Monte Carlo Photoionization Simulations of

Diffuse Ionized Gas

Kenneth Wood

University of St Andrews

In collaboration withJohn Mathis, Barbara Ercolano, Ron Reynolds,

Torsten Elwert, Matt Haffner, Greg Madsen

Milky Way’s DIG: Recap

• Wisconsin H Mapper: DIG everywhere

• n(z) ~ 0.2 exp(-|z|/H) cm-3, ff ~ 0.2

• H ~ 1 kpc, if isothermal T ~ 8000 K

Reynolds, Tufte, Haffner, Madsen,…

Line Ratios: Physical Conditions

• [N II]/H increases with height => T increases

• Problem for spherically averaged models

• Extra heating (Reynolds et al. 1999)Haffner et al. (1999)

H [N II]/H

[S II]/H [S II]/[N II]

Scatter Plots

• [N II]/H large where Hfaint

• Note tightness of correlation

Haffner et al. (1999)

[N II]/H H(R)

[S I

I]/H

[N I

I]/H

Need 3D Photoionization Codes

Monte Carlo PhotoionizationWood, Mathis, & Ercolano (2004)

• 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

• See also: Och, Lucy, Rosa (1998)Ercolano et al. (2003)

Lexington H II Benchmarks• T*=40000K, Q(H)=4.26E49 s-1, n(H)=100 cm-3

+ Monte Carlo( CLOUDY

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)

z (k

pc)

2D Models: Line Ratios

• [N II]/H, [S II]/H increase with height

• Highest energy photons penetrate to high z

• Harder radiation field at large distances from source

Wood & Mathis (2004)

z (k

pc)

Scatter Plots

• 1D models predict tight correlation: each sightline samples same temperature and ionization structure

Elwert & Dettmar (2004)

[N I

I]/H

H(R)

• 2D models show scatter: sightlines probe different temperatures and ionization

• Slope change in [S II]/H – [N II]/H: interfaces, not seen in Milky Way’s DIG

Wood & Mathis (2004)

Scatter Plots: 3D Structure?

• Multiple sources with different spectra

• 3D Density structure

• Strategy: Planar emission at z = 0Repeating boundaries in x, ySmooth and two-phase densitiesVary Q, n(z) to fit H(z)

• What is [N II]/H, extra heating?

Two Phase Density

• Dense grid cells with filling factor 0.01 < ff < 1

• Minimum “clump” size set by grid resolution

Smooth Model

• n(z) = 0.1 exp(-|z|/1.3)

• Large Q to ionize grid: high ionization parameter

• N mostly N++ at low z: [N II]/H too low at large H

Clumpy Models

• Decrease ff => lower U => less N++ => higher [N II]/H

ff =80% ff =40%

ff =10% ff =5%

Summary

• Photoionization heating explains most line ratios

• Extra heating ~ 10-25 ne erg cm-3 s-1 for largest line ratios, [N II]/H

• Smooth models: too low [N II]/Hat large H• Clumpy models with ff ~ 0.2 look good• Caution: 3D Toy Model!

Future Work

• More 3D models: lots of parameter space

• Apply this to WHAM B star H II regions

• Constrain models with additional WHAM data: [S II], [O I], [O II], [O III], He I

• Need S dielectronic recombination rates

• Merge 3D photoionization with MHD…

Future Work

• Take 3D density from MHD simulation

• 3D ionization simulation

Density from De Avillez & Berry (2001)

log n