1. Radiative transfer in galactic disks…
Simone Bianchi
INAF-Osservatorio Astrofisico
di Arcetri, Firenze
… using TRADING Transfer of RAdiation through
Dust IN Galaxies (Bianchi 2008)

2. RT and the Monte Carlo Method
The path of a phothon through dust
Optimizations
Extinction:
scattering or
absorption ?
Out of dust:
observed
Emission
from stars
Photon weights
Scattering
phase function
Forced Scattering
(Cashwell & Everett 1959)
Several images
Peeling-off
(Yusef-Zadeh et al. 1984)
Continuous
Absorption
(Lucy 1999)

3. RT and the Monte Carlo Method
Emission from dust
Local ISRF
J = W /(4 )
Absorbed energy
Mean absorption
coefficient j
Dust Emission Coefficient
Thermal & Stochastic heating
Dust self absorption
(iteratively)
l.o.s. integration
Scheme similar to DIRTY (Misselt et al. 2001)

4. Radiative Transfer models for a spiral (disk) galaxy
Smooth exponential stellar disk
+ spheroidal bulge
i = 40˚
i = 90˚
edge-on
i = 85˚
i = 60˚
i = 80˚
i = 0˚
face-on
+ smooth exponential dust disk
MC model for dust extinction Bianchi, Ferrara & Giovanardi (1996)
Dust emission at thermal equilibrium Bianchi, Davies & Alton (2000)
Extinction & Clumping Bianchi et al. (2000)
On a regular grid

5. Real galaxies vs models
+ 6 other objects
Bianchi (2007)
NGC 5746
V band
model
K’ band
model

6. TRADING: an adaptive grid for a clumpy medium
3-D Tree structure
Octree
Finer grid on a denser
(more opaque) spot
1500 clouds
Clumps of different size
Resolution within the clump

7. Effective extinction in a clumpy ball
Asmooth = 2000
ff=0.05
ff=0.025
400 and 1600 clumps, density contrast ^4, density in clumps ^-6 g/cm^3
A ≈ 1

8. Images Example: model of an NGC891-like galaxy seen at i=86 (Bianchi 2008)
B, V, R
About 1500 clumps, I would have needed 2048^3/20=4.3D8 regular cells instead than 3.6D6
B, V, R
70m, 160m, 850m

9. Spectral Energy Distribution
Draine & Li (2007)
dust

10. Summary of TRADING features
An adaptive grid for dust
Stellar emission from diffuse /point-like sources
of different spectra
Thermal & stochastic dust emission
from a distribution of grain sizes and material
Dust self-absorption.
Model of the
Cartwheel galaxy
(Mapelli et al. 2008)