Efficient Monte Carlo continuum radiative transfer with SKIRT Maarten Baes 2 nd East-Asia Numerical Astrophysics Meeting, Daejeon, Korea 3 November 2006

Brussels

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

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

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

Radiative transfer equation we take into account the effects of - extinction - multiple anisotropic scattering - thermal dus t re-emission, assuming thermal equilibrium condition of thermal equilibrium:

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

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 the radiation field is reconstructed by classifying the photons by position, propagation direction, wavelength…

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 !? Monte Carlo radiative transfer

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

SKIRT strongly optimized through the use of deterministic elements - forced (first) scattering Witt peeling-off technique Yusef-Zadeh et al continuous absorption Lucy partly polychromatic 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

SKIRT in LTE mode LTE radiative transfer: - radiative equilibrium: energy absorbed = energy emitted - the absorbed energy determines the dust temperature frequency distribution adjustment technique Bjorkman & 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

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

Polychromatic photon packages (re-)emission each photon package initially contains photons of all wavelengths exit if it leaves the galaxy: contribution to the SED at all wavelengths scattering loss of polychromatism minimal computational overhead significant gain in efficiency Baes 2006, MNRAS, submitted

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

SKIRT 2D benchmark SKIRT 3D vs benchmark

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 MBy SKIRT 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

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 tempera t ure - MIR-submm SED and VLTI data suggest circumbinary discs

question: how do the temperature distribution and the emerging radiation field depend on the structure of the circumstellar medium ? Application 1: Circumstellar discs 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. density temperature

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

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

R-band images Spitzer MIPS 160 μm images

Conclusions 2 modes: LTE and KINE SKIRT = efficient 3D Monte Carlo radiative transfer code 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 ???

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