Simulations of the formation and evolution of isolated dwarf galaxies

We present new fully self-consistent models of the formation and evolution of isolated dwarf galaxies. We have used the publicly available N-body/SPH code HYDRA, to which we have added a set of star formation criteria, and prescriptions for chemical enrichment (taking into account contributions from both SNIa and SNII), supernova feedback, and gas cooling. The models follow the evolution of an initially homogeneous gas cloud collapsing in a pre-existing dark-matter halo. These simplified initial conditions are supported by the merger trees of isolated dwarf galaxies extracted from the milli-Millennium Simulation. The star-formation histories of the model galaxies exhibit burst-like behaviour. These bursts are a consequence of the blow-out and subsequent in-fall of gas. The amount of gas that leaves the galaxy for good is found to be small, in absolute numbers, ranging between 3x10^7 Msol and 6x10^7 Msol . For the least massive models, however, this is over 80 per cent of their initial gas mass. The local fluctuations in gas density are strong enough to trigger star-bursts in the massive models, or to inhibit anything more than small residual star formation for the less massive models. Between these star-bursts there can be time intervals of several Gyrs. We have compared model predictions with available data for the relations between luminosity and surface brightness profile, half-light radius, central velocity dispersion, broad band colour (B-V) and metallicity, as well as the location relative to the fundamental plane. The properties of the model dwarf galaxies agree quite well with those of observed dwarf galaxies.Comment: 16 pages, 20 figures, accepted for publication in MNRA

Efficient 3D NLTE dust radiative transfer with SKIRT

We present an updated version of SKIRT, a 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, 2D, LTE model to a fully 3D model that includes NLTE dust emission and a clumpy structure of the dusty ISM. 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.Comment: 15 pages, 7 figures, accepted for publication in ApJ

Simulations of the formation and evolution of dwarf galaxies

Abstract We present models of the formation and evolution of isolated dwarf galaxies. The models follow the evolution of an initially homogeneous gas cloud collapsing in a pre-existing dark-matter halo. These simplified initial conditions are supported by the merger trees of isolated dwarf galaxies extracted from the milli-Millennium Simulation. An extensive comparison of the models to observations was made. The models' surface brightness profiles are well fitted by Sérsic profiles and the correlations between the models' Sérsic parameters and luminosity agree with the observations. We have also compared model predictions for the half-light radius R_{e} , central velocity dispersion σ\rs{c}, broad band colour B-V, metallicity [Z/Z\sun] versus luminosity relations and for the location relative to the fundamental plane with the available data. In all cases the models give the correct slope, in most cases we also get the zero-point right

Flat metallicity profiles in rotating dwarf galaxies

dIrrs and flat, rotating dEs generally possess flat metallicity profiles while round dEs show strong metallicity gradients [8]. Unlike dEs, dIrrs also exhibit ongoing star formation (SF) [4], compatible with a continuous star formation history (SFH). We show results based on a large suite of Nbody-SPH simulations of isolated flat dwarf galaxies (DG), both rotating and non-rotating. These simulations show that using rotation to flatten a dwarf galaxy is particularly efficient in turning a so-called "breathing" SFH [15] into a more continuous SFH, and in producing flat metallicity profiles. Non-rotating dEs in a flattened dark-matter halo are not able to reproduce this. Thus it appears that angular momentum is a key factor in DG behaviour. Rotation causes a `centrifugal barrier' which slows down the infall of gas, so the low-level SF (and feedback) is not centrally concentrated but occurs galaxy-wide, and prevents large-scale oscillations in the SFR. This mechanism of smearing out SF in time and space proves to be the principal reason for flat metallicity profiles, instead of the often referred to `fountain mechanism' [1, 3, 5, 10]. We therefore propose a `centrifugal barrier mechanism' which is able to explain the observations

Flat metallicity profiles in rotating dwarf galaxies

Dwarf irregulars (dlrr) and flat, rotating dwarf ellipticals (dE) generally possess flat metallicity profiles while round dEs show strong metallicity gradients (Koleva et al. 2009). Unlike dEs, dlrrs also exhibit ongoing star formation (SF) (Dolphin et al. 2005), in most cases compatible with a continuous star formation history (SFH). We show results based on a large suite of Nbody-SPH simulations of flat dwarf galaxies, both rotating and non-rotating, performed with a modified version of GADGET2. They have a range of masses, flattenings and rotations speeds and are based on the spherical models of (Valcke et al. 2008). Specifically, we want to see if it is possible to reproduce these characteristics in isolated DG models. These simulations show that using rotation to flatten a dwarf galaxy is particularly efficient in turning a so-called "breathing" SFH (Valcke et al. 2008) into a more continuous SFH, and in producing flat metallicity profiles. Non-rotating dEs in a flattened dark-matter halo are not able to reproduce these characteristics. Thus, it appears that rotation is key to reproducing the observed characteristics. Rotation causes a "centrifugal barrier" which slows down the infall of gas, so that the low-level star formation is not centrally concentrated but occurs galaxy-wide, and in this way also prevents large-scale oscillations in the SFR. This mechanism of smearing out the star formation in time and space proves to be the principal reason for the flat metallicity profiles, instead of the often referred to "fountain mechanism" (De Young & Heckman 1994; Barazza & Binggeli 2002; Mac Low & Ferrara 1999; Ferrara & Tolstoy 2000). We therefore propose a "centrifugal barrier mechanism" which is able to explain the observations

Kelvin-Helmholtz instabilities in smoothed particle hydrodynamics

We have investigated whether Smoothed Particle Hydrodynamics (SPH), equipped with artificial conductivity, 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 (LMI), 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. We show that setting up initial conditions with a suitably smoothed density gradient dramatically improves results. 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 (LIQ) kernel. Furthermore, we present new Artificial Conductivity 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

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