ALBERT

Description: We present an implementation of smoothed particle hydrodynamics (SPH) with improved accuracy for simulations of galaxies and the large-scale structure. In particular, we implement and test a vast majority of SPH improvement in the developer version of gadget -3. We use the Wendland kernel functions, a particle wake-up time-step limiting mechanism and a time-dependent scheme for artificial viscosity including high-order gradient computation and shear flow limiter. Additionally, we include a novel prescription for time-dependent artificial conduction, which corrects for gravitationally induced pressure gradients and improves the SPH performance in capturing the development of gas-dynamical instabilities. We extensively test our new implementation in a wide range of hydrodynamical standard tests including weak and strong shocks as well as shear flows, turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas clouds. We jointly employ all modifications; however, when necessary we study the performance of individual code modules. We approximate hydrodynamical states more accurately and with significantly less noise than standard gadget -SPH. Furthermore, the new implementation promotes the mixing of entropy between different fluid phases, also within cosmological simulations. Finally, we study the performance of the hydrodynamical solver in the context of radiative galaxy formation and non-radiative galaxy cluster formation. We find galactic discs to be colder and more extended and galaxy clusters showing entropy cores instead of steadily declining entropy profiles. In summary, we demonstrate that our improved SPH implementation overcomes most of the undesirable limitations of standard gadget -SPH, thus becoming the core of an efficient code for large cosmological simulations.

Description: We have simulated the formation of a massive galaxy cluster ( $M_{200}^{\rm crit}$ = 1.1 x 10 15 h –1 M ) in a cold dark matter universe using 10 different codes ( ramses , 2 incarnations of arepo and 7 of gadget ), modelling hydrodynamics with full radiative subgrid physics. These codes include smoothed-particle hydrodynamics (SPH), spanning traditional and advanced SPH schemes, adaptive mesh and moving mesh codes. Our goal is to study the consistency between simulated clusters modelled with different radiative physical implementations – such as cooling, star formation and thermal active galactic nucleus (AGN) feedback. We compare images of the cluster at z = 0, global properties such as mass, and radial profiles of various dynamical and thermodynamical quantities. We find that, with respect to non-radiative simulations, dark matter is more centrally concentrated, the extent not simply depending on the presence/absence of AGN feedback. The scatter in global quantities is substantially higher than for non-radiative runs. Intriguingly, adding radiative physics seems to have washed away the marked code-based differences present in the entropy profile seen for non-radiative simulations in Sembolini et al.: radiative physics + classic SPH can produce entropy cores, at least in the case of non cool-core clusters. Furthermore, the inclusion/absence of AGN feedback is not the dividing line -as in the case of describing the stellar content – for whether a code produces an unrealistic temperature inversion and a falling central entropy profile. However, AGN feedback does strongly affect the overall stellar distribution, limiting the effect of overcooling and reducing sensibly the stellar fraction.

Description: We present an on-the-fly geometrical approach for shock detection and Mach number calculation in simulations employing smoothed particle hydrodynamics (SPH). We utilize pressure gradients to select shock candidates and define up- and downstream positions. We obtain hydrodynamical states in the up- and downstream regimes with a series of normal and inverted kernel weightings parallel and perpendicular to the shock normals. Our on-the-fly geometrical Mach detector incorporates well within the SPH formalism and has low computational cost. We implement our Mach detector into the simulation code gadget and alongside many SPH improvements. We test our shock finder in a sequence of shock tube tests with successively increasing Mach numbers exceeding by far the typical values inside galaxy clusters. For all shocks, we resolve the shocks well and the correct Mach numbers are assigned. An application to a strong magnetized shock tube gives stable results in full magnetohydrodynamic setups. We simulate a merger of two idealized galaxy clusters and study the shock front. Shock structures within the merging clusters as well as the cluster shock are well captured by our algorithm and assigned correct Mach numbers.

Description: We have simulated the formation of a galaxy cluster in a cold dark matter universe using 13 different codes modelling only gravity and non-radiative hydrodynamics ( ramses , ART, arepo , hydra and nine incarnations of gadget ). This range of codes includes particle-based, moving and fixed mesh codes as well as both Eulerian and Lagrangian fluid schemes. The various gadget implementations span classic and modern smoothed particle hydrodynamics (SPH) schemes. The goal of this comparison is to assess the reliability of cosmological hydrodynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at z  = 0, global properties such as mass and radial profiles of various dynamical and thermodynamical quantities. The underlying gravitational framework can be aligned very accurately for all the codes allowing a detailed investigation of the differences that develop due to the various gas physics implementations employed. As expected, the mesh-based codes ramses , art and arepo form extended entropy cores in the gas with rising central gas temperatures. Those codes employing classic SPH schemes show falling entropy profiles all the way into the very centre with correspondingly rising density profiles and central temperature inversions. We show that methods with modern SPH schemes that allow entropy mixing span the range between these two extremes and the latest SPH variants produce gas entropy profiles that are essentially indistinguishable from those obtained with grid-based methods.

Description: We present a model for the seeding and evolution of magnetic fields in protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy self-consistently provide magnetic seed fields, which are subsequently amplified by compression, shear flows and random motions. Our model explains the origin of strong magnetic fields of μG amplitude within the first star-forming protogalactic structures shortly after the first stars have formed. We implement the model into the magnetohydrodynamics (MHD) version of the cosmological N -body/smoothed-particle hydrodynamics (SPH) simulation code gadget and couple the magnetic seeding directly to the underlying multi-phase description of star formation. We perform simulations of Milky-Way-like galactic halo formation using a standard CDM cosmology and analyse the strength and distribution of the subsequent evolving magnetic field. Within star-forming regions and given typical dimensions and magnetic field strengths in canonical SN remnants, we inject a dipole-shaped magnetic field at a rate of 10 –9  G Gyr –1 . Subsequently, the magnetic field strength increases exponentially on time-scales of a few tens of millions of years within the innermost regions of the halo. Furthermore, turbulent diffusion, shocks and gas motions transport the magnetic field towards the halo outskirts. At redshift z 0, the entire galactic halo is magnetized and the field amplitude is of the order of a few μG in the centre of the halo and 10 –9  G at the virial radius. Additionally, we analyse the intrinsic rotation measure (RM) of the forming galactic halo over a range of redshift. The mean halo intrinsic RM peaks between redshifts z 4 and z 2 and reaches absolute values around 1000 rad m –2 . While the halo virializes towards redshift z 0, the intrinsic RM values decline to a mean value below 10 rad m –2 . At high redshifts, the distribution of individual star-forming and thus magnetized regions is widespread. This leads to a widespread distribution of large intrinsic RM values. In our model for the evolution of galactic magnetic fields, the seed magnetic field amplitude and distribution are no longer free parameters, but determined self-consistently by the star formation process occurring during the formation of cosmic structures. Thus, this model provides a solution to the seed field problem.

Description: We examine subhaloes and galaxies residing in a simulated cold dark matter galaxy cluster ( $M^{\rm crit}_{200}=1.1\times 10^{15}\,h^{-1}\,\mathrm{M}_{\odot }$ ) produced by hydrodynamical codes ranging from classic smooth particle hydrodynamics (SPH), newer SPH codes, adaptive and moving mesh codes. These codes use subgrid models to capture galaxy formation physics. We compare how well these codes reproduce the same subhaloes/galaxies in gravity-only, non-radiative hydrodynamics and full feedback physics runs by looking at the overall subhalo/galaxy distribution and on an individual object basis. We find that the subhalo population is reproduced to within 10 per cent for both dark matter only and non-radiative runs, with individual objects showing code-to-code scatter of 0.1 dex, although the gas in non-radiative simulations shows significant scatter. Including feedback physics significantly increases the diversity. Subhalo mass and V max distributions vary by 20 per cent. The galaxy populations also show striking code-to-code variations. Although the Tully–Fisher relation is similar in almost all codes, the number of galaxies with 10 9 h – 1 M M * 10 12 h – 1 M can differ by a factor of 4. Individual galaxies show code-to-code scatter of ~0.5 dex in stellar mass. Moreover, systematic differences exist, with some codes producing galaxies 70 per cent smaller than others. The diversity partially arises from the inclusion/absence of active galactic nucleus feedback. Our results combined with our companion papers demonstrate that subgrid physics is not just subject to fine-tuning, but the complexity of building galaxies in all environments remains a challenge. We argue that even basic galaxy properties, such as stellar mass to halo mass, should be treated with errors bars of ~0.2–0.4 dex.

Description: Building on the initial results of the nIFTy simulated galaxy cluster comparison, we compare and contrast the impact of baryonic physics with a single massive galaxy cluster, run with 11 state-of-the-art codes, spanning adaptive mesh, moving mesh, classic and modern smoothed particle hydrodynamics (SPH) approaches. For each code represented we have a dark-matter-only (DM) and non-radiative (NR) version of the cluster, as well as a full physics (FP) version for a subset of the codes. We compare both radial mass and kinematic profiles, as well as global measures of the cluster (e.g. concentration, spin, shape), in the NR and FP runs with that in the DM runs. Our analysis reveals good consistency 20 per cent) between global properties of the cluster predicted by different codes when integrated quantities are measured within the virial radius R 200 . However, we see larger differences for quantities within R 2500 , especially in the FP runs. The radial profiles reveal a diversity, especially in the cluster centre, between the NR runs, which can be understood straightforwardly from the division of codes into classic SPH and non-classic SPH (including the modern SPH, adaptive and moving mesh codes); and between the FP runs, which can also be understood broadly from the division of codes into those that include active galactic nucleus feedback and those that do not. The variation with respect to the median is much larger in the FP runs with different baryonic physics prescriptions than in the NR runs with different hydrodynamics solvers.

Description: We examine the properties of the galaxies and dark matter haloes residing in the cluster infall region surrounding the simulated cold dark matter galaxy cluster studied by Elahi et al. at z = 0. The 1.1  x 10 15 h –1 M galaxy cluster has been simulated with eight different hydrodynamical codes containing a variety of hydrodynamic solvers and sub-grid schemes. All models completed a dark-matter-only, non-radiative and full-physics run from the same initial conditions. The simulations contain dark matter and gas with mass resolution m DM = 9.01  x 10 8 h –1 M and m gas = 1.9  x 10 8 h –1 M , respectively. We find that the synthetic cluster is surrounded by clear filamentary structures that contain ~60 per cent of haloes in the infall region with mass ~10 12.5 –10 14 h –1 M , including 2–3 group-sized haloes (〉10 13 h –1 M ). However, we find that only ~10 per cent of objects in the infall region are sub-haloes residing in haloes, which may suggest that there is not much ongoing pre-processing occurring in the infall region at z = 0. By examining the baryonic content contained within the haloes, we also show that the code-to-code scatter in stellar fraction across all halo masses is typically ~2 orders of magnitude between the two most extreme cases, and this is predominantly due to the differences in sub-grid schemes and calibration procedures that each model uses. Models that do not include active galactic nucleus feedback typically produce too high stellar fractions compared to observations by at least ~1 order of magnitude.

Description: Author(s): A. Schmidt, P. Achenbach, J. Ahrens, H. J. Arends, R. Beck, A. M. Bernstein, V. Hejny, M. Kotulla, B. Krusche, V. Kuhr, R. Leukel, I. J. D. MacGregor, J. C. McGeorge, V. Metag, V. M. Olmos de León, F. Rambo, U. Siodlaczek, H. Ströher, Th. Walcher, J. Weiß, F. Wissmann, and M. Wolf [Phys. Rev. Lett. 110, 039903] Published Thu Jan 17, 2013

Description: We present constraints on the origins of fast radio bursts (FRBs) using large cosmological simulations. We calculate contributions to FRB dispersion measures (DMs) from the Milky Way, from the Local Universe, from cosmological large-scale structure, and from potential FRB host galaxies, and then compare these simulations to the DMs of observed FRBs. We find that the Milky Way contribution has previously been underestimated by a factor of ~2, and that the foreground-subtracted DMs are consistent with a cosmological origin, corresponding to a source population observable to a maximum redshift z  ~ 0.6–0.9. We consider models for the spatial distribution of FRBs in which they are randomly distributed in the Universe, track the star formation rate of their host galaxies, track total stellar mass, or require a central supermassive black hole. Current data do not discriminate between these possibilities, but the predicted DM distributions for different models will differ considerably once we begin detecting FRBs at higher DMs and higher redshifts. We additionally consider the distribution of FRB fluences, and show that the observations are consistent with FRBs being standard candles, each burst producing the same radiated isotropic energy. The data imply a constant isotropic burst energy of ~7  x  10 40  erg if FRBs are embedded in host galaxies, or ~9  x  10 40  erg if FRBs are randomly distributed. These energies are 10–100 times larger than had previously been inferred. Within the constraints of the available small sample of data, our analysis favours FRB mechanisms for which the isotropic radiated energy has a narrow distribution in excess of 10 40  erg.