Monte Carlo Photoionization
Simulations of Diffuse Ionized Gas
Wood, K & Mathis, S
astro-ph/0402409
Diffuse Ionized Gas (DIG)
Ne=0.025 cm-3
Te=103-104 K
DIG in the Milky Way
Formation (current model)
SNe in the galactic
disk create chimney
structures through
which ionized gas
can reach the halo
OB star
Unanswered problems
How is the DIG ionized and how is its
ionization maintained far away from any
ionizing sources?
Observations
• [S II]/Ha and [N II]/Ha
increase with height above
the plane.
• [S II]/[N II] is uniform with
height.
• He is underionized with
respect to H.
Tüllmann & Dettmar 2000
Difficulty in 1-D model
Additional heating is needed.
For a smooth density distribution, the
mean free path of Lya is very small. It is
difficult to ionize the extensive gas.
Argument of this paper
2-DMonte Carlo code (Wood, Mathis &
Ercolano, 2004)
The ionizationspectrum is from leaking from H II
regions instead of O star. So it is harder and He-
ionizing photons are suppressed.
Density can vary.
Density profile
Ingredients of the ionization
model
Monte Carlo model
Elemental abundance (uniform)
ISM density structure
Ionizing source (single)
Results
H-ionization edge He-ionization edge
|z|=0
|z|=2 kpc
Varying ionizing luminosity
low
high
Varying ionizing spectra
hard
soft
Input spectra
leakage factor:
a)100%
b)60%
c)30%
d)15%
HeI/HI vs z in different leak
100%
He/Ha
15%
Conclusion
Multi-density yields harder ionization spectrum
reaching larger distance. This accounts for the
increasing [N II]/Ha and [S II]/Ha ratios with
distance to the disk.
Radiation leaking out of HII region explains the
underionization of He with respect to H.
A 3-D model is in process.
End
Thank you!