Showing papers by "Oliver Hahn published in 2010"

Halo mass function and scale-dependent bias from N-body simulations with non-Gaussian initial conditions

Abstract: We perform a series of high-resolution N-body simulations of cosmological structure formation starting from Gaussian and non-Gaussian initial conditions. We adopt the best-fitting cosmological parameters from the third- and fifth-year data releases of the Wilkinson Microwave Anisotropy Probe, and we consider non-Gaussianity of the local type parametrized by eight different values of the non-linearity parameter f NL . Building upon previous work based on the Gaussian case, we show that, when expressed in terms of suitable variables, the mass function of friends-of-friends haloes is approximately universal (i.e. independent of redshift, cosmology and matter transfer function) to good precision (nearly 10 per cent) also in non-Gaussian scenarios. We provide fitting formulae for the high-mass end (M > 10 13 h -1 M ⊙ ) of the universal mass function in terms of f NL , and we also present a non-universal fit in terms of bothfN L and z to be used for applications requiring higher accuracy. For Gaussian initial conditions, we extend our fit to a wider range of halo masses (M > 2.4 x 10 10 h -1 M ⊙ ) and we also provide a consistent fit of the linear halo bias. We show that, for realistic values of f NL , the matter power spectrum in non-Gaussian cosmologies departs from the Gaussian 1 by up to 2 per cent on the scales where the baryonic-oscillation features are imprinted on the two-point statistics. Finally, using both the halo power spectrum and the halo-matter cross spectrum, we confirm the strong k-dependence of the halo bias on large scales (k < 0.05 h Mpc 1- ) which was already detected in previous studies. However, we find that commonly used parametrizations based on the peak-background split do not provide an accurate description of our simulations which present extra dependencies on the wavenumber, the non-linearity parameter and, possibly, the clustering strength. We provide an accurate fit of the simulation data that can be used as a benchmark for future determinations of f NL with galaxy surveys.

The large‐scale orientations of disc galaxies

Abstract: We use a 380-h−1 pc resolution hydrodynamic adaptive mesh refinement (AMR) simulation of a cosmic filament to investigate the orientations of a sample of ∼100 well-resolved galactic discs spanning two orders of magnitude in both stellar and halo mass. We find: (i) at z= 0, there is an almost perfect alignment at a median angle of 18°, in the inner dark matter halo regions where the discs reside, between the spin vector of the gaseous and stellar galactic discs and that of their inner host haloes. The alignment between galaxy spin and spin of the entire host halo is however significantly weaker, ranging from a median of ∼ 46° at z= 1 to ∼ 50° at z= 0. (ii) The most massive galaxy discs have spins preferentially aligned so as to point along their host filaments. (iii) The spin of discs in lower mass haloes shows, at redshifts above z∼ 0.5 and in regions of low environmental density, a clear signature of alignment with the intermediate principal axis of the large-scale tidal field. This behaviour is consistent with predictions of linear tidal torque theory. This alignment decreases with increasing environmental density, and vanishes in the highest density regions. Non-linear effects in the high-density environments are plausibly responsible for establishing this density-alignment correlation. We expect that our numerical results provide important insights for both understanding intrinsic alignment in weak lensing from the astrophysical perspective and formation and evolution processes of galactic discs in a cosmological context.

The buildup of the hubble sequence in the cosmos field

Abstract: We use similar to 8600 COSMOS galaxies at mass scales \textgreater 5 x 10(10)M(circle dot) to study how the morphological mix of massive ellipticals, bulge-dominated disks, intermediate-bulge disks, disk-dominated galaxies, and irregular systems evolves from z = 0.2 to z = 1. The morphological evolution depends strongly on mass. At M \textgreater 3 x 10(11) M(circle dot), no evolution is detected in the morphological mix: ellipticals dominate since z = 1, and the Hubble sequence has quantitatively settled down by this epoch. At the 10(11)M(circle dot) mass scale, little evolution is detected, which can be entirely explained by major mergers. Most of the morphological evolution from z = 1 to z = 0.2 takes place at masses 5 x 10(10)-10(11) M(circle dot), where (1) the fraction of spirals substantially drops and the contribution of early types increases. This increase is mostly produced by the growth of bulge-dominated disks, which vary their contribution from similar to 10% at z = 1 to \textgreater30% at z = 0.2 (for comparison, the elliptical fraction grows from similar to 15% to similar to 20%). Thus, at these masses, transformations from late to early types result in diskless elliptical morphologies with a statistical frequency of only 30%-40%. Otherwise, the processes which are responsible for the transformations either retain or produce a non-negligible disk component. (2) The disk-dominated galaxies, which contribute similar to 15% to the intermediate-mass galaxy population at z = 1, virtually disappear by z = 0.2. The merger rate since z = 1 is too low to account for the disappearance of these massive disk-dominated systems, which most likely grow a bulge via secular evolution.