Progenitor Halos Research Articles

PINOCCHIO and the hierarchical build-up of dark matter haloes

We study the ability of PINOCCHIO (PINpointing Orbit-Crossing Collapsed HIerarchical Objects) to predict the merging histories of dark matter (DM) haloes, comparing the PINOCCHIO predictions with the results of two large N-body simulations run from the same set of initial conditions. We focus our attention on the quantities most relevant to galaxy formation and large-scale structure studies. PINOCCHIO is able to predict the statistics of merger trees with a typical accuracy of 20 per cent. Its validity extends to higher-order moments of the distribution of progenitors. The agreement is also valid at the object-by-object level, with 70–90 per cent of the progenitors cleanly recognized when the parent halo is cleanly recognized itself. Predictions are also presented for quantities that are usually not reproduced by semi-analytic codes, such as the two-point correlation function of the progenitors of massive haloes and the distribution of initial orbital parameters of merging haloes. For the accuracy of the prediction and for the facility with which merger histories are produced, PINOCCHIO provides a means to generate catalogues of DM haloes, which is extremely competitive with large-scale N-body simulations, making it a suitable tool for galaxy formation and large-scale structure studies.

Dark matter in Draco and the Local Group: Implications for direct detection experiments

We use a cosmological simulation of the Local Group to make quantitative and speculative predictions for direct detection experiments. Cold dark matter (CDM) halos form via a complex series of mergers, accretion events and violent relaxation which precludes the formation of significant caustic features predicted by axially symmetric collapse. The halo density profiles are combined with observational constraints on the galactic mass distribution to constrain the local density of cold dark matter to lie in the range 0.18 <~ rho_CDM(R_solar)/GeV cm^-3 <~ 0.30. In velocity space, coherent streams of dark matter from tidally disrupted halos fill the halo and provide a tracer of the merging hierarchy. The particle velocities within triaxial CDM halos cannot be approximated by a simple Maxwellian distribution and is radially biased at the solar position. The detailed phase space structure within the solar system will depend on the early merger history of the progenitor halos and the importance of major mergers over accretion dominated growth. We follow the formation of a ``Draco'' sized dSph halo of mass 10^8M_solar with several million particles and high force accuracy. Its internal structure and substructure resembles that of galactic or cluster mass halos: the density profile has a singular central cusp and it contains thousands of sub-halos orbiting within its virial radius demonstrating a self-similar nature to collisionless dark matter sub-clustering. The singular cores of substructure halos always survive complete tidal disruption although mass loss is continuous and rapid. Extrapolating wildly to earth mass halos with velocity dispersion of 1 m s^-1 (roughly equal to the free streaming scale for neutralinos) we find that most of the dark matter may remain attached to bound subhalos. (Abridged)

An excursion set model of hierarchical clustering: ellipsoidal collapse and the moving barrier

The excursion set approach allows one to estimate the abundance and spatial distribution of virialized dark matter haloes efficiently and accurately. The predictions of this approach depend on how the non-linear processes of collapse and virialization are modelled. We present simple analytic approximations that allow us to compare the excursion set predictions associated with spherical and ellipsoidal collapse. In particular, we present formulae for the universal unconditional mass function of bound objects and the conditional mass function which describes the mass function of the progenitors of haloes in a given mass range today. We show that the ellipsoidal collapse based moving barrier model provides a better description of what we measure in the numerical simulations than the spherical collapse based constant barrier model, although the agreement between model and simulations is better at large lookback times. Our results for the conditional mass function can be used to compute accurate approximations to the local-density mass function, which quantifies the tendency for massive haloes to populate denser regions than less massive haloes. This happens because low-density regions can be thought of as being collapsed haloes viewed at large lookback times, whereas high-density regions are collapsed haloes viewed at small lookback times. Although we have applied our analytic formulae only to two simple barrier shapes, we show that they are, in fact, accurate for a wide variety of moving barriers. We suggest how they can be used to study the case in which the initial dark matter distribution is not completely cold.

Merging History as a Function of Halo Environment

According to the hierarchical scenario, galaxies form via merging and accretion of small objects. Using N-body simulations, we study the frequency of merging events in the history of the halos. We find that at z 2 the merging rate of the overall halo population can be described by a simple power law, (1 + z)3. The main emphasis of this paper is on the effects of environment of halos at the present epoch (z = 0). We find that the halos located inside clusters have formed earlier (Δz ≈ 1) than isolated halos of the same mass. At low redshifts (z < 1), the merger rate of cluster halos is 3 times lower than that of isolated halos and twice as low as that of halos that end up in groups by z = 0. At higher redshifts (z ~ 1-4), progenitors of cluster and group halos have 3-5 times higher merger rates than isolated halos. We briefly discuss the implications of our results for galaxy evolution in different environments.

Resolving the Structure of Cold Dark Matter Halos

We examine the effects of mass resolution and force softening on the density profiles of cold dark matter halos that form within cosmological N-body simulations. As we increase the mass and force resolution, we resolve progenitor halos that collapse at higher redshifts and have very high densities. At our highest resolution we have nearly 3×106 particles within the virial radius, which is several orders of magnitude more than previously used, and we can resolve more than 1000 surviving dark matter halos within this single virialized system. The halo profiles become steeper in the central regions, and we may not have achieved convergence to a unique slope within the inner 10% of the virialized region. Results from two very high resolution halo simulations yield steep inner density profiles, ρ(r)~r−1.4. The abundance and properties of arcs formed within this potential will be different from calculations based on lower resolution simulations. The kinematics of disks within such a steep potential may prove problematic for the cold dark matter model when compared with the observed properties of halos on galactic scales.

The assembly of matter in galaxy clusters

We study the merging history of dark matter haloes that end up in rich clusters, using N-body simulations of a scale-free universe. We compare the predictions of the extended Press & Schechter (PS) formalism (Bond et al. 1991; Bower 1991; Lacey & Cole 1993) with several conditional statistics of the proto-cluster matter: the mass distribution and relative abundance of progenitor haloes at different redshifts, the infall rate of progenitors within the proto-cluster, the formation redshift of the most massive cluster progenitor, and the accretion rates of other haloes onto it. The high quality of our simulations allows an unprecedented resolution in the mass range of the studied distributions. We also present the global mass function for the same cosmological model. We find that the PS formalism and its extensions cannot simultaneously describe the global evolution of clustering and its evolution in a proto-cluster environment. The best-fit PS model for the global mass function is a poor fit to the statistics of cluster progenitors. This discrepancy is in the sense of underpredicting the number of high-mass progenitors at high redshift. Although the PS formalism can provide a good qualitative description of the global evolution of hierarchical clustering, particular attention is needed when applying the theory to the mass distribution of progenitor objects at high redshift.

Dark matter haloes within clusters

We examine the properties of dark matter haloes within a rich galaxy cluster using a high-resolution simulation that captures the cosmological context of a cold dark matter universe. The mass and force resolution permit the resolution of 150 haloes with circular velocities larger than 80 km s−1 within the cluster virial radius of 2 Mpc (with Hubble constant H0 = 50 km s−1 Mpc−1). This enables an unprecedented study of the statistical properties of a large sample of dark matter haloes evolving in a dense environment. The cumulative fraction of mass attached to these haloes varies from close to zero per cent at 200 kpc to 13 per cent at the virial radius. Even at this resolution the overmerging problem persists; haloes that pass within 100–200 kpc of the cluster centre are tidally disrupted. Additional substructure is lost at earlier epochs within the massive progenitor haloes. The median ratio of apocentric to pericentric radii is 6:1, so that the orbital distribution is close to isotropic, circular orbits are rare and radial orbits are common. The orbits of haloes are unbiased with respect to both position within the cluster and the orbits of the smooth dark matter background, and no velocity bias is detected. The tidal radii of surviving haloes are generally well-fitted using the simple analytic prediction applied to their orbital pericentres. Haloes within clusters have higher concentrations than those in the field. Within the cluster, halo density profiles can be modified by tidal forces and individual encounters with other haloes that cause significant mass loss —‘galaxy harassment’. Mergers between haloes do not occur inside the cluster virial radius.

Evolution of the Hubble sequence in hierarchical models for galaxy formation

We present a model for the broad morphological distinction between the disk and spheroidal components of galaxies. Elaborating on the hierarchical clustering scheme of galaxy formation proposed by Cole et al., we assume that galaxies form stars quiescently in a disk until they are disrupted into a spheroidal configuration by mergers. We calculate formation and merging histories, and the evolution in colour, luminosity and morphology of the galaxy populations in different environments. Roughly $ 50\ %$ of ellipticals in our model have undergone a major merger since $z = 0.5$, yet in spite of this we find that cluster ellipticals have colour-magnitude diagrams with remarkably small scatter. The morphological mix of galaxies that become rich cluster members at high redshift is dominated by spiral galaxies, due to the long timescale for galaxy mergers compared with the timescale for cluster assembly at high redshift. The assembly of low redshift clusters is slower, allowing more galaxy mergers to occur in the progenitor halos. As a result $z=0$ rich clusters become E/S0 dominated and we find a ``Butcher-Oemler'' effect that becomes weaker for poorer groups at high redshift. The field luminosity function of red galaxies shows little evolution out to $z\simeq 1$ and the reddest galaxies at these redshifts are as bright as their local counterparts. The blue luminosity function, on the other hand, evolves rapidly with redshift, increasing its characteristic luminosity and becoming steeper at the faint end. These trends are similar to those recently observed in the Canada-France Redshift Survey.

The merging history of dark matter haloes in a hierarchical universe

We have developed an algorithm to investigate the merging history of present-day dark matter haloes in a universe where structure is built up hierarchically. The algorithm constructs merging history trees which can be used to trace the merging path of every halo mass element through all the progenitor haloes from which the present object formed. These trees follow merging histories from high redshift until z = 0 and so can be used to study the formation and merging of galaxies within the material which constitutes a single present-day group or cluster of galaxies. We have tested our results against those derived from numerical simulations of gravitational clustering and find that our algorithm correctly reproduces the mass distribution of the two largest halo progenitors at a series of redshifts

Magnetic fields in damped Ly-alpha systems

The incidence of Faraday rotation in a sample of radio-selected QSOs is combined with the incidence of foreground metal-line absorption to compute the probability for Faraday rotation in various types of metal- line absorbers. We divide the sample into subsets with and without damped Ly α absorption. The probability for Faraday rotation is significantly higher in the damped subset. Moreover, the probability is higher in the damped subset than in nondamped subsets selected on the basis of Mg II or C IV absorption. Owing to evidence linking damped systems to the progenitors of galactic disks and the Mg II systems to the progenitors of galactic halos, we conclude that magnetic fields were significantly higher in protogalactic disks than in proto-galactic halos. We estimate that the B fields in two damped Ly α systems with z ~ 2 are a few microgauss. The existence of microgauss magnetic fields in rotating galactic disks at these redshifts is extremely difficult to understand if magnetic field amplification is due to a global galactic dynamo. This is because there is not enough time available for amplification of an infinitesimal seed field of B ~ 10^-21^ G to B ~ 10^-6^ G by z ~ 2 in any standard Friedmann cosmology. The observations thus indicate either a more rapid amplification mechanism, possibly by a rapid local dynamo, or the presence of a finite primordial B field.