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Clusters of galaxies: the soft X-ray excess and Sunyaev-Zel’dovich effect. Richard Lieu Department of Physics University of Alabama, Huntsville. Another cosmological problem. Have we observationally accounted for all the atoms and molecules of the universe? . Another cosmological problem.

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Clusters of galaxies: the soft X-ray excess and Sunyaev-Zel’dovich effect


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    1. Clusters of galaxies: the soft X-ray excess and Sunyaev-Zel’dovich effect Richard Lieu Department of Physics University of Alabama, Huntsville

    2. Another cosmological problem • Have we observationally accounted for all the atoms and molecules of the universe?

    3. Another cosmological problem • Have we observationally accounted for all the atoms and molecules of the universe? • Are there significant amounts of thermal and non-thermal particles that evaded detection?

    4. Hinshaw et al 2007 ratio of 2nd to 1st acoustic peaks gives baryon to dark matter fraction.

    5. The WHIM (Warm Hot Intergalactic Medium) Gas between from a cosmological simulation (from R. Cen’s homepage)

    6. Cen & Ostriker ApJ 1999, on `Where are the baryons?’

    7. The discovery of cluster soft excess as extra photon emission in the 0.2 – 0.5 keV range above the level expected from the low energy tail of the virialised intracluster gas at X-ray temperatures was made by the EUVE mission in 1995 Coma Cluster in the EUV

    8. as measured by ASCA

    9. Virial speed of mass in the cluster is given by Virial theorem For a rich cluster like Coma, A proton moving in this potential has kinetic energy

    10. The discovery of cluster soft excess as extra photon emission in the 0.2 – 0.5 keV range above the level expected from the low energy tail of the virialised intracluster gas at X-ray temperatures was made by the EUVE mission in 1995 Coma Cluster in the EUV

    11. Coma Cluster 6’ – 9’ ROSAT and EUVE DS ROSAT PSPC EUVE Solid line is the expected emission spectrum of the hot ICM at kT = 8.7 +/- 0.4 keV and A = 0.3 solar, as measured by ASCA.

    12. Central soft excess (no background issues) for Coma XMM ROSAT XMM

    13. Best 3 Temperature model for the soft excess of Coma’s 6’-9’ region IS THIS A PHYSICALLY SENSIBLE MODEL?

    14. Is this the gas that `hides’ all the missing baryons?

    15. Physical constraints on the model For intracluster origin of the WHIM If we take Radiative cooling time is important For WHAT SUSTAINS THE WARM GAS AGAINST SUCH RAPID RADIATIVE COOLING?

    16. Thermal (mekal) model kT

    17. Coma cluster 0.5 – 2 keV with XMM-Newton pointings

    18. RGS spectra of X-Comae which lies behind the Coma cluster. Shown are the individual spectra from three separate observations of X-Comae together with the position of the OVII line at the redshift of Coma and of the Galaxy. The expected absorption from the CSE cannot be seen

    19. Same as previous slide but now all observations have been added. Again, there is no line at the expected redshift of Coma

    20. Mass fraction Nicastro et al. (LETG) A2218 (Suzaku) Coma (XMM) Baryon overdensity • No convincing detection of the WHIM so far. • Dense WHIM clouds can be detected with currently available detectors. • Larger sample of cluster vicinities will give us strong observational constraints.

    21. Non-thermal interpretation of the cluster soft excess Hwang, C.-Y., 1997, Science, 278, 191 Ensslin, T.A. & Biermann, P.L., 1998, A&A, 330, 20 Sarazin, C.L. & Lieu, R., 1998, ApJ, 494, L177 Proposed the origin of the cluster soft excess emission as due to inverse-Compton scattering between intracluster cosmic rays (relativistic electrons with Lorentz factors of a few hundred) and the cosmic microwave background HOW LARGE A COSMIC-RAY (CR) POPULATION DO WE NEED TO ACCOUNT FOR THE SOFT EXCESS BRIGHTNESS?

    22. CLUSTER SOFT EXCESS DIAGNOSTICS • Inside a cluster’s core the soft excess of clusters is likely to be • non-thermal in its nature, due to problems with rapidly cooling • of the thermal baryons, and absence of the OVII line. These non-thermal electrons can also be tested by means of the Sunyaev-Zel’dovich effect. • The soft excess outside clusters’ cores might still be • of thermal origin.

    23. The gas cooling problem is less serious if the warm gas lies in the outer parts of a cluster and emits soft X-rays from there. • We do observe soft excess from the outer parts of Coma.

    24. Giant ¼ keV Halo centered at Coma (as detailed by the ROSAT sky survey)

    25. ROSAT/PSPC Radial surface brightness of Coma (Bonamente, Joy, & Lieu, 2003, ApJ, 585, 722) ¾ keV ¼ keV

    26. ROSAT/PSPC data of the Coma cluster (50’-70’ annulus) Fitting the excess with a 2nd component X-ray thermal model (kT~8keV) Hot ICM + power-law Hot ICM + warm component Unlike the cluster core, strong soft excess at the outskirts of Coma. Statistically the thermal model is preferred (to a power law). The warm gas here may be part of the WHIM (e.g. Cen & Ostriker 1999) not in physical contact with the hot ICM. XMM-Newton confirmation of the Coma soft excess halo.

    27. Other efforts in search of the missing baryons also look promising.

    28. Zappacosta filament strikes back

    29. The Diffuse X-ray Background NH NH Local Bubble Unresolved point sources Galactic Halo WHIM Solar Wind Charge Exchange ~40 AU Detector Background ~100 pc ~2-5 kpc ~z<1

    30. Control Observations NH NH MBM20 Eridanus Hole Local Bubble Unresolved point sources Galactic Halo WHIM Solar Wind Charge Exchange ~40 AU Detector Background ~100 pc ~2-5 kpc ~z<1

    31. Control Observations • MBM20 • l=211º24’15.7”, b=-36º33’46.7” • NH=15.9*1020 cm-2 • Absorption=75% at ¾ keV • 112±15 pc < d < 161±21 pc • Eridanus Hole • l=213º25’52.3”, b=-39º5’26.6” • NH=0.86*1020 cm-2 • Absorption=8% at ¾ keV IRAS 100 m map of MBM20 and surroundings with the two XMM pointing used in our investigation.

    32. Control Observations Calculated AcF for the two control targets: MBM20 (red) and the Eridanus hole (blue) ON-CLOUD: c2=16, n=19 OFF-CLOUD: c2=60, n=19

    33. Results Several sigma detection of the WHIM signature in the AcF of XMM-Newton blank fields. The fraction of X-rays due to the WHIM in the energy band 0.4-0.6 keV is 18±5%of the total diffuse X-ray emission.

    34. CLUSTER SOFT EXCESS DIAGNOSTICS • The soft excess outside clusters’ cores might still be • of thermal origin. • Inside a cluster’s core the strong soft excess is likely • non-thermal in nature, due to short lifetime of the thermal • gas and absence of the OVII line.

    35. Giant radio (1.4 GHz) arcs of A3376, Bagchi et al, Science (2006),astro-ph/0611297

    36. In fact, one ought to ask the original question `what fraction of a cluster’s X-rays is thermal’ ?

    37. The original SZ paper (1972)

    38. Central soft excess (no background issues) for Coma XMM ROSAT XMM

    39. To Observer Sunyaev-Zel’dovich Effect: Compton up scattering of CMB photons by cluster hot electrons

    40. The point made by SZ is that only hot thermal gas can cause a significant decrease in the brightness (hence temperature) of the CMB.

    41. Basics of the Sunyaev-Zeldovich Effect CMB The electron density of the hot gas is obtained by fitting ROSAT X-ray surface brightness profiles with the 2 parameter isothermal -model ignoring the central cooling flow The decrement in TCMB is then given by with

    42. WMAP surveys the sky in three cosmological filter passbands: • Q band (31 GHz) • V band (64 GHz) • W band (96 GHz)