30 research outputs found
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
Black Holes in our Galactic Halo: Compatibility with FGST and PAMELA Data and Constraints on the First Stars
10 to 10^5 solar mass black holes with dark matter spikes that formed in
early minihalos and still exist in our Milky Way Galaxy today are examined in
light of recent data from the Fermi Gamma-Ray Space Telescope (FGST). The dark
matter spikes surrounding black holes in our Galaxy are sites of significant
dark matter annihilation. We examine the signatures of annihilations into
gamma-rays, electrons and positrons, and neutrinos. We find that some
significant fraction of the point sources detected by FGST might be due to dark
matter annihilation near black holes in our Galaxy. We obtain limits on the
properties of dark matter annihilations in the spikes using the information in
the FGST First Source Catalog as well as the diffuse gamma-ray flux measured by
FGST. We determine the maximum fraction of high redshift minihalos that could
have hosted the formation of the first generation of stars and, subsequently,
their black hole remnants. The strength of the limits depends on the choice of
annihilation channel and black hole mass; limits are strongest for the heaviest
black holes and annhilation to and final states. The
larger black holes considered in this paper may arise as the remnants of Dark
Stars after the dark matter fuel is exhausted and thermonuclear burning runs
its course; thus FGST observations may be used to constrain the properties of
Dark Stars. Additionally, we comment on the excess positron flux found by
PAMELA and its possible interpretation in terms of dark matter annihilation
around these black hole spikes.Comment: 34 pages, 11 figures. v2: typos corrected, references added. v3:
updated to match published versio
Radial distribution and strong lensing statistics of satellite galaxies and substructure using high resolution LCDM hydrodynamical simulations
We analyse the number density and radial distribution of substructures and
satellite galaxies using cosmological simulations that follow the gas dynamics
of a baryonic component, including shock heating, radiative cooling and star
formation within the hierarchical concordance LCDM model. We find that the
dissipation of the baryons greatly enhances the survival of subhaloes,
expecially in the galaxy core, resulting in a radial distribution of satellite
galaxies that closely follows the overall mass distribution in the inner part
of the halo. Hydrodynamical simulations are necessary to resolve the adiabatic
contraction and dense cores of galaxies, resulting in a total number of
satellites a factor of two larger than found in pure dark matter simulation, in
good agreement with the observed spatial distribution of satellite galaxies
within galaxies and clusters. Convergence tests show that the cored
distribution found by previous authors in pure N-body simulations was due to
physical overmerging of dark matter only structures.
We proceed to use a ray-shooting technique in order to study the impact of
these additional substructures on the number of violations of the cusp caustic
magnification relation. We develop a new approach to try to disentangle the
effect of substructures from the intrinsic discreteness of N-Body simulations.
Even with the increased number of substructures in the centres of galaxies, we
are not able to reproduce the observed high numbers of discrepancies observed
in the flux ratios of multiply lensed quasars.Comment: 11 pages, 15 figures, comparison with previous works updated, one
more plot added, minor changes to match the accepted version by MNRA
Dark matter substructure and gamma-ray annihilation in the Milky Way halo
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 Mvir=1.8x10^12 Msun 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(>Msub)= 0.0064 (Msub/Mvir)^(-1) down
to Msun=4x10^6 Msun. 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 (5 km/s) is 124 (812):
of these, 5 (26) are found within 0.1 Rvir, a region that appeared practically
smooth in previous simulations. The neutralino self-annihilation gamma-ray
emission from dark matter clumps is approximately constant per subhalo mass
decade. Therefore, while in our run the contribution of substructure to the
gamma-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 gamma-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.Comment: 15 pages, 10 figures. ApJ accepted. Movies, images and a version with
higher resolution figures are available at
http://www.ucolick.org/~diemand/vl
Two-body relaxation in cold dark matter simulations
N-body simulations of the hierarchical formation of cosmic structures suffer from the problem that the first objects to form always contain just a few particles. Although relaxation is not an issue for virialized objects containing millions of particles, collisional processes will always dominate within the first structures that collapse. First we quantify how the relaxation varies with resolution, softening and radius within isolated equilibrium and non-equilibrium cuspy haloes. We then attempt to determine how this numerical effect propagates through a merging hierarchy by measuring the local relaxation rates of each particle throughout the hierarchical formation of a dark matter halo. The central few per cent of the final structures - a region that one might naively think is well resolved at the final time since the haloes contain â106 particles - suffer from high degrees of relaxation. It is not clear how to interpret the effects of the accumulated relaxation rate, but we argue that it describes a region within which one should be careful about trusting the numerical results. Substructure haloes are most affected by relaxation since they contain few particles at a constant energy for the entire simulation. We show that relaxation will flatten a cusp in just a few mean relaxation times of a halo. We explore the effect of resolution on the degree of relaxation, and we find that increasing N slowly reduces the degree of relaxation âNâ0.25 rather than proportional to N as expected from the collisionless Boltzmann equation. Simulated with the same relative mass resolution (i.e. equal numbers of particles), cluster mass objects suffer significantly more relaxation than galaxy mass objects since they form relatively late and therefore more of the particles spend more time in small-N haloe
Redefining the Missing Satellites Problem
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