Photoionization Studies of HII Regions and Diffuse Ionized Gas in

Photoionization Studies of the
Turbulent Interstellar Medium
Kenny Wood
University of St Andrews
in collaboration with
Alex Hill, Matt Haffner, Ron Reynolds, John Mathis, Bob Benjamin
Rich Rand, Ryan Joung, Mordecai MacLow,
Andrew Schechtman-Rook, Matt Bershady
Galactic Star Formation Factories
Warm Diffuse H II
Courtesy of FIMS (Berkeley)
WHAM
n0 ~ 0.03 cm-3; ff ~ 0.2; H ~ 1 kpc
How do photons get there?
Perseus Arm
Hα
[N II]/Hα
[S II]/Hα
[S II]/[N II]
6000 K < T < 12000 K
How is the gas heated?
Haffner et al. (2009)
3D Photoionization Models of
1.  Propagation of ionizing photons & 3D
structure of the interstellar medium
2.  Extra heating sources of diffuse ionized
gas in Milky Way and NGC891
3.  Structure of Galactic HII regions
4.  Ionization of high velocity clouds
Stromgren Volumes
Uniform medium:
Stromgren spheres
Stratified medium:
Stromgren volumes
•  Stratified density gives larger volume of ionized
gas at large |z|
•  DIG is superposition of overlapping Stromgren
volumes
Franco, Tenorio-Tagle, & Bodenheimer (1989); Miller & Cox (1993);
Dove & Shull (1994); Dove, Shull, & Ferrara (2000)
Stromgren Volumes
•  Miller & Cox (1993) reproduce EM and DM
–  Stratified ISM density + clouds
–  Locations/luminosities of O stars in Galaxy
•  Significant Hα from cloud faces
•  BUT their H0 density lower than “standard”
Dickey-Lockman disk
•  ISM must have low density paths to allow leakage
of LyC to large |z|
•  Need 3D radiation transfer
Monte Carlo Photoionization
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3D density structure and radiation transfer
Ions:
H, He, C, N, O, Ne, S
Stellar and diffuse photons in Cartesian grid
Input:
ionizing spectrum from source(s)
Output: 3D temperature & ionization structure
Emissivities, emission line maps, line ratios
Wood, Mathis, & Ercolano (2003)
Lexington H II Benchmarks
•  T*=40000K, Q(H)=4.26E49 s-1, n(H)=100 cm-3
+ Monte Carlo
CLOUDY
Wood, Mathis, & Ercolano (2003)
Ionizing a Smooth and Fractal ISM
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Initial H0: Dickey-Lockman disk + Reynolds layer
Fractal algorithm: Elmegreen (1997)
Source: Q(H0) = 1e49, 3e49, 5.e49, 1.e50
Contours: slices showing edge of H+ volume
Hα from smooth component and also cloud faces
M51: O stars can’t ionize the gas?
Seon (2009)
•  LyC propagation: FLyC (r) ~ e - τ(HI) / r 2
•  Need N(HI) ~ 10-5 observed values
•  Two effects:
–  ionized gas has f (H0) < 10-3
–  3D ISM provides low density paths
M51: Ionization of a 3D ISM
log (EM)
Wood & Seon (2011)
•  Fractal ISM allows LyC penetration
3D Models: NGC891 & M51
Schechtman-Rook, Wood, & Bershady (2011)
Need 3D dynamical simulations of ISM…
log n
Ionize this…!
De Avillez & Berry (2001)
Supernovae Driven Turbulent ISM
ρ
T
P vz ΝΗ
1 GSN
8 GSN
64 GSN
512 GSN
GSN = 258 SNe Myr-1 kpc-3
Joung et al. (2009)
Ionization Simulations
•  LyC can propagate to
large heights
•  Ionize gas with
Galactic LyC budget
Wood et al. (2010)
EM Distributions
•  Model distributions too
broad
•  Need to smooth out
density variations
•  Magnetic fields…?
Hill et al. (2009)
Wood et al. (2010)
MHD Simulations
•  Smooth out large density
contrasts, narrower EM?
•  Pressure support of gas at
large heights?
•  First results… 7 µG too large,
confines density, need very
large QH to ionize grid
Hill et al. (2011)
Summary so far
•  3D turbulent ISM allows for production of
DIG, but EM distributions too broad
•  B-fields may help narrow EM distribution
•  What about temperatures and line ratios…?
DIG Spectrum
•  Data: Hα, Hβ, He I, [S II], [N II], [O I], [O II], [O III],
radio recombination lines
•  Models: mostly 1D spherical volume averaged line ratios
(Mathis 1986; Domgoergen & Mathis 1994; Mathis 2000; Sembach et
al. 2000; Hoopes & Walterbos 2003)
I[NII] / IHα
•  More realistic models: rays piercing ionized volumes
(Bland-Hawthorn, Freeman, & Quinn 1998, Elwert 2003, Wood &
Mathis 2004)
I[N II](ξ, ψ) / IHα(ξ, ψ)
Extra Heating
•  [N II]/Hα rises with |z| in Milky Way and other
edge-on galaxies (Rand 1997; Haffner et al. 1999; Otte et al. 2002)
–  H II regions: [N II]/Hα ~ 0.2
–  DIG: [N II]/Hα ~ 0.5 - 1.7
•  [N II]/Hα probes temperature structure
•  1D spherically averaged models can produce large
[N II]/Hα if include additional heating
•  ν3 frequency dependence of H0 and He0 opacity
gives natural hardening of radiation field and
increasing temperatures away from source: (BlandHawthorn, Freeman, & Quinn 1998; Elwert 2003, Wood & Mathis 2004)
Density Bounded,
Leaky H II Region
Direct escape
H-recombs
He-recombs
•  Leaky H II regions may help [N II]/Hα, He+/H+ problems
•  Test with other diagnostic line ratios (Hoopes & Walterbos 2003)
z (kpc)
2D Ionization & Temperature
•  Point source, Q = 6 1049 s-1, n(z) ~ exp(-|z|/H)
•  Slices through grid in x-z plane
•  Temperature rises away from source
Wood & Mathis (2004)
•  [N II]/Hα, [S II]/Hα
increase with height
•  Can get [N II]/Hα ~ 0.7,
need extra heating for
largest observed values
•  Extra heating dominates
photoionization (n2) at
large |z| => G1ne (Reynolds,
Haffner, & Tufte 1999)
•  Required values: G1ne ~
10-25 ergs/s/cm3 (Reynolds,
Haffner, & Tufte 1999; Wood &
Mathis 2004)
z (kpc)
2D Models:
Line Ratios
Extra Heating Sources/Diagnosis
•  Ionization structure not changed, gas
temperature increased
•  Dust photoelectric heating (Reynolds & Cox 1992)
•  Turbulent dissipation (Minter & Spangler 1998)
•  SNe, shocks, cosmic rays, …
•  [O II] / Hα very sensitive to extra heating
(Mathis 2000, Elwert 2003, Otte et al. 2003)
NGC891 Line Ratios
•  Spitzer spectrum: IR Ne lines,
no extinction
•  Radiation hardening cannot
give rising [NeIII]/[NeII]
•  Extra source of high energy
photons: sdO stars?
Rand, Wood, & Benjamin (2009, 2010)
Hot ionizing sources
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“Cool” + “Hot”
sdO stars, Teff = 50kK
Scaleheight: z ~ 1kpc
Ionizing luminosity ~7%
Rand, Wood, & Benjamin (2009, 2010)
Summary of temperature structure
•  Additional heating above photoionization
•  Hot stars with large scaleheight, other
mechanical heating,…?
•  WIYN Grad-Queue observations of M51
Porosity of H II Regions
ζ Oph H II Region
Single star: 09.5V
Wood et al. (2005)
Porosity of H II Regions
Fractal models: vary
porosity to fit radial Hα
intensity and line ratios
Wood et al. (2005)
Porosity of H II Regions
Smooth models:
Hα too steep at edge
[N II] / Hα rapid increase at edge
Wood et al. (2005)
Clump volume filling factor
ff = 35%
Wood et al. (2005)
Clump volume filling factor
ff = 55%
Wood et al. (2005)
Clump volume filling factor
ff = 80%
Wood et al. (2005)
Clump volume filling factor
ff = 99%
Wood et al. (2005)
Porosity of ζ Oph H II Region
• Hα intensity and line ratios: porosity ~ 50%
• Lyman continuum leakage ~ 3% - 15%
• DIG requires 15% of LyC from OB stars
• DIG ionizing sources must reside in more
porous regions of the ISM
Wood et al. (2005)
Survival of neutral clouds
•  Cloud at z = 1.5 kpc, rc = 50pc
•  n = 0.01, 0.05, 0.1, 0.5 cm-3
•  Neutral for n > 0.1 cm-3
Wood et al. (2010)
Smith Cloud
Lockman et al. (2008)
Hill et al. (2009)
•  Cloud colliding with Galaxy, v ~ 75 km/s
•  M(H0) ~ M(H+) ~ 106 Msun
•  N / H ~ 0.2 solar
Ionization of Smith Cloud
•  n ~ 0.01 cm-3; ionized skin
Wood & Hill et al. (2011)
Summary
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3D MHD allows DIG
Extra heating to explain line ratios
Porosity of ISM
Ionization and metallicity of HVCs
Next…
•  Diagnostics of extra heating sources
•  3D MHD of HII regions
•  High resolution MCRT: interfaces
Importance of Interfaces
Planetary Nebula NGC6583: Hα, [O I]
•  [O I] emission from ionized/
neutral interface
•  Needs neutral gas and high T
•  Gas turning neutral and
temperature increasing in
narrow zone
•  PNe, ISM, AGN,…
•  Need adaptive mesh
radiation transfer code…