1,721,718 research outputs found
Inferring the dark matter power spectrum from the Lyman-alpha forest in high-resolution QSO absorption spectra
Full text linkWe use the LUQAS sample, a set of 27 high-resolution and high signal-to-noise ratio quasi-stellar object (QSO) absorption spectra at a median redshift of z=2.25, and the data from Croft et al. at a median redshift of z=2.72, together with a large suite of high-resolution large box-size hydrodynamical simulations, to estimate the linear dark matter power spectrum on scales 0.003<k<0.03 s km(-1). Our reanalysis of the Croft et al. data agrees well with their results if we assume the same mean optical depth and gas temperature-density relation. The inferred linear dark matter power spectrum at z=2.72 also agrees with that inferred from LUQAS at lower redshift if we assume that the increase of the amplitude is due to gravitational growth between these redshifts. We further argue that the smaller mean optical depth measured from high-resolution spectra is more accurate than the larger value obtained from low-resolution spectra by Press et al. which Croft et al. used. For the smaller optical depth we obtain a approximate to20 per cent higher value for the rms fluctuation amplitude of the matter density. By combining the amplitude of the matter power spectrum inferred from the Lyalpha forest with the amplitude on large scales inferred from measurements of the CMB we obtain constraints on the primordial spectral index n and the normalization sigma(8). For values of the mean optical depth favoured by high-resolution spectra, the inferred linear power spectrum is consistent with a LambdaCDM model with a scale-free (n=1) primordial power spectrum
The formation of disc galaxies in high-resolution moving-mesh cosmological simulations
No full textWe present cosmological hydrodynamical simulations of eight Milky Way-sized haloes that have been previously studied with dark matter only in the Aquarius project. For the first time, we employ the moving-mesh code arepo in zoom simulations combined with a comprehensive model for galaxy formation physics designed for large cosmological simulations. Our simulations form in most of the eight haloes strongly disc-dominated systems with realistic rotation curves, close to exponential surface density profiles, a stellar mass to halo mass ratio that matches expectations from abundance matching techniques, and galaxy sizes and ages consistent with expectations from large galaxy surveys in the local Universe. There is no evidence for any dark matter core formation in our simulations, even so they include repeated baryonic outflows by supernova-driven winds and black hole quasar feedback. For one of our haloes, the object studied in the recent ‘Aquila’ code comparison project, we carried out a resolution study with our techniques, covering a dynamic range of 64 in mass resolution. Without any change in our feedback parameters, the final galaxy properties are reassuringly similar, in contrast to other modelling techniques used in the field that are inherently resolution dependent. This success in producing realistic disc galaxies is reached, in the context of our interstellar medium treatment, without resorting to a high density threshold for star formation, a low star formation efficiency, or early stellar feedback, factors deemed crucial for disc formation by other recent numerical studies
Magnetic Fields in Cosmological Simulations of Disk Galaxies
No full textObservationally, magnetic fields reach equipartition with thermal energy and cosmic rays in the interstellar medium of disk galaxies such as the Milky Way. However, thus far cosmological simulations of the formation and evolution of galaxies have usually neglected magnetic fields. We employ the moving-mesh code AREPO to follow for the first time the formation and evolution of a Milky Way-like disk galaxy in its full cosmological context while taking into account magnetic fields. We find that a prescribed tiny magnetic seed field grows exponentially by a small-scale dynamo until it saturates around z = 4 with a magnetic energy of about 10% of the kinetic energy in the center of the galaxy's main progenitor halo. By z = 2, a well-defined gaseous disk forms in which the magnetic field is further amplified by differential rotation, until it saturates at an average field strength of ~6 μG in the disk plane. In this phase, the magnetic field is transformed from a chaotic small-scale field to an ordered large-scale field coherent on scales comparable to the disk radius. The final magnetic field strength, its radial profile, and the stellar structure of the disk compare well with observational data. A minor merger temporarily increases the magnetic field strength by about a factor of two, before it quickly decays back to its saturation value. Our results are highly insensitive to the initial seed field strength and suggest that the large-scale magnetic field in spiral galaxies can be explained as a result of the cosmic structure formation process
Testing the accuracy of the Hydro-PM approximation in numerical simulations of the Lyman-alpha forest
No full textWe implement the hydrodynamic particle-mesh (HPM) technique in the hydrodynamical simulation code GADGET-2 and quantify the differences between this approximate method and full hydrodynamical simulations of the Lyman alpha forest in a concordance Lambda CDM (cold dark matter) model. At redshifts z= 3 and 4, the differences between the gas and dark matter distributions, as measured by the one-point distribution of density fluctuations, the density power spectrum and the flux power spectrum, systematically decrease with increasing resolution of the HPM simulation. However, reducing these differences to less than a few per cent requires a significantly larger number of grid cells than particles, with a correspondingly larger demand for memory. Significant differences in the flux decrement distribution remain even for very high-resolution HPM simulations, particularly at low redshift. At z= 2, the differences between the flux power spectra obtained from HPM simulations and full hydrodynamical simulations are generally large and of the order of 20-30 per cent, and do not decrease with increasing resolution of the HPM simulation. This is due to the presence of large amounts of shock-heated gas, a situation which is not adequately modelled by the HPM approximation. We confirm the results of Gnedin & Hui that the statistical properties of the flux distribution are discrepant by greater than or similar to 5-20 per cent when compared to full hydrodynamical simulations. The discrepancies in the flux power spectrum are strongly scale- and redshift-dependent and extend to large scales. Considerable caution is needed in attempts to use calibrated HPM simulations for quantitative predictions of the flux power spectrum and other statistical properties of the Lyman alpha forest
Modeling the co-evolution of supermassive black holes and galaxies: I. Black hole scaling relations and the Active Galactive Nuclei luminosity function
No full textWe model the cosmological co-evolution of galaxies and their central supermassive black holes (BHs) within a semi-analytical framework developed on the outputs of the Millennium Simulation. This model, described in detail by Croton et al. and De Lucia and Blaizot, introduces a `radio mode' feedback from active galactic nuclei (AGN) at the centre of X-ray emitting atmospheres in galaxy groups and clusters. Thanks to this mechanism, the model can simultaneously explain: (i) the low observed mass dropout rate in cooling flows; (ii) the exponential cut-off in the bright end of the galaxy luminosity function and (iii) the bulge-dominated morphologies and old stellar ages of the most massive galaxies in clusters. This paper is the first of a series in which we investigate how well this model can also reproduce the physical properties of BHs and AGN. Here we analyse the scaling relations, the fundamental plane and the mass function of BHs, and compare them with the most recent observational data. Moreover, we extend the semi-analytic model to follow the evolution of the BH mass accretion and its conversion into radiation, and compare the derived AGN bolometric luminosity function with the observed one. While we find for the most part a very good agreement between predicted and observed BH properties, the semi-analytic model underestimates the number density of luminous AGN at high redshifts, independently of the adopted Eddington factor and accretion efficiency. However, an agreement with the observations is possible within the framework of our model, provided it is assumed that the cold gas fraction accreted by BHs at high redshifts is larger than at low redshifts
Collisional dark matter and the structure of dark halos
No full textWe study how the internal structure of dark halos is affected if cold dark matter particles are assumed to have a large cross section for elastic collisions. We identify a cluster halo in a large cosmological N-body simulation and resimulate its formation with progressively increasing resolution. We compare the structure found in the two cases in which dark matter is treated as collisionless or as a fluid. For the collisionless case, the overall ellipticity of the cluster, the central density cusp, and the amount of surviving substructure are all similar to those found in earlier high-resolution simulations. Collisional dark matter results in a cluster that is more nearly spherical at all radii, has a steeper central density cusp, and has less-but still substantial-surviving substructure. As in the collisionless case, these results for a ``fluid'' cluster halo are expected to carry over approximately to smaller mass systems. The observed rotation curves of dwarf galaxies then argue that self-interacting dark matter can only be viable if intermediate cross sections produce structure that does not lie between the extremes we have simulated
Accurately simulating anisotropic thermal conduction on a moving mesh
No full textWe present a novel implementation of an extremum preserving anisotropic diffusion solver for thermal conduction on the unstructured moving Voronoi mesh of the Arepo code. The method relies on splitting the one-sided facet fluxes into normal and oblique components, with the oblique fluxes being limited such that the total flux is both locally conservative and extremum preserving. The approach makes use of harmonic averaging points and a simple, robust interpolation scheme that works well for strong heterogeneous and anisotropic diffusion problems. Moreover, the required discretization stencil is small. Efficient fully implicit and semi-implicit time integration schemes are also implemented. We perform several numerical tests that evaluate the stability and accuracy of the scheme, including applications such as point explosions with heat conduction and calculations of convective instabilities in conducting plasmas. The new implementation is suitable for studying important astrophysical phenomena, such as the conductive heat transport in galaxy clusters, the evolution of supernova remnants, or the distribution of heat from black hole-driven jets into the intracluster medium
Stellar feedback by radiation pressure and photoionization
No full textThe relative impact of radiation pressure and photoionization feedback from young stars on surrounding gas is studied with hydrodynamic radiative transfer (RT) simulations. The calculations focus on the single-scattering (direct radiation pressure) and optically thick regime, and adopt a moment-based RT-method implemented in the moving-mesh code arepo. The source luminosity, gas density profile and initial temperature are varied. At typical temperatures and densities of molecular clouds, radiation pressure drives velocities of the order of ∼20 km s−1 over 1–5 Myr; enough to unbind the smaller clouds. However, these estimates ignore the effects of photoionization that naturally occur concurrently. When radiation pressure and photoionization act together, the latter is substantially more efficient, inducing velocities comparable to the sound speed of the hot ionized medium (10–15 km s−1) on time-scales far shorter than required for accumulating similar momentum with radiation pressure. This mismatch allows photoionization to dominate the feedback as the heating and expansion of gas lowers the central densities, further diminishing the impact of radiation pressure. Our results indicate that a proper treatment of the impact of young stars on the interstellar medium needs to primarily account for their ionization power whereas direct radiation pressure appears to be a secondary effect. This conclusion may change if extreme boosts of the radiation pressure by photon trapping are assumed
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