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The Effect of Escaping Galactic Radiation on the Ionization of High-Velocity Clouds

The Effect of Escaping Galactic Radiation on the Ionization of High-Velocity Clouds. Andrew Fox, UW-Madison STScI, 8 th March 2005. Collaborators. Blair Savage, Bart Wakker (UW-Madison) Ken Sembach (STScI) Todd Tripp (UMass) Joss Bland-Hawthorn (AAO). Outline.

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The Effect of Escaping Galactic Radiation on the Ionization of High-Velocity Clouds

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  1. The Effect of Escaping Galactic Radiation on the Ionization of High-Velocity Clouds Andrew Fox, UW-Madison STScI, 8th March 2005

  2. Collaborators • Blair Savage, Bart Wakker (UW-Madison) • Ken Sembach (STScI) • Todd Tripp (UMass) • Joss Bland-Hawthorn (AAO)

  3. Outline • Introduction to High-Velocity Clouds • Diskhalo model of Galactic radiation field • Apply this radiation field to CLOUDY photoionization modelling of UV absorption line observations (FUSE/HST) • Determine physical conditions in HVCs • How to test the diskhalo model to constrain photon escape fraction

  4. High Velocity Clouds in H I HVCs: H I moving at |vLSR|>100 km s-1 Several processes occur to create HVCs Courtesy Bart Wakker

  5. High Velocity Clouds in O VI Color scale: HV HI 21cm Circles: HV O VI 1031Å (Sembach et al. 2003) • Highly ionized gas exists in HVCs • Generally good correlation between H I and O VI

  6. HVCs for this Study • QSO HE 0226-4110 (z=0.495): • Sight line lies a few degrees of the edge of the Magellanic Stream • We observe many ionic species in absorption with FUSE and HST/STIS. • 4 components seen (see color) Fox et al. 2005

  7. Column Density Measurements • H I comes from absorption in Lyman series lines • In 3 of the 4 HE 0226-4110 clouds: • Metallicity [Z/H] = –0.5 ± 0.2 suggestive of Magellanic Stream  d = 50 kpc

  8. DiskHalo model • Diskhalo is a 3-D model of the interstellar radiation field in the Milky Way • Bland-Hawthorn et al. (1998) • Bland-Hawthorn & Maloney (1999, 2001) • Bland-Hawthorn & Putman (2002) • Includes radiation from • Spiral arm O-star distrib. (hard photons) (Vacca, Garmany & Shull 2002) • Thin disk (soft photons) - observed • Stellar bulge (post-AGB stars) • Hot corona • Opacity includes spiral distribution for dust • Photon escape fraction is constrained by Ha brightness of HVCs (6% of ionizing photons escape in z-direction)

  9. Isoflux contours near the Galaxy • Plot on right shows integrated ionizing flux as function of distance. Contours are log F (cm-2 s-1) • Blue l<912Å (hard) • Red l>912Å (soft) • Galaxy dominates EGB inside log F<912 =4.0. • Dot-dashed lines show directions where we have UV spectra.

  10. Spectral Shape • Top green line is naked ionizing spectrum in the disk • Opacity is higher for hard photons than soft photons • Blue line shows extragalactic background of Haardt & Madau (1996) (Fn=4pJn)

  11. Ionization Patterns • Use CLOUDY photoionization code (Ferland et al. 1998) • In: - incident radiation field - observed ionic column densities • Out:- Best fit values of U (ng/nH), [Z/H], ionization balance, temperature - Calculate pressure, cloud size, Ha intensity • This technique used by Sembach et al. (2001), Tripp et al. (2003), Collins et al. (2003, 2004), Ganguly et al. (2005) • MW field predicts different ionic ratios (e.g. Si III/Si II; C III/ C II)

  12. Match with Data c2minimization used to find best-fit U and [Z/H]

  13. Results • HVCs have high overall level of ionization • Pressures imply clouds would be close to pressure equilibrium in 106 K halo (Sternberg et al. 2002)

  14. CLOUDY models underpredict high ions (O VI, C IV) by several orders of magnitude, with either EGB or MW. • Collisional ionization is required to create the high ions (conductive interfaces and shocks can reproduce ionic ratios and kinematics) High Ions Not Photoionized C IV/ O VI N V/ O VI

  15. Testing the Diskhalo model • Ha observations can constrain the ionizing flux, and hence the photon escape fraction • I(Ha) F (assumes ionization followed by recombination and cascade) • Greg Madsen has program to search for Ha from HVC Complex A using WHAM see poster by Madsen et al.

  16. Conclusions • Photons escaping from the Galactic disk can ionize HVCs. • The ionization balance in these clouds is determined by the shape of the emerging radiation. • Photoionization modelling of HVCs can be used to solve for P, n. • High ions in HVCs (O VI , C IV) are not explained by photoionization - collisionally ionized boundary layers (conductive, shocked). • If confirmed using Ha observations, model predicts that Galactic photons do escape the clumpy ISM, and will contribute to the extragalactic background.

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