© Lloyd

1. On the Origins of the Diffuse H α Emission: Ionized Gas or Dust-Scattered H α Halos? - http://crescent.astro.illinois.edu/IAUSpS12/TALKS/SeonSpS12.pdf
2. Origins of the diffuse H α emission - The diffuse H α emission is known to trace the Warm Ionized Medium (WIM) or Diffuse Ionized Gas (DIG) [].
3. Only the O stars meet and surpass the power requirements to ionize the diffuse ISM.\
4. Density bounded (leaky) H II regions - Turbulent or clumpy morphology of the ISM - Existence of enormous, H I free bubbles/holes surrounding the O stars
5. However - How can the ionizing photons (Lyman continuum; E > 13.6 eV) from O stars travel hundred[s] of pc through the disk and into the halo?
6. Photo absorption optical depth ~ 1 @ N (H I) = 10 17 cm 2 N (H I) > 10 20 cm 2 toward most of line of sights
7. The WMAP data shows that the observed ratio of free [] radio continuum to H α is at least twice smaller than the expected value.
8. Summary - Dust scattering cannot be ruled out
9. The observed optical line ratios can be well reproduced by the dust scattered halo scenario.
10. ① The optical lines originate from H II regions ionized by late O or early B type stars in the media with abundances close to WNM and scattered off by interstellar dust.
11. ② Some part of the enhancement or [f]luctuation in the line ratios may be attributed to the variation of clumpiness from sightline to sightline.
12. ③ [N II] / H α in the diffuse ISM regions is highly likely to be consistent with that in bright H II regions, if the underlying Balmer absorption lines are properly taken into account.
13. The global H α morphology accords well with the dust scattered halos.