The formation of CDM haloes I: Collapse thresholds and the ellipsoidal collapse model

Abstract

In the excursion set approach to structure formation initially spherical regions of the linear density field collapse to form haloes of mass $M$ at redshift $z_{\mathrm{id}}$ if their linearly extrapolated density contrast, averaged on that scale, exceeds some critical threshold, ${\delta }_{\mathrm{c}}(z_{\mathrm{id}})$. The value of ${\delta }_{\mathrm{c}}(z_{\mathrm{id}})$ is often calculated from the spherical or ellipsoidal collapse model, which provide well-defined predictions given auxiliary properties of the tidal field at a given location. We use two cosmological simulations of structure growth in a $\Lambda$ cold dark matter scenario to quantify ${\delta }_{\mathrm{c}}(z_{\mathrm{id}})$, its dependence on the surrounding tidal field, as well as on the shapes of the Lagrangian regions that collapse to form haloes at $z_{\mathrm{id}}$. Our results indicate that the ellipsoidal collapse model provides an accurate description of the mean dependence of ${\delta }_{\mathrm{c}}(z_{\mathrm{id}})$ on both the strength of the tidal field and on halo mass. However, for a given $z_{\mathrm{id}}$, ${\delta }_{\mathrm{c}}(z_{\mathrm{id}})$ depends strongly on the halo's characteristic formation redshift: the earlier a halo forms, the higher its initial density contrast. Surprisingly, the majority of haloes forming $today$ fall below the ellipsoidal collapse barrier, contradicting the model predictions. We trace the origin of this effect to the non-spherical shapes of Lagrangian haloes, which arise naturally due to the asymmetry of the linear tidal field. We show that a modified collapse model, that accounts for the triaxial shape of protohaloes, provides a more accurate description of the measured minimum overdensities of recently collapsed objects.

MNRAS in press

References in corpus (8)

• Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation
• The spin and shape of dark matter haloes in the Millennium simulation of a LambdaCDM universe
• The Unorthodox Orbits of Substructure Halos
• Environmental Dependence of Cold Dark Matter Halo Formation
• Ellipsoidal halo finders and implications for models of triaxial halo formation
• The spatial and velocity bias of linear density peaks and proto-haloes in the Lambda cold dark matter cosmology
• The locations of halo formation and the peaks formalism
• Assembly Bias of Dwarf-sized Dark Matter Haloes

Cited by in corpus (33)

• Large-Scale Galaxy Bias
• The Mass-Concentration-Redshift Relation of Cold and Warm Dark Matter Halos
• ZOMG-I. How the cosmic web inhibits halo growth and generates assembly bias
• How does the cosmic web impact assembly bias?
• Galaxies flowing in the oriented saddle frame of the cosmic web
• The Hunt for Primordial Interactions in the Large Scale Structures of the Universe
• Concentration, Ellipsoidal Collapse, and the Densest Dark Matter haloes
• Deviations from tidal torque theory: evolution of the halo spin-filament alignment
• Evolution and Statistics of Non-Sphericity of Dark Matter Halos from Cosmological N-Body Simulation
• Non-thermal radio supernova remnants of exiled Wolf-Rayet stars
• When do cosmic peaks, filaments or walls merge? A theory of critical events in a multi-scale landscape
• The universal multiplicity function: counting halos and voids
• Optimal Transport Reconstruction of Baryon Acoustic Oscillations
• Excursion set peaks in energy as a model for haloes
• Confrontation of Top-Hat Spherical Collapse against Dark Halos from Cosmological N-Body Simulations
• Bianchi IX gravitational collapse of matter inhomogeneities
• Gas accretion and Ram Pressure Stripping of Haloes in Void Walls
• Getting in shape with minimal energy. A variational principle for protohaloes
• The large-separation expansion of peak clustering in Gaussian random fields
• Constrained simulations and excursion sets: understanding the risks and benefits of `genetically modified' haloes
• Scale-dependent bias from an inflationary bispectrum: the effect of a stochastic moving barrier
• Evaluating backreaction with the ellipsoidal collapse model
• Ellipticity and prolaticity of the initial gravitational-shear field at the position of density maxima
• Accurate halo mass functions from the simplest excursion set theory
• Mass accretion rates and multi-scale halo environment in cold and warm dark matter cosmologies
• A spherical hydrodynamical model of cosmic voids in ΛCDM and beyond
• The energy shear of protohaloes
• Mean maps for cosmic web structures in cosmological initial conditions
• The connection between halo concentrations and assembly histories: a probe of gravity?
• Approximate methods for the generation of dark matter halo catalogs in the age of precision cosmology
• Impact of the shape of the prestellar density fluctuations on the core mass function
• Segmenting proto-halos with vision transformers
• The role of energy shear in the collapse of protohaloes