Formation and evolution of galaxy dark matter halos and their substructure

We use the ``Via Lactea'' simulation to study the co-evolution of a Milky Way-size LambdaCDM halo and its subhalo population. While most of the host halo mass is accreted over the first 6 Gyr in a series of major mergers, the physical mass distribution [not M_vir(z)] remains practically constant since z=1. The same is true in a large sample of LambdaCDM galaxy halos. Subhalo mass loss peaks between the turnaround and virialization epochs of a given mass shell, and declines afterwards. 97% of the z=1 subhalos have a surviving bound remnant at the present epoch. The retained mass fraction is larger for initially lighter subhalos: satellites with maximum circular velocities Vmax=10 km/s at z=1 have today about 40% of their mass back then. At the first pericenter passage a larger average mass fraction is lost than during each following orbit. Tides remove mass in substructure from the outside in, leading to higher concentrations compared to field halos of the same mass. This effect, combined with the earlier formation epoch of the inner satellites, results in strongly increasing subhalo concentrations towards the Galactic center. We present individual evolutionary tracks and present-day properties of the likely hosts of the dwarf satellites around the Milky Way. The formation histories of ``field halos'' that lie today beyond the Via Lactea host are found to strongly depend on the density of their environment. This is caused by tidal mass loss that affects many field halos on eccentric orbits.Comment: 20 pages, 18 figures. Figures 6,7 and 8 corrected in this version, for details see the erratum in ApJ 679, 1680 and http://www.ucolick.org/~diemand/vl/publ/vlevolerr.pdf. Data, movies and images are available at http://www.ucolick.org/~diemand/vl

The Dark Matter Annihilation Signal from Galactic Substructure: Predictions for GLAST

We present quantitative predictions for the detectability of individual Galactic dark matter subhalos in gamma-rays from dark matter pair annihilations in their centers. Our method is based on a hybrid approach, employing the highest resolution numerical simulations available (including the recently completed one billion particle Via Lactea II simulation) as well as analytical models for the extrapolation beyond the simulations' resolution limit. We include a self-consistent treatment of subhalo boost factors, motivated by our numerical results, and a realistic treatment of the expected backgrounds that individual subhalos must outshine. We show that for reasonable values of the dark matter particle physics parameters (M_X ~ 50 - 500 GeV and ~ 10^-26 - 10^-25 cm^3/s) GLAST may very well discover a few, even up to several dozen, such subhalos, at 5 sigma significance, and some at more than 20 sigma. We predict that the majority of luminous sources would be resolved with GLAST's expected angular resolution. For most observer locations the angular distribution of detectable subhalos is consistent with a uniform distribution across the sky. The brightest subhalos tend to be massive (median Vmax of 24 km/s) and therefore likely hosts of dwarf galaxies, but many subhalos with Vmax as low as 5 km/s are also visible. Typically detectable subhalos are 20 - 40 kpc from the observer, and only a small fraction are closer than 10 kpc. The total number of observable subhalos has not yet converged in our simulations, and we estimate that we may be missing up to 3/4 of all detectable subhalos.Comment: 19 pages, 12 figures, ApJ accepted, a version with higher resolution figures can be downloaded from http://www.sns.ias.edu/~mqk/transfer/VL2_GLAST_predictions.pd

Dark matter subhalos and the dwarf satellites of the Milky Way

The Via Lactea simulation of the dark matter halo of the Milky Way predicts the existence of many thousands of bound subhalos distributed approximately with equal mass per decade of mass. Here we show that: a) a similar steeply rising subhalo mass function is also present at redshift 0.5 in an elliptical-sized halo simulated with comparable resolution in a different cosmology. Compared to Via Lactea, this run produces nearly a factor of two more subhalos with large circular velocities; b) the fraction of Via Lactea mass brought in by subhalos that have a surviving bound remnant today with present-day peak circular velocity Vmax>2 km/s (>10 km/s) is 45% (30%); c) because of tidal mass loss, the number of subhalos surviving today that reached a peak circular velocity of >10 km/s throughout their lifetime exceeds half a thousand, five times larger than their present-day abundance and more than twenty times larger than the number of known satellites of the Milky Way; e) unless the circular velocity profiles of Galactic satellites peak at values significantly higher that expected from the stellar line-of-sight velocity dispersion, only about one in five subhalos with Vmax>20 km/s today must be housing a luminous dwarf; f) small dark matter clumps appear to be relatively inefficient at forming stars even well beyond the virial radius; g) the observed Milky Way satellites appear to follow the overall dark matter distribution of Via Lactea, while the largest simulated subhalos today are found preferentially at larger radii; h) subhalos have central densities that increase with Vmax and reach 0.1-0.3 Msun/pc3 comparable to the central densities inferred in dwarf spheroidals with core radii >250 pc.Comment: 14 pages, 8 figures, ApJ in press. A few typos correcte

On the age-radius relation and orbital history of cluster galaxies

We explore the region of influence of a galaxy cluster using numerical simulations of cold dark matter halos. Many of the observed galaxies in a cluster are expected to be infalling for the first time. Half of the halos at distances of one to two virial radii today have previously orbited through the cluster, most of them have even passed through the dense inner regions of the cluster. Some halos at distances of up to three times the virial radius have also passed through the cluster core. We do not find a significant correlation of ``infall age'' versus present day position for substructures and the scatter at a given position is very large. This relation may be much more significant if we could resolve the physically overmerged galaxies in the central region.Comment: To appear in the proceedings of IAU Colloquium 195: "Outskirts of galaxy clusters: intense life in the suburbs", Torino, Italy, March 12-16, 200

Earth-mass dark-matter haloes as the first structures in the early Universe

The Universe was nearly smooth and homogeneous before a redshift of z = 100, about 20 million years after the Big Bang. After this epoch, the tiny fluctuations imprinted upon the matter distribution during the initial expansion began to collapse because of gravity. The properties of these fluctuations depend on the unknown nature of dark matter, the determination of which is one of the biggest challenges in present-day science. Here we report supercomputer simulations of the concordance cosmological model, which assumes neutralino dark matter (at present the preferred candidate), and find that the first objects to form are numerous Earth-mass dark-matter haloes about as large as the Solar System. They are stable against gravitational disruption, even within the central regions of the Milky Way. We expect over 10^15 to survive within the Galactic halo, with one passing through the Solar System every few thousand years. The nearest structures should be among the brightest sources of gamma-rays (from particle-particle annihilation).Comment: 11 pages, 3 figures, published in Nature, January 27, 200

Molecular dynamics simulations of bubble nucleation in dark matter detectors

Bubble chambers and droplet detectors used in dosimetry and dark matter particle search experiments use a superheated metastable liquid in which nuclear recoils trigger bubble nucleation. This process is described by the classical heat spike model of F. Seitz [Phys. Fluids (1958-1988) 1, 2 (1958)], which uses classical nucleation theory to estimate the amount and the localization of the deposited energy required for bubble formation. Here we report on direct molecular dynamics simulations of heat-spike-induced bubble formation. They allow us to test the nanoscale process described in the classical heat spike model. 40 simulations were performed, each containing about 20 million atoms, which interact by a truncated force-shifted Lennard-Jones potential. We find that the energy per length unit needed for bubble nucleation agrees quite well with theoretical predictions, but the allowed spike length and the required total energy are about twice as large as predicted. This could be explained by the rapid energy diffusion measured in the simulation: contrary to the assumption in the classical model, we observe significantly faster heat diffusion than the bubble formation time scale. Finally we examine {\alpha}-particle tracks, which are much longer than those of neutrons and potential dark matter particles. Empirically, {\alpha} events were recently found to result in louder acoustic signals than neutron events. This distinction is crucial for the background rejection in dark matter searches. We show that a large number of individual bubbles can form along an {\alpha} track, which explains the observed larger acoustic amplitudes.Comment: 7 pages, 5 figures, accepted for publication in Phys. Rev. E, matches published versio

Bound and unbound substructures in Galaxy-scale Dark Matter haloes

We analyse the coarse-grained phase-space structure of the six Galaxy-scale dark matter haloes of the Aquarius Project using a state-of-the-art 6D substructure finder. Within r_50, we find that about 35% of the mass is in identifiable substructures, predominantly tidal streams, but including about 14% in self-bound subhaloes. The slope of the differential substructure mass function is close to -2, which should be compared to around -1.9 for the population of self-bound subhaloes. Near r_50 about 60% of the mass is in substructures, with about 30% in self-bound subhaloes. The inner 35 kpc of the highest resolution simulation has only 0.5% of its mass in self-bound subhaloes, but 3.3% in detected substructure, again primarily tidal streams. The densest tidal streams near the solar position have a 3-D mass density about 1% of the local mean, and populate the high velocity tail of the velocity distribution.Comment: Submitted to MNRAS on 12/10/2010, 11 pages, 10 figure

Carbon-Enhanced Metal-Poor Stars, the Cosmic Microwave Background, and the Stellar IMF in the Early Universe

The characteristic mass of stars at early times may have been higher than today owing to the cosmic microwave background (CMB). This study proposes that (1) the testable predictions of this "CMB-IMF" hypothesis are an increase in the fraction of carbon-enhanced metal-poor (CEMP) stars with declining metallicity and an increase from younger to older populations at a single metallicity (e.g. disk to halo), and (2) these signatures are already seen in recent samples of CEMP stars and can be better tested with anticipated data. The expected spatial variation may explain discrepancies of CEMP frequency among published surveys. The ubiquity and time dependence of the CMB will substantially alter the reconstruction of star formation histories in the Local Group and early Universe.Comment: 7 pages emulateapj format, three figures, accepted for ApJ Letter

The Graininess of Dark Matter Haloes

We use the recently completed one billion particle Via Lactea II LambdaCDM simulation to investigate local properties like density, mean velocity, velocity dispersion, anisotropy, orientation and shape of the velocity dispersion ellipsoid, as well as structure in velocity space of dark matter haloes. We show that at the same radial distance from the halo centre, these properties can deviate by orders of magnitude from the canonical, spherically averaged values, a variation that can only be partly explained by triaxiality and the presence of subhaloes. The mass density appears smooth in the central relaxed regions but spans four orders of magnitude in the outskirts, both because of the presence of subhaloes as well as of underdense regions and holes in the matter distribution. In the inner regions the local velocity dispersion ellipsoid is aligned with the shape ellipsoid of the halo. This is not true in the outer parts where the orientation becomes more isotropic. The clumpy structure in local velocity space of the outer halo can not be well described by a smooth multivariate normal distribution. Via Lactea II also shows the presence of cold streams made visible by their high 6D phase space density. Generally, the structure of dark matter haloes shows a high degree of graininess in phase space that cannot be described by a smooth distribution function.Comment: 22 pages, 14 figures, 1 table, accepted for publication by MNRA