We use the "Via Lactea" simulation to study the co-evolution of a Milky Way-size ΛCDM 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 ΛCDM 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 V max = 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.

This paper describes the open-source code Enzo, which uses block-structured adaptive mesh refinement to provide high spatial and temporal resolution for modeling astrophysical fluid flows. The code is Cartesian, can be run in 1, 2, and 3 dimensions, and supports a wide variety of physics including hydrodynamics, ideal and non-ideal magnetohydrodynamics, N-body dynamics (and, more broadly, self-gravity of fluids and particles), primordial gas chemistry, optically-thin radiative cooling of primordial and metal-enriched plasmas (as well as some optically-thick cooling models), radiation transport, cosmological expansion, and models for star formation and feedback in a cosmological context. In addition to explaining the algorithms implemented, we present solutions for a wide range of test problems, demonstrate the code's parallel performance, and discuss the Enzo collaboration's code development methodology.

We present initial results from "Via Lactea", the highest resolution simulation to date of Galactic CDM substructure. It follows the formation of a Milky Way-size halo with M halo = 1.8 × 10 12 M ⊙ in a WMAP 3-year cosmology, using 234 million particles. Over 10,000 subhalos can be identified at z=0: Their cumulative mass function is well-fit by N (> M sub ) = 0.0064 (M sub /M halo ) −1 down to M sub = 4 × 10 6 M ⊙ . The total mass fraction in subhalos is 5.3%, while the fraction of surface mass density in substructure within a projected distance of 10 kpc from the halo center is 0.3%. Because of the significant contribution from the smallest resolved subhalos, these fractions have not converged yet. Sub-substructure is apparent in all the larger satellites, and a few dark matter lumps are resolved even in the solar vicinity. The number of dark satellites with peak circular velocities above 10 km s −1 (5 km s −1 ) is 124 (812): of these, 5 (26) are found within 0.1 r vir , a region that appeared practically smooth in previous simulations. The neutralino self-annihilation γ-ray emission from dark matter clumps is approximately constant per subhalo mass decade. Therefore, while in our run the contribution of substructure to the γ-ray luminosity of the Galactic halo amounts to only 40% of the total spherically-averaged smooth signal, we expect this fraction to grow significantly as resolution is increased further. An all-sky map of the expected annihilation γ-ray flux reaching a fiducial observer at 8 kpc from the Galactic center shows that at the current resolution a small number of subhalos start to be bright enough to be visible against the background from the smooth density field surrounding the observer.

Recent observations have constrained the galaxy ultraviolet (UV) luminosity function up to z ∼ 10. However, these observations alone allow for a wide range of reionization scenarios due to uncertainties in the abundance of faint galaxies and the escape fraction of ionizing photons. We show that requiring continuity with post-reionization (z < 6) measurements, where the Lyα forest provides a complete probe of the cosmological emissivity of ionizing photons, significantly reduces the permitted parameter space. Models that are simultaneously consistent with the measured UV luminosity function, the Thomson optical depth to the microwave background and the Lyα forest data require either (1) extrapolation of the galaxy luminosity function down to very faint UV magnitudes M lim ∼ −10, corresponding roughly to the UV background suppression scale; (2) an increase of the escape fraction by a factor 10 from z = 4 (where the best fit is 4 per cent) to 9; or (3) more likely, a hybrid solution in which undetected galaxies contribute significantly and the escape fraction increases more modestly. Models in which star formation is strongly suppressed in low-mass, reionizationepoch haloes of mass up to M h ∼ 10 10 M (e.g. owing to a metallicity dependence) are only allowed for extreme assumptions for the redshift evolution of the escape fraction. However, variants of such models in which the suppression mass is reduced (e.g. assuming an earlier or higher metallicity floor) are in better agreement with the data. Interestingly, concordance scenarios satisfying the available data predict a consistent redshift of 50 per cent ionized fraction z reion (50 per cent) ∼ 10. On the other hand, the duration of reionization is sensitive to the relative contribution of bright versus faint galaxies, with scenarios dominated by faint galaxies predicting a more extended reionization event. Scenarios relying too heavily on highredshift dwarfs are disfavoured by kinetic Sunyaev-Zeldovich measurements, which prefer a short reionization history.

We perform a series of simulations of a Galactic mass dark matter halo at different resolutions: our largest uses over 3 billion particles and has a mass resolution of 1000 M⊙. We quantify the structural properties of the inner dark matter distribution and study how they depend on numerical resolution. We can measure the density profile to a distance of 120 pc (0.05 per cent of Rvir), where the logarithmic slope is −0.8 and −1.4 at (0.5 per cent of Rvir). We propose a new two‐parameter fitting function that has a linearly varying logarithmic density gradient over the resolved radii which fits the GHALO and VL2 density profiles extremely well. Convergence in the halo shape is achieved at roughly three times the convergence radius for the density profile at which point the halo becomes more spherical due to numerical resolution. The six‐dimensional phase‐space profile is dominated by the presence of the substructures and does not follow a power law, except in the central few kpc which is devoid of substructure even at this resolution. The quantity, ρ/σ3, which is often used as a proxy for the six‐dimensional phase‐space density should be used with caution.

Abstract:The velocity distribution function of dark matter particles is expected to show significant departures from a Maxwell-Boltzmann distribution. This can have profound effects on the predicted dark matter -nucleon scattering rates in direct detection experiments, especially for dark matter models in which the scattering is sensitive to the high velocity tail of the distribution, such as inelastic dark matter (iDM) or light (few GeV) dark matter (LDM), and for experiments that require high energy recoil events, such as many directionally sensitive experiments. Here we determine the velocity distribution functions from two of the highest resolution numerical simulations of Galactic dark matter structure (Via Lactea II and GHALO), and study the effects for these scenarios. For directional detection, we find that the observed departures from Maxwell-Boltzmann increase the contrast of the signal and change the typical direction of incoming DM particles. For iDM, the expected signals at direct detection experiments are changed dramatically: the annual modulation can be enhanced by more than a factor two, and the relative rates of DAMA compared to CDMS can change by an order of magnitude, while those compared to CRESST can change by a factor of two. The spectrum of the signal can also change dramatically, with many features arising due to substructure. For LDM the spectral effects are smaller, but changes do arise that improve the compatibility with existing experiments. We find that the phase of the modulation can depend upon energy, which would help discriminate against background should it be found.

Numerical simulations of Milky-Way size Cold Dark Matter (CDM) halos predict a steeply rising mass function of small dark matter subhalos and a substructure count that greatly outnumbers the observed satellites of the Milky Way. Several proposed explanations exist, but detailed comparison between theory and observation in terms of the maximum circular velocity (Vmax) of the subhalos is hampered by the fact that Vmax for satellite halos is poorly constrained. We present comprehensive mass models for the well-known Milky Way dwarf satellites, and derive likelihood functions to show that their masses within 0.6 kpc (M_0.6) are strongly constrained by the present data. We show that the M_0.6 mass function of luminous satellite halos is flat between ~ 10^7 and 10^8 M_\odot. We use the ``Via Lactea'' N-body simulation to show that the M_0.6 mass function of CDM subhalos is steeply rising over this range. We rule out the hypothesis that the 11 well-known satellites of the Milky Way are hosted by the 11 most massive subhalos. We show that models where the brightest satellites correspond to the earliest forming subhalos or the most massive accreted objects both reproduce the observed mass function. A similar analysis with the newly-discovered dwarf satellites will further test these scenarios and provide powerful constraints on the CDM small-scale power spectrum and warm dark matter models.Comment: 8 pages, 6 figure

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 1 billion particle Via Lactea II simulation), as well as analytical models, for extrapolating beyond the simulations' resolution limit. We include a selfconsistent 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 $ 50Y500 GeV and hvi $ 10 À26 Y10 À25 cm 3 s À1 ) GLAST may very well discover a few, even up to several dozen, such subhalos at 5 significance, and some at more than 20 . 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 V max of 24 km s À1 ) and therefore likely hosts of dwarf galaxies, but many subhalos with V max as low as 5 km s À1 are also visible. Typically detectable subhalos are 20Y40 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.