Efficient Monte Carlo continuum radiative transfer with SKIRT

1. Efficient Monte Carlo continuum radiative transfer with SKIRT Maarten Baes 2nd East-Asia Numerical Astrophysics Meeting, Daejeon, Korea3 November 2006

4. Brussels

5. Why continuum radiative transfer… • the ISM is extremely dusty • dust strongly affects the radiation field at all wavelengths - X-ray: scattering - UV and optical: extinction - IR and submm: emission Detailed continuum radiative transfer simulations are necessary to investigate the effect of dust on observable properties of all dusy systems…

6. Radiative transfer equation • we take into account the effects of - extinction • condition of thermal equilibrium:

7. Radiative transfer equation • we take into account the effects of - extinction - multiple anisotropic scattering • condition of thermal equilibrium:

8. Radiative transfer equation • we take into account the effects of - extinction - multiple anisotropic scattering - thermal dust re-emission, assuming thermal equilibrium • condition of thermal equilibrium:

9. Radiative transfer equation • we take into account the effects of - extinction - multiple anisotropic scattering - thermal dust re-emission, assuming thermal equilibrium - multiple dust grain populations • condition of thermal equilibrium:

10. the radiation field is reconstructed by classifying the photons by position, propagation direction, wavelength… Monte Carlo radiative transfer • probabilisitic technique >< deterministic technique • RT simulations in which a large number of photons are followed individually through the dusty medium • the trajectory of each photon is determined by (pseudo) random numbers

11. Monte Carlo radiative transfer • ADVANTAGES • conceptually simple, natural treatment of emission, absorption and scattering • all geometries possible (3D simulations) • rather economic in memory → large grids are possible • very flexible: multiple anisotropic scattering, polarization, kinematics, dust clumping… • DISADVANTAGES • Poisson noise • error analysis is difficult • accuracy goes as N-1/2→ efficiency !?

12. Steinacker et al. 2003 SKIRT • Stellar Kinematics Including Radiative Transfer • allows all geometries for sources and sinks: dust cells • several dust cell geometries: spherical, cylindrical, cuboidal,…

13. SKIRT • strongly optimized through the use of deterministic elements • forced (first) scattering Witt 1977 • peeling-off technique Yusef-Zadeh et al. 1984 • continuous absorption Lucy 1999 • partlypolychromatic photon packages Baes 2006, MNRAS, submitted • computing power: dedicated cluster with 16 x 2 Gb memory • two major modes: • LTE → modelling the dust temperature distribution and the SED of dusty systems • KINE → modelling the observed kinematics of dusty galaxies

14. SKIRT in LTE mode • LTE radiative transfer: • radiative equilibrium: energy absorbed = energy emitted • the absorbed energy determines the dust temperature • frequency distribution adjustment techniqueBjorkman & Wood 2001 Baes et al. 2005, NewA, 10, 523 • no iteration is necessary • immediate re-emission: guaranteed flux conservation • works with all optical depths • polychromatic photon packages: very efficient

15. 1D benchmark tests • Ivezić et al. (1997) benchmark tests • star + spherical envelope • V-band optical depths 1-1000

16. Polychromatic photon packages (re-)emissioneach photon package initially contains photons of all wavelengths exitif it leaves the galaxy: contribution to the SED at all wavelengths scatteringloss of polychromatism minimal computational overheadsignificant gain in efficiency Baes 2006, MNRAS, submitted

17. 2D benchmark tests • Pascucci et al. (2004) benchmark tests • star + axisymmetric envelope • V-band optical depths 0.1-100

18. SKIRT 2D benchmark SKIRT 3D vs benchmark

19. Efficiency of Monte Carlo RT • “common wisdom” about Monte Carlo RT: numerically demanding • comparison between SKIRT and other codes used in Pascucci et al. SKIRT 2D: 2.5 MBySKIRT 3D: 58 MBy MC RT codes can be very efficient when modern optimization techniques are used. Limited memory usage is extra advantage when moving to 3D Baes 2006, MNRAS, submitted

20. Application 1: Circumstellar discs • large homogeneous survey of post-AGB stars - they all seem to be binary systems - they have a MIR excess due to dust starting at the sublimation temperature - MIR-submm SED and VLTI data suggest circumbinary discs

21. Application 1: Circumstellar discs • question: how do the temperature distribution and the emerging radiation field depend on the structure of the circumstellar medium ? density temperature We can see some systematic effects, but in general the structure of the dust temperature distribution is rather insensitive to the structure of the ISM.

22. Application 2: spiral galaxy atlas • simulation of a large set of spiral galaxy models • - images at various inclinations and passbands - global and spatially resolved spectral energy distributions- attenuation maps- dust temperature distributions • scientific goals • - investigate the systematic effects of physical parameters on the observables (luminosity, dust content, bulge-to-disc ratio, inclination…) • - construct an optimized galaxy dust mass estimator for IRAS, Spitzer, Akari,… data • - provide a database for statistical / cosmological applications

23. Application 2: spiral galaxy atlas Optical depth Bulge luminosity Bulge luminosity Optical depth

24. R-band images Spitzer MIPS 160 μm images

25. Conclusions • SKIRT = efficient 3D Monte Carlo radiative transfer code • 2 modes: LTE and KINE • uses efficient optimization techniques • reproduces the 1D and 2D benchmark test easily • ready to go…. • - models for circumbinary discs around post-AGB stars - atlas of dusty spiral galaxy models - simulations of accretion discs in the centre of AGNs - kinematics of dusty galaxies and galactic nuclei - your radiative transfer problem ???

26. Thank you… EANAM 2008China EANAM 2004Japan EANAM 2012Belgium! EANAM 2006Korea EANAM 2010Iran ? See you there !