Sander Valcke

EFFICIENT THREE-DIMENSIONAL NLTE DUST RADIATIVE TRANSFER WITH SKIRT

We present an updated version of SKIRT, a three-dimensional (3D) Monte Carlo radiative transfer code developed to simulate dusty galaxies. The main novel characteristics of the SKIRT code are the use of a stellar foam to generate random positions, an efficient combination of eternal forced scattering and continuous absorption, and a new library approach that links the radiative transfer code to the DustEM dust emission library. This approach enables a fast, accurate, and self-consistent calculation of the dust emission of arbitrary mixtures of transiently heated dust grains and polycyclic aromatic hydrocarbons, even for full 3D models containing millions of dust cells. We have demonstrated the accuracy of the SKIRT code through a set of simulations based on the edge-on spiral galaxy UGC 4754. The models we ran were gradually refined from a smooth, two-dimensional, local thermal equilibrium (LTE) model to a fully 3D model that includes non-LTE (NLTE) dust emission and a clumpy structure of the dusty interstellar medium. We find that clumpy models absorb UV and optical radiation less efficiently than smooth models with the same amount of dust, and that the dust in clumpy models is on average both cooler and less luminous. Our simulations demonstrate that, given the appropriate use of optimization techniques, it is possible to efficiently and accurately run Monte Carlo radiative transfer simulations of arbitrary 3D structures of several million dust cells, including a full calculation of the NLTE emission by arbitrary dust mixtures.

Simulations of the formation and evolution of isolated dwarf galaxies – II. Angular momentum as a second parameter

We show results based on a large suite of N-body/smoothed particle hydrodynamics simulations of isolated, flat dwarf galaxies, both rotating and non-rotating. The main goal is to investigate possible mechanisms to explain the observed dichotomy in radial stellar metallicity profiles of dwarf galaxies: dwarf irregulars (dIrrs) and flat, rotating dwarf ellipticals (dEs) generally possess flat metallicity profiles, while rounder and non-rotating dEs show strong negative metallicity gradients. These simulations show that flattening by rotation is key to reproducing the observed characteristics of flat dwarf galaxies, proving particularly efficient in erasing metallicity gradients. We propose a ‘centrifugal barrier mechanism’ as an alternative to the previously suggested ‘fountain mechanism’ for explaining the flat metallicity profiles of dIrrs and flat, rotating dEs. While only flattening the dark matter halo has little influence, the addition of angular momentum slows down the infall of gas, so that star formation (SF) and the ensuing feedback are less centrally concentrated, occurring galaxy-wide. Additionally, this leads to more continuous star formation histories by preventing large-scale oscillations in the star formation rate (‘breathing’), and creates low-density holes in the interstellar medium, in agreement with observations of dIrrs. Our general conclusion is that rotation has a significant influence on the evolution and appearance of dwarf galaxies, and we suggest angular momentum as a second parameter (after galaxy mass as the dominant parameter) in dwarf galaxy evolution. Angular momentum differentiates between SF modes, making our fast rotating models qualitatively resemble dIrrs, which does not seem possible without rotation.

Kelvin-Helmholtz instabilities in Smoothed Particle Hydrodynamics

In this paper we investigate whether smoothed particle hydrodynamics (SPH), equipped with artificial conductivity (AC), is able to capture the physics of density/energy discontinuities in the case of the so-called shearing layers test, a test for examining Kelvin–Helmholtz (KH) instabilities. We can trace back each failure of SPH to show KH rolls to two causes: (i) shock waves travelling in the simulation box and (ii) particle clumping, or more generally, particle noise. The probable cause of shock waves is the local mixing instability, previously identified in the literature. Particle noise on the other hand is a problem because it introduces a large error in the SPH momentum equation. The shocks are hard to avoid in SPH simulations with initial density gradients because the most straightforward way of removing them, i.e. relaxing the initial conditions, is not viable. Indeed, by the time sufficient relaxing has taken place the density and energy gradients have become prohibitively wide. The particle disorder introduced by the relaxation is also a problem. We show that setting up initial conditions with a suitably smoothed density gradient dramatically improves results: shock waves are reduced whilst retaining relatively sharp gradients and avoiding unnecessary particle disorder. Particle clumping is easy to overcome, the most straightforward method being the use of a suitable smoothing kernel with non-zero first central derivative. We present results to that effect using a new smoothing kernel: the linear quartic kernel. We also investigate the role of AC. Although AC is necessary in the simulations to avoid ‘oily’ features in the gas due to artificial surface tension, we fail to find any relation between using AC and the appearance of seeded KH rolls. Including AC is necessary for the long-term behaviour of the simulation (e.g. to get λ= 1/2, 1 KH rolls). In sensitive hydrodynamical simulations great care is however needed in selecting the AC signal velocity, with the default formulation leading to too much energy diffusion. We present new signal velocities that lead to less diffusion. The effects of the shock waves and of particle disorder become less important as the time-scale of the physical problem (for the shearing layers problem: lower density contrast and higher Mach numbers) decreases. At the resolution of current galaxy formation simulations mixing is probably not important. However, mixing could become crucial for next-generation simulations.

Hubble Space Telescope survey of the Perseus cluster – II. Photometric scaling relations in different environments

We investigate the global photometric scaling relations traced by early-type galaxies in different environments, ranging from dwarf spheroidals, over dwarf elliptical galaxies (dEs), up to giant ellipticals (-8mag greater than or similar to M-V greater than or similar to -24 mag). These results are based, in part, on our new Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) F555W and F814W imagery of dwarf spheroidal galaxies in the Perseus cluster. The full sample, built from our HST images and from data taken from the literature, comprises galaxies residing in the Local Group; the Perseus, Antlia, Virgo and Fornax clusters; and the NGC 5898 and NGC 5504 groups. Photometric parameters, such as the half-light radius, the central surface brightness and the Sersic exponent n, are used to parametrize the light distributions and sizes of early-type galaxies. All these parameters vary in a continuous fashion with galaxy luminosity over a range of more than six orders of magnitude in luminosity. We also find that all early-type galaxies follow a single colour-magnitude relation (CMR), which we interpret as a luminosity metallicity relation for old stellar populations. These scaling relations are almost independent of environment, with Local Group and cluster galaxies coinciding in the various diagrams. As an example, due to the presence of a population of very low surface brightness dwarf spheroidal galaxies (dSphs) in the Fornax cluster, which may be tidally heated dwarf galaxies, the Fornax dwarf spheroidal galaxy (dSph) population is on average only 0.2 mag arcsec(-2) fainter than the Local Group dSph populations. This offset is much too small to destroy the global relation between luminosity and central surface brightness. We show that at M-V similar to -14 mag, the slopes of the photometric scaling relations involving the Sersic parameters change significantly. This contradicts previous claims that the relations involving Sersic parameters are pure power laws for all early-type galaxies and are, therefore, more fundamental than other photometric scaling relations derived from them. We argue that these changes in slope reflect the different physical processes that dominate the evolution of early-type galaxies in different mass regimes. As such, these scaling relations contain a wealth of information that can be used to test models for the formation of early-type galaxies.

Tidal Stripping of Dwarf Spheroidal Galaxies

We have constructed a series of models to investigate the influence of tidal forces on the formation and evolution of dwarf spheroidal galaxies. These Nbody/SPH models consist of a model for an isolated dwarf spheroidal galaxy which is placed on an orbit in a Milky‐Way sized—analytical—dark matter halo. We have varied orbital parameters, time of entrance into the potential and the dwarf progenitor total mass, totalling sixty models.The results indicate that dynamically hot dwarf spheroidal galaxies are robust against tides, unless they are on very close and eccentric orbits. In general there is an increase in star formation, caused by additional gas being funneled in to the galaxy due to tidal compression. The morphology of the galaxies (again, unless on very close and eccentric orbits) is however virtually unaltered.

Truncated star formation in dwarf spheroidal galaxies and photometric scaling relations

We investigate the global photometric scaling relations traced by early-type galaxies in different environments, ranging from dwarf spheroidals, over dwarf elliptical galaxies, up to giant, ellipticals (-8 mag greater than or similar to M-V greater than or similar to -24 mag). These results are based in part on our new HST/ACS F555W and F814W imagery of dwarf spheroidal galaxies in the Perseus Cluster. We show that at MV similar to -14 mag, the slopes of the photometric scaling relations involving the Sersic parameters change significantly. We argue that, these changes in slope reflect the different physical processes that dominate the evolution of early-type galaxies in different mass regimes. We present N-body/SPH simulations of the formation and evolution of dwarf spheroidals that reproduce these slope changes, and discuss the underlying physics. As such, these scaling relations contain a wealth of information that can be used to test models for the formation of early-type galaxies.