Federico Stasyszyn

An MHD gadget for cosmological simulations

Various radio observations have shown that the hot atmospheres of galaxy clusters are magnetized. However, our understanding of the origin of these magnetic fields, their implications on structure formation and their interplay with the dynamics of the cluster atmosphere, especially in the centers of galaxy clusters, is still very limit ed. In preparation for the upcoming new generation of radio telescopes (like EVLA, LWA, LOFAR and SKA), a huge effort is being made to learn more about cosmological magnetic fields f rom the observational perspective. Here we present the implementation of magneto-hydrodynamics in the cosmological SPH code GADGET (Springel et al. 2001; Springel 2005). We discuss the details of the implementation and various schemes to suppress numerical instabilities as well as regularization schemes, in the context of cosmological simulations. The performance of the SPH-MHD code is demonstrated in various one and two dimensional test problems, which we performed with a fully, three dimensional setup to test the code under reali stic circumstances. Comparing solutions obtained using ATHENA (Stone et al. 2008), we find exc ellent agreement with our SPH-MHD implementation. Finally we apply our SPH-MHD implementation to galaxy cluster formation within a large, cosmological box. Performing a resolution study we demonstrate the robustness of the predicted shape of the magnetic field pr ofiles in galaxy clusters, which is in good agreement with previous studies.

Angular momentum-large-scale structure alignments in ΛCDM models and the SDSS

We study the alignments between the angular momentum of individual objects and the large-scale structure in cosmological numerical simulations and real data from the Sloan Digital Sky Survey, Data Release 6 (SDSS-DR6). To this end, we measure anisotropies in the two point cross-correlation function around simulated haloes and observed galaxies, studying separately the one- and two-halo regimes. The alignment of the angular momentum of dark-matter haloes in A cold dark matter (ACDM) simulations is found to be dependent on scale and halo mass. At large distances (two-halo regime), the spins of high-mass haloes are preferentially oriented in the direction perpendicular to the distribution of matter; lower mass systems show a weaker trend that may even reverse to show an angular momentum in the plane of the matter distribution. In the one-halo term regime, the angular momentum is aligned in the direction perpendicular to the matter distribution; the effect is stronger than for the one-halo term and increases for higher mass systems. On the observational side, we focus our study on galaxies in the SDSS-DR6 with elongated apparent shapes, and study alignments with respect to the major semi-axis. We study five samples of edge-on galaxies; the full SDSS-DR6 edge-on sample, bright galaxies, faint galaxies, red galaxies and blue galaxies (the latter two consisting mainly of ellipticals and spirals, respectively). Using the two-halo term of the projected correlation function, we find an excess of structure in the direction of the major semi-axis for all samples; the red sample shows the highest alignment (2.7 ± 0.8 per cent) and indicates that the angular momentum of flattened spheroidals tends to be perpendicular to the large-scale structure. These results are in qualitative agreement with the numerical simulation results indicating that the angular momentum of galaxies could be built up as in the Tidal Torque scenario. The one-halo term only shows a significant alignment for blue spirals (1.0 ± 0.4 per cent), consistent with the one-halo results from the simulation but with a lower amplitude. This could indicate that even though the structure traced by galaxies is adequate to study large-scale structure alignments, this would not be the case for the inner structure of low-mass haloes, M ≤ 10 13 h -1 M ⊙ , an effect apparently more important around red g - r > 0.7 galaxies.

Magnetic field structure due to the global velocity field in spiral galaxies

We present a set of global, self-consistent N-body/smoothed particle hydrodynamic (SPH) simulations of the dynamic evolution of galactic discs with gas, including magnetic fields. We have implemented a description to follow the evolution of magnetic fields with the ideal induction equation in the SPH part of the vine code. Results from a direct implementation of the field equations are compared to a representation by Euler potentials, which pose a ∇·B-free description, a constraint not fulfilled for the direct implementation. All simulations are compared to an implementation of magnetic fields in the gadget code which also includes cleaning methods for ∇·B. Starting with a homogeneous seed field, we find that by differential rotation and spiral structure formation of the disc the field is amplified by one order of magnitude within five rotation periods of the disc. The amplification is stronger for higher numerical resolution. Moreover, we find a tight connection of the magnetic field structure to the density pattern of the galaxy in our simulations, with the magnetic field lines being aligned with the developing spiral pattern of the gas. Our simulations clearly show the importance of non-axisymmetry for the evolution of the magnetic field.

Protostellar collapse and fragmentation using an MHD gadget

Although the influence of magnetic fields is regarded as vital in the star formation process, only a few magnetohydrodynamics (MHD) simulations have been performed on this subject within the smoothed particle hydrodynamics method. This is largely due to the unsatisfactory treatment of non-vanishing divergence of the magnetic field. Recently smoothed particle magnetohydrodynamics (SPMHD) simulations based on Euler potentials have proven to be successful in treating MHD collapse and fragmentation problems, however these methods are known to have some intrinsic difficulties. We have performed SPMHD simulations based on a traditional approach evolving the magnetic field itself using the induction equation. To account for the numerical divergence, we have chosen an approach that subtracts the effects of numerical divergence from the force equation, and additionally we employ artificial magnetic dissipation as a regularization scheme. We apply this realization of SPMHD to a widely known setup, a variation of the ‘Boss and Bodenheimer standard isothermal test case’, to study the impact of the magnetic fields on collapse and fragmentation. In our simulations, we concentrate on setups, where the initial magnetic field is parallel to the rotation axis. We examine different field strengths and compare our results to other findings reported in the literature. We are able to confirm specific results found elsewhere, namely the delayed onset of star formation for strong fields, accompanied by the tendency to form only single stars. We also find that the ‘magnetic cushioning effect’, where the magnetic field is wound up to form a ‘cushion’ between the binary, aids binary fragmentation in a case where previously only formation of a single protostar was expected.

Galactic ménage à trois: simulating magnetic fields in colliding galaxies

We present high resolution simulations of a multiple merger of three disk galaxies including the evolution of magnetic fields performed with the N-body/SPH code Gadget. For the first time, we embed the galaxies in a magnetized, low-density medium, thus modeling an ambient IGM. The simulations include radiative cooling and a model for star formation and supernova feedback. Magnetohydrodynamics is followed using the SPH method. The progenitor disks have initial magnetic seed fields in the range of 10 9 to 10 6 G and the IGM has initial fields of 10 12 to 10 9 G. The simulations are compared to a run excluding magnetic fields. We show that the propagation of interaction-driven shocks depends significantly on the initial magnetic field strength. The shocks propagate faster in simulations with stronger initial field, suggesting that the shocks are supported by magnetic pressure. The Mach numbers of the shocks range from approximately M = 1:5 for the non-magnetized case up to M = 6 for the highest initial magnetization, resulting in higher temperatures of the shock heated IGM gas. The magnetic field in the system saturates rapidly after the mergers at � 10 6 G within the galaxies and � 10 8 G in the IGM independent of the initial value. These field strengths agree with observed values and correspond to the equipartition value of the magnetic pressure with the turbulent pressure in the system. We also present synthetic radio and polarization maps for different phases of the evolution showing that shocks driven by the interaction produce a high amount of polarized emission. These idealized simulations indicate that magnetic fields play an important role for the hydrodynamics of the IGM during galactic interactions. We also show that even weak seed fields are efficiently amplified during multiple galactic mergers. This interaction driven amplification might have been a key process for the magnetization of the Universe.

The nature of assembly bias – III. Observational properties

We analyse galaxies in groups in the Sloan Digital Sky Survey (SDSS) and find a weak but significant assembly-type bias, where old central galaxies have a higher clustering amplitude (61

On the magnetic fields in voids

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Magnetic field amplification and X-ray emission in galaxy minor mergers

9 per cent) at scales > 1 Mpc than young central galaxies of equal host halo mass (

Measuring cosmic magnetic fields by rotation measure–galaxy cross-correlations in cosmological simulations

M_{h} \sim 10^{11.8} h^{-1}