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

Andrew Fox, UW-MadisonSTScI, 8th March 2005

Collaborators

• Blair Savage, Bart Wakker (UW-Madison)

• Ken Sembach (STScI)• Todd Tripp (UMass)• Joss Bland-Hawthorn (AAO)

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

High Velocity Clouds in H I

Courtesy Bart WakkerHVCs: H I moving at |vLSR|>100 km s-1

Several processes occur to create HVCs

High Velocity Clouds in O VI

(Sembach et al. 2003)

•Highly ionized gas exists in HVCs

•Generally good correlation between H I and O VI

Color scale:

HV HI 21cm

Circles:

HV O VI 1031Å

HVCs for this Study

Fox et al. 2005

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)

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

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 H brightness of HVCs (6% of ionizing photons escape in z-direction)

Isoflux contours near the Galaxy

Plot on right shows integrated ionizing flux as function of distance. Contours are log (cm-2 s-1)

•Blue <912Å (hard)

•Red >912Å (soft)

Galaxy dominates EGB inside log =4.0.

Dot-dashed lines show directions where we have UV spectra.

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) (F=4J)

Ionization Patterns

•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)

•Use CLOUDY photoionization code (Ferland et al. 1998)•In: - incident radiation field

- observed ionic column densities

•Out:- Best fit values of U (n/nH), [Z/H], ionization balance, temperature

- Calculate pressure, cloud size, H intensity

Match with Data

2 minimization used to find best-fit U and [Z/H]

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)

C IV

/ O

VI

N V/ O VI

•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

Testing the Diskhalo model

•H observations can constrain the ionizing flux, and hence the photon escape fraction

•I(H) (assumes ionization followed by recombination and cascade)

•Greg Madsen has program to search for Hfrom HVC Complex A using WHAM

see poster by Madsen et al.

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 H observations, model predicts that Galactic photons do escape the clumpy ISM, and will contribute to the extragalactic background.