715 research outputs found
The Dark Disk of the Milky Way
Massive satellite accretions onto early galactic disks can lead to the
deposition of dark matter in disk-like configurations that co-rotate with the
galaxy. This phenomenon has potentially dramatic consequences for dark matter
detection experiments. We utilize focused, high-resolution simulations of
accretion events onto disks designed to be Galaxy analogues, and compare the
resultant disks to the morphological and kinematic properties of the Milky
Way's thick disk in order to bracket the range of co-rotating accreted dark
matter. We find that the Milky Way's merger history must have been unusually
quiescent compared to median LCDM expectations and therefore its dark disk must
be relatively small: the fraction of accreted dark disk material near the Sun
is about 20% of the host halo density or smaller and the co-rotating dark
matter fraction near the Sun, defined as particles moving with a rotational
velocity lag less than 50 km/s, is enhanced by about 30% or less compared to a
standard halo model. Such a dark disk could contribute dominantly to the low
energy (of order keV for a dark matter particle with mass 100 GeV) nuclear
recoil event rate of direct dectection experiments, but it will not change the
likelihood of detection significantly. These dark disks provide testable
predictions of weakly-interacting massive particle dark matter models and
should be considered in detailed comparisons to experimental data. Our findings
suggest that the dark disk of the Milky Way may provide a detectable signal for
indirect detection experiments, contributing up to about 25% of the dark matter
self-annihilation signal in the direction of the center of the Galaxy, lending
the signal a noticeably oblate morphology.Comment: 11 pages, 6 figures, 1 table; submitted to Ap
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Heated Disc Stars in the Stellar Halo
Minor accretion events with mass ratio M_sat : M_host ~ 1:10 are common in
the context of LCDM cosmology. We use high-resolution simulations of
Galaxy-analogue systems to show that these mergers can dynamically eject disk
stars into a diffuse light component that resembles a stellar halo both
spatially and kinematically. For a variety of orbital configurations, we find
that ~3-5e8 M_sun of primary stellar disk material is ejected to a distance
larger than 5 kpc above the galactic plane. This ejected contribution is
similar to the mass contributed by the tidal disruption of the satellite galaxy
itself, though it is less extended. If we restrict our analysis to the
approximate solar neighborhood in the disk plane, we find that ~1% of the
initial disk stars in that region would be classified kinematically as halo
stars. Our results suggest that the inner parts of galactic stellar halos
contain ancient disk stars and that these stars may have been liberated in the
very same events that delivered material to the outer stellar halo.Comment: 9 pages, 5 figures; MNRAS accepte
The Destruction of Thin Stellar Disks Via Cosmologically Common Satellite Accretion Events
Most Galaxy-sized systems (M_host ~ 10^12 M_sun) in the LCDM cosmology are
expected to have accreted at least one satellite with a total mass M_sat ~
10^11 M_sun = 3M_disk in the past 8 Gyr. Analytic and numerical investigations
suggest that this is the most precarious type of merger for the survival of
thin galactic disks because more massive accretion events are relatively rare
and less massive ones preserve thin disk components. We use high-resolution,
dissipationless N-body simulations to study the response of an initially-thin,
fully-formed Milky-Way type stellar disk to these cosmologically common events
and show that the thin disk does not survive. Regardless of orbital
configuration, the impacts transform the disks into structures that are roughly
three times as thick and more than twice as kinematically hot as the observed
dominant thin disk component of the Milky Way. We conclude that if the Galactic
thin disk is a representative case, then the presence of a stabilizing gas
component is the only recourse for explaining the preponderance of disk
galaxies in an LCDM universe; otherwise, the disk of the Milky Way must be
uncommonly cold and thin for its luminosity, perhaps as a consequence of an
unusually quiescent accretion history.Comment: 6 pages, 4 figures; accepted by ApJL; high-resolution version and
example movie of simulation available at
http://web.me.com/purcellgalaxies/workshop/log/Entries/2008/10/15_The_Destruction_of_thin_stellar_disks_via_cosmologically_common_mergers.htm
Vertical density waves in the Milky Way disc induced by the Sagittarius Dwarf Galaxy
Recently, Widrow and collaborators announced the discovery of vertical
density waves in the Milky Way disk. Here we investigate a scenario where these
waves were induced by the Sagittarius dwarf galaxy as it plunged through the
Galaxy. Using numerical simulations, we find that the Sagittarius impact
produces North-South asymmetries and vertical wave-like behavior that
qualitatively agrees with what is observed. The extent to which vertical modes
can radially penetrate into the disc, as well as their amplitudes, depend on
the mass of the perturbing satellite. We show that the mean height of the disc
is expected to vary more rapidly in the radial than in the azimuthal direction.
If the observed vertical density asymmetry is indeed caused by vertical
oscillations, we predict radial and azimuthal variations of the mean vertical
velocity, correlating with the spatial structure. These variations can have
amplitudes as large as 8 km/s.Comment: 6 pages, 6 figures, MNRAS accepted. Revised to reflect final versio
Shredded Galaxies as the Source of Diffuse Intrahalo Light On Varying Scales
We make predictions for diffuse stellar mass fractions in dark matter halos
from the scales of small spiral galaxies to those of large galaxy clusters. We
use an extensively-tested analytic model for subhalo infall and evolution and
empirical constraints from galaxy survey data to set the stellar mass in each
accreted subhalo to model diffuse light. We add stellar mass to the diffuse
light as subhalos become disrupted due to interactions within their host halos.
We predict that the stellar mass fraction in diffuse, intrahalo light should
rise on average from ~0.5% to approximately 20% from small galaxy halos to poor
groups. The trend with mass flattens considerably beyond the group scale,
increasing weakly from a fraction of ~20% in poor galaxy clusters (~10^14
M_sun) to roughly ~30% in massive clusters (~10^15 M_sun). The mass-dependent
diffuse light fraction is governed primarily by the empirical fact that the
mass-to-light ratio in galaxy halos must vary as a function of halo mass.
Galaxy halos have little diffuse light because they accrete most of their mass
in small subhalos that themselves have high mass-to-light ratios; stellar halos
around galaxies are built primarily from disrupted dwarf-irregular-type
galaxies with M*~10^8.5 M_sun. The diffuse light in group and cluster halos is
built from satellite galaxies that form stars efficiently and have
correspondingly low mass-to-light ratios; intracluster light is dominated by
material liberated from massive galaxies with M*~10^11 M_sun. Our results are
consistent with existing observations spanning the galaxy, group, and cluster
scale; however, they can be tested more rigorously in future deep surveys for
faint diffuse light.Comment: version accepted for publication in ApJ; details clarified and
robustness discussion expanded, with results and conclusions unchanged; 15
pages, 10 figure
Signatures of minor mergers in the Milky Way disc I: The SEGUE stellar sample
It is now known that minor mergers are capable of creating structure in the
phase-space distribution of their host galaxy's disc. In order to search for
such imprints in the Milky Way, we analyse the SEGUE F/G-dwarf and the Schuster
et al. (2006) stellar samples. We find similar features in these two completely
independent stellar samples, consistent with the predictions of a Milky Way
minor-merger event. We next apply the same analyses to high-resolution,
idealised N-body simulations of the interaction between the Sagittarius dwarf
galaxy and the Milky Way. The energy distributions of stellar particle samples
in small spatial regions in the host disc reveal strong variations of structure
with position. We find good matches to the observations for models with a mass
of Sagittarius' dark matter halo progenitor M.
Thus, we show that this kind of analysis could be used to provide
unprecedentedly tight constraints on Sagittarius' orbital parameters, as well
as place a lower limit on its mass.Comment: 14 pages, 9 figures, 2 tables. Revised to reflect accepted versio
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