of only some clusters, thus it may only work for the inner regions of some objects.

18. Way out: hiding the flow Differential Absorption The absorber could be patchy and concentrated near the center and perhapes absorb the gas producing the lines which are not seen. A few of the clusters observed so far have at their center an AGN visible at X-ray wavelengths. The spectra of these AGN do not show any evidence of absorption.

19. Way out: hiding the flow Bi-modal metallicity (very ingenuous!) If metal distribution is highly bimodal (e.g. 10% of gas with Z=3 and 90% of gas with Z=0), with Z rich gas in small clumps (r < 1 kpc), for T> 2 keV Z rich and poor gas would cool togheter, for T< 2 keV, when line emission becomes an important coolant, Z rich gas would cool much more rapidly than Z poor gas Observed spectra are not well fit with bi-modal spectral models.

20. Consequences

21. Multiphaseness • Gas is NOT multiphase, at least not in the sense required by the standard multi-phase CF model • Multiphaseness is or was a fundamental ingredient of the CF model, without it the model falls!

22. Intrinsic Absorption • Cooling flow spectrum characterized by intense soft emission • Spectrum with Tmin~ 1 keV not as much • If you want to reproduce a spectrum with a Tmin~ 1 keV • with a CF model you have to get rid of the soft emission • A possible way is to assume intrinsic absorption • The large absorption column depths inferred by previous • analysis are an artifact resulting from the application of an • incorrect model to the data

23. Mass Deposition • Little evidence of gas cooler than 1-2 keV • anywhere • If gas does not cool below 1-2 keV it will not be • deposited as cold gas • The gas could still be multi-phase on scales we do • not resolve with XMM/EPIC • However at least in the case of M87 our resolution • is of a few kpc

24. Mass Deposition • RGS detects only very weak lines from gas cooler than ~ 1 keV (Peterson et al. 2003) • occurs on scales which are not spatially resolved • by EPIC Mass deposition is smaller and confined to smaller scales than previously thought The old problem of not finding abundant cooled gas is solved

25. Now that we have brought the house down it is time to think about rebuilding Let’s try with a fresh point of view, let us go to Chandra observation. Chandra has a very sharp eye, PSF better than 1 arcsec about 10 times better than XMM-Newton EPIC

26. The Chandra View Hydra A Chandra finds what appear to be holes “cavities”. Radio lobes are conicident with X-ray cavities Radio lobes inflated by jets appear to be making their way pushing aside the X-ray emitting plasma McNamara et al. (2001)

27. The Chandra View Abell 2052 Blanton et al. (2001) Radio lobes fill X-ray cavities Cavities are surrounded by denser & cooler gas

28. The Chandra View Centaurus, Sanders et al. (2001), Taylor et al. (2001) Radio X-ray interaction produces an unusual radio source with small bent lobes

29. The Chandra View Perseus, Fabian et al. (2000) Radio lobes fill X-ray cavities. Inner cavities surrounded by denser & cooler gas. Holes appear to be devoid of ICM, Schmidt et al. (2002) If we assume that the radio lobes are in pressure equilibrium with the surrounding ICM, this is reasonable as no shocks are observed, then it is easy to show that the lobes filled with B field and relativistc particles have a smaller specific weight than surrounding ICM and should therefore detach and rise buoyantly.

30. The Chandra View Abell 2597, McNamara et al. (2001) Expanded viewof the central regionof Abell 2597 after subtracting a smooth backgroundcluster model. The 8.44GHz radio contours are superposed VLA1.4 GHz image ofAbell 2597 at 11’’×6’’ resolution Cavities in Abell 2597are not coincident with bright radio lobes. Instead, they are associated withfaint extended radio emissionseen in a deepVery Large Array radiomap. Ghost cavities are likely buoyantlyrising relics of aradio outburst that occurredbetween 50 and 100Myr ago.

31. The overall picture The core of CF clusters is far from being relaxed, interaction with AGN is clearly present. Radio Jets open up as radio lobes plowing their way through the ICM. Radio lobes are in pressure equilibrium with ICM, no strong shocks observed. Bubbles of low density plasma rise through cluster atmosphere. Rising speed ~ a fraction of vs, rise timescales of the order of 108 yrs, ~ to cooling timescale Higher density gas at rims, either dragged out from center or compressed by uprising bubbles

32. Heating the flow • Feedback from AGN may provide a valid heating mechanism • Mixing of relativistic plasma with ICM • Adiabatic expansion of bubbles as they rise through cluster atmosphere • Dragging out of dense gas from core which expands adiabatically becomes less dense and eventually mixes with ambient gas

33. Heating the flow • Feedback from AGN • Gas cools everywhere in the core, cavities are not seen everywhere • Some cores do not host an active radio source (e.g. A2597) • A possible solution is to assume a duty cycle, note that the timescale over which radio lobes evolve must be ~ a few times shorter than the cooling time scale. • The energetic requirements, at least in some cases could be met.

34. Heating the flow Feedback from AGN The total energy required to quench a flow can be consider-able. Take total cooling energy, determined from L(< rcool )•tHubble for a set of clusters and compare it with the total energy emitted by an AGN over tHubble. The more luminous cores imply very large black-hole masses Fabian et al. (2002)

35. Heating the flow Conduction Large heat reservoir in outer regions of cluster Conduction and B Field. If the B field ischaotic over a widerange of length scalesas might happen with MHD turbulence conductivity can be as high as 1/3 Spitzer (Narayan & Medvedev 2001). Large ΔT/T in cores => conduction should be efficent

36. Heating the flow Conduction Effective conduction Most clusters have κeff between κs and 1/10κs Conduction is potentially important Fabian et al. (2002)

37. Heating the flow Conduction From 3d structure it is possible to derive estimate of κeff(r) For outer regions κeff is between κs and 1/3κs , for innermost regions κeff exceeds κs. Conduction cannot quench CF in the very center Ghizzardi et al. (2003), Voigt et al. (2002)

38. Heating the flow Mixed models Some authors have considered “mixed” models Heating from conduction in outerparts Heating from AGN within 20-30 kpc

39. Summary The old revered Cooling-Flow model has fallen under the weight of new observations obtained mainly with XMM-Newton. Chandra images show that the cores of clusters far from being relaxed, are the sight of much dynamical activity. Interaction between radio lobes and ICM Current efforts are concentrated on finding plausible heating sources to balance the CF AGN play a role in the innermost 20-30 kpc and conduction may operate at larger radii