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Halo Metallicity in NGC 891 An X-ray/UV Perspective

Halo Metallicity in NGC 891 An X-ray/UV Perspective. Edmund Hodges- Kluck Joel Bregman University of Michigan. How do baryons cycle in and out of galaxies, and how does this impact galaxy evolution? What happened to the “missing” baryons?

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Halo Metallicity in NGC 891 An X-ray/UV Perspective

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  1. Halo Metallicity in NGC 891An X-ray/UV Perspective Edmund Hodges-KluckJoel Bregman University of Michigan

  2. How do baryons cycle in and out of galaxies, and how does this impact galaxy evolution? • What happened to the “missing” baryons? • Are contemporary halos formed by infall from the IGM or galactic fountains?

  3. High Z Low Z Metallicity is a distinguishing factor

  4. QSO Absorption Lines • column density of halo metals • Dust Extinction/Scattering/Emission • dust type depends on Z • X-ray Emission • Z from simple thermal model How do we Measure Halo Metallicity?

  5. Case Study: NGC 891 • Edge-on Milky Way analog • Nearby (10 Mpc) • Bright X-ray halo • Giant HI halo

  6. Halo Metallicity from the X-rays • Clean test possible in the “outer halo” seen in 100 ks with XMM-Newton

  7. Halo Metallicity from the X-rays • 1T fits to XMM+CXO spectra prefer Z < Zto 3σ (5σ joint fit)Hodges-Kluck+Bregman (2013)

  8. Halo Metallicity from the UV • UV photons leaking out of the disk scatter off dust grains in the halo with metallicity-dependent spectrum

  9. Galex and Swift UVOT sensitivity limited by foreground fluctuations (sky background is low) • Swift UVOT has persistent scattered-light artifacts, but these can be corrected by subtracting scaled templates in each filter

  10. NGC 891

  11. UV SED favors Milky Way-type dust over LMC or SMC dust, but need HST data to bracket UV bump (UVW1 filter has red leak)

  12. QSO Absorption Lines • SOLAR (Bregman+2013) • Dust Extinction/Scattering/Emission • SOLAR (Rand+2011, Hodges-Kluck+2013, in prep) • X-ray Emission • SUBSOLAR (Hodges-Kluck+Bregman 2013) How do we Measure Halo Metallicity?

  13. Hard to eject cold gas without ejecting hot gas, too • But, 109 M HI halo (Oosterloo+07) could not have cooled from the 3x108 M hot component at current cooling rate of <0.5 M/yr • Halo may not be in a steady state • X-rays could trace steady hot accretion; cooler gas might be from a galactic fountain or wind fallback Different Components?

  14. Halo metallicity distinguishes between infall and galactic fountain scenarios • In NGC 891, cold gas seems to have solar metallicity, hot gas subsolar • Hot, cold gas may have different origin • New method in UV using Swiftmay be fruitful alternative to QSO absorption Summary

  15. kT = 0.5 keV NH = 1021 cm2 100 10 Photon Flux (arbitrary) 1.0 0.1 Z = 0.1 Zʘ Z = 1.0 Zʘ 0.01 0.3 1.0 2.0 0.3 1.0 2.0 Energy (keV) Energy (keV) • Metallicity is a key indicator that is directly measurable in the X-rays • At CCD resolution, good S/N needed to distinguish spectral shape

  16. From a fitting perspective, low metallicity results from flux below 0.6 keV The absorbing column in the outer halo is constrained well by other observations

  17. Low metallicity not likely caused by systematics: • Unaccounted bkg • Abundance table • Absorption model • Calibration issues

  18. But, the data also admit a 2-T model where both components have solar metallicity. This is in the outer halo

  19. Two “reality checks” favor accretion • Limit on cooling rate from UV —O vi from Otte et al. 2003 indicates < 2-3 Mʘ/yr • Observed vs. expected scale height —Hobs measured assuming hydrostatic equilibrium—Hexp from cooling time

  20. Clearly, the scattered light can bias a search for extended emission as objects of interest are usually centered on the chip

  21. Bkg. Sub. Ring Count Rate [cts/s/pixel] Edge Bkg. Count Rate [cts/s/pixel]

  22. UV – r color is too blue in most cases to be produced by an old stellar halo population But, a “stellar fountain” may be possible

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