467 research outputs found
Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical Satellite Accretion
(Abridged) We conduct a series of high-resolution, dissipationless N-body
simulations to investigate the cumulative effect of substructure mergers onto
thin disk galaxies in the context of the LCDM paradigm of structure formation.
Our simulation campaign is based on a hybrid approach. Substructure properties
are culled directly from cosmological simulations of galaxy-sized cold dark
matter (CDM) halos. In contrast to what can be inferred from statistics of the
present-day substructure populations, accretions of massive subhalos onto the
central regions of host halos, where the galactic disk resides, since z~1
should be common occurrences. One host halo merger history is subsequently used
to seed controlled numerical experiments of repeated satellite impacts on an
initially-thin Milky Way-type disk galaxy. We show that these accretion events
produce several distinctive observational signatures in the stellar disk
including: a ring-like feature in the outskirts; a significant flare; a central
bar; and faint filamentary structures that (spuriously) resemble tidal streams.
The final distribution of disk stars exhibits a complex vertical structure that
is well-described by a standard ``thin-thick'' disk decomposition. We conclude
that satellite-disk encounters of the kind expected in LCDM models can induce
morphological features in galactic disks that are similar to those being
discovered in the Milky Way, M31, and in other disk galaxies. These results
highlight the significant role of CDM substructure in setting the structure of
disk galaxies and driving galaxy evolution. Upcoming galactic structure surveys
and astrometric satellites may be able to distinguish between competing
cosmological models by testing whether the detailed structure of galactic disks
is as excited as predicted by the CDM paradigm.Comment: Accepted version to appear in ApJ, 24 pages, 8 figures, LaTeX (uses
emulateapj.cls). Comparison between the simulated ring-like features and the
Monoceros ring stellar structure in the Milky Way performed; conclusions
unaltere
Density Profiles of Cold Dark Matter Substructure: Implications for the Missing Satellites Problem
The structural evolution of substructure in cold dark matter (CDM) models is
investigated combining ``low-resolution'' satellites from cosmological N-body
simulations of parent halos with N=10^7 particles with high-resolution
individual subhalos orbiting within a static host potential. We show that, as a
result of mass loss, convergence in the central density profiles requires the
initial satellites to be resolved with N=10^7 particles and parsec-scale force
resolution. We find that the density profiles of substructure halos can be well
fitted with a power-law central slope that is unmodified by tidal forces even
after the tidal stripping of over 99% of the initial mass and an exponential
cutoff in the outer parts. The solution to the missing-satellites problem
advocated by Stoehr et al. in 2002 relied on the flattening of the dark matter
(DM) halo central density cusps by gravitational tides, enabling the observed
satellites to be embedded within DM halos with maximum circular velocities as
large as 60 km/s. In contrast, our results suggest that tidal interactions do
not provide the mechanism for associating the dwarf spheroidal satellites
(dSphs) of the Milky Way with the most massive substructure halos expected in a
CDM universe. We compare the predicted velocity dispersion profiles of Fornax
and Draco to observations, assuming that they are embedded in CDM halos. Models
with isotropic and tangentially anisotropic velocity distributions for the
stellar component fit the data only if the surrounding DM halos have maximum
circular velocities in the range 20-35 km/s. If the dSphs are embedded within
halos this large then the overabundance of satellites within the concordance
LCDM cosmological model is significantly alleviated, but this still does not
provide the entire solution.Comment: Accepted for publication in ApJ, 17 pages, 9 figures, LaTeX (uses
emulateapj5.sty
The formation of ultra-compact dwarf galaxies and nucleated dwarf galaxies
Ultra-compact dwarf galaxies (UCDs) have similar properties as massive globular clusters or the nuclei of nucleated galaxies. Recent observations suggesting a high dark matter content and a steep spatial distribution within groups and clusters provide new clues as to their origins. We perform high-resolution N-body/smoothed particle hydrodynamics simulations designed to elucidate two possible formation mechanisms for these systems: the merging of globular clusters in the centre of a dark matter halo, or the massively stripped remnant of a nucleated galaxy. Both models produce density profiles as well as the half-light radii that can fit the observational constraints. However, we show that the first scenario results to UCDs that are underluminous and contain no dark matter. This is because the sinking process ejects most of the dark matter particles from the halo centre. Stripped nuclei give a more promising explanation, especially if the nuclei form via the sinking of gas, funnelled down inner galactic bars, since this process enhances the central dark matter content. Even when the entire disc is tidally stripped away, the nucleus stays intact and can remain dark matter dominated even after severe stripping. Total galaxy disruption beyond the nuclei only occurs on certain orbits and depends on the amount of dissipation during nuclei formation. By comparing the total disruption of cold dark matter subhaloes in a cluster potential, we demonstrate that this model also leads to the observed spatial distribution of UCDs which can be tested in more detail with larger data set
Atmospheric effects of volcanic eruptions as seen by famous artists and depicted in their paintings
International audiencePaintings created by famous artists, representing sunsets throughout the period 1500?1900, provide proxy information on the aerosol optical depth following major volcanic eruptions. This is supported by a statistically significant correlation coefficient (0.8) between the measured red-to-green ratios of a few hundred paintings and the dust veil index. A radiative transfer model was used to compile an independent time series of aerosol optical depth at 550 nm corresponding to Northern Hemisphere middle latitudes during the period 1500?1900. The estimated aerosol optical depths range from 0.05 for background aerosol conditions, to about 0.6 following the Tambora and Krakatau eruptions and cover a period practically outside of the instrumentation era
Comparative breeding ecology of the little egret (Egretla g. garzetla) in the Axios delta (Greece) and the Camargue (France)
Des données sur la biologie de reproduction de l'Aigrette garzette (Egretta garzetta) sont comparées entre deux zones humides méditerranéennes d'importance internationale pour les oiseaux d'eau: la Camargue en France et le delta
de l'Axios en Grèce. Pendant l'incubation et l'élevage des poussins, les aigrettes de Camargue s'alimentent principalement dans les marais d'eau douce et, dans une moindre mesure, dans les lagunes d'eau saumâtre et salée. Dans l'Axios, les rizières constituent des lieux d'alimentation
particulièrement importants, les milieux d'eau saumâtre et salée se plaçant en deuxième position. Les différences dans l'exploitation des divers types de milieux d'alimentation apparaissent dans le régime des poussins. En Camargue, les poissons d'eau douce constituent l'essentiel (en nombre)
de leur alimentation alors qu'en Grèce, le régime est surtout composé d'invertébrés et de tétards provenant des rizières ainsi que de poissons d'eau salée. La taille des pontes est significativement plus élevée dans l'Axios qu'en Camargue. Ce résultat pourrait refléter une situation
trophique favorable offerte par les marais salants côtiers de l'Axios, principaux milieux exploités par les aigrettes pendant la saison pré-nuptiale, avant la mise en eau des rizières. En Camargue, les nichées sont significativement plus élevées que dans l'Axios, une proportion plus élevée
de nichées de quatre poussins ayant été enregistrée en France. De plus, à l'époque où les poussins ont besoin d'un maximum de nourriture pour survivre, vers deux semaines, ceux de Camargue sont en meilleure condition que ceux de Grèce. Par conséquent, à partir d'une ponte moins élevée que
celle enregistrée dans l'Axios, les aigrettes de Camargue élèvent plus de jeunes qui sont en meilleure condition à l'époque cruciale de l'élevage. Ces différences dans le succès de la reproduction sont liées à une situation trophique très favorable en Camargue en période d'élevage. Dans
l'Axios, il est essentiel que les rizières soient intégrées dans les programmes futurs de conservation de cette importante zone humide
Central mass and luminosity of Milky Way satellites in the LCDM model
It has been pointed out that the Galactic satellites all have a common mass
around 1e7 Msun within 300 pc (M03), while they span almost four order of
magnitudes in luminosity (Mateo et al. 1993, Strigari et al. 2008). It is
argued that this may reflect a specific scale for galaxy formation or a scale
for dark matter clustering. Here we use numerical simulations coupled with a
semi-analytic model for galaxy formation, to predict the central mass and
luminosity of galactic satellites in the LCDM model. We show that this common
mass scale can be explained within the Cold Dark Matter scenario when the
physics of galaxy formation is taken into account. The narrow range of M03
comes from the narrow distribution of circular velocities at time of accretion
(peaking around 20 km/s) for satellites able to form stars and the not tight
correlation between halo concentration and circular velocity. The wide range of
satellite luminosities is due to a combination of the mass at time of accretion
and the broad distribution of accretion redshifts for a given mass. This causes
the satellites baryonic content to be suppressed by photo-ionization to very
different extents. Our results favor the argument that the common mass M03
reflects a specific scale (circular velocity ~ 20 km/s) for star formation.Comment: 5 pages, 3 figures. References added, discussion enlarged, new
version of Figure 3. Minor changes to match the version accepted for
publication on ApJ Letter
Halo Shapes, Dynamics and Environment
In the hierarchical structure formation model cosmic halos are supposed to
form by accretion of smaller units along anisotropic direction, defined by
large-scale filamentary structures. After the epoch of primary mass aggregation
(which depend on the cosmological model), violent relaxation processes will
tend to alter the halo phase-space configuration producing quasi-spherical
halos with a relatively smooth density profiles.
Here we attempt to investigate the relation between halos shapes, their
environment and their dynamical state. To this end we have run a large ( Mpc, particles) N-body simulation of a flat low-density
cold dark matter model with a matter density , Hubble constant km s
Mpc and a normalization parameter of . The particle mass
is comparable to the mass of
one single galaxy. The halos are defined using a friends-of-friend algorithm
with a linking length given by where is the mean
density. This linking length corresponds to an overdensity at the present epoch () and the total number of halos
with more than 130 particles () is 57524.Comment: To be published in "Groups Of Galaxies In The Nearby Universe", held
in Chile, December 2005, edited by I.Saviane, V.Ivanov and J.Borissova.
Springer-Verlag series "ESO Astrophysics Symposia
Evolution of the Dark Matter Phase-Space Density Distributions of LCDM Halos
We study the evolution of phase-space density during the hierarchical
structure formation of LCDM halos. We compute both a spherically-averaged
surrogate for phase-space density (Q) and the coarse-grained distribution
function f(x,v) for dark matter particles that lie within~2 virial radii of
four Milky-Way-sized dark matter halos. The estimated f(x,v) spans over four
decades at any radius. Dark matter particles that end up within two virial
radii of a Milky-Way-sized DM halo at have an approximately Gaussian
distribution in log(f) at early redshifts, but the distribution becomes
increasingly skewed at lower redshifts. The value corresponding to the peak of
the Gaussian decreases as the evolution progresses and is well described by a
power-law in (1+z). The highest values of f are found at the centers of dark
matter halos and subhalos, where f can be an order of magnitude higher than in
the center of the main halo. The power-law Q(r) profile likely reflects the
distribution of entropy (K = sigma^2/rho^{2/3} \propto r^{1.2}), which dark
matter acquires as it is accreted onto a growing halo. The estimated f(x, v),
on the other hand, exhibits a more complicated behavior. Although the median
coarse-grained phase-space density profile F(r) can be approximated by a
power-law in the inner regions of halos and at larger radii the profile
flattens significantly. This is because phase-space density averaged on small
scales is sensitive to the high-f material associated with surviving subhalos,
as well as relatively unmixed material (probably in streams) resulting from
disrupted subhalos, which contribute a sizable fraction of matter at large
radii. (ABRIDGED)Comment: Closely matches version accepted for publicatio
The formation of ultra-compact dwarf galaxies and nucleated dwarf galaxies
Ultra compact dwarf galaxies (UCDs) have similar properties as massive
globular clusters or the nuclei of nucleated galaxies. Recent observations
suggesting a high dark matter content and a steep spatial distribution within
groups and clusters provide new clues as to their origins. We perform
high-resolution N-body / smoothed particle hydrodynamics simulations designed
to elucidate two possible formation mechanisms for these systems: the merging
of globular clusters in the centre of a dark matter halo, or the massively
stripped remnant of a nucleated galaxy. Both models produce density profiles as
well as the half light radii that can fit the observational constraints.
However, we show that the first scenario results to UCDs that are underluminous
and contain no dark matter. This is because the sinking process ejects most of
the dark matter particles from the halo centre. Stripped nuclei give a more
promising explanation, especially if the nuclei form via the sinking of gas,
funneled down inner galactic bars, since this process enhances the central dark
matter content. Even when the entire disk is tidally stripped away, the nucleus
stays intact and can remain dark matter dominated even after severe stripping.
Total galaxy disruption beyond the nuclei only occurs on certain orbits and
depends on the amount of dissipation during nuclei formation. By comparing the
total disruption of CDM subhaloes in a cluster potential we demonstrate that
this model also leads to the observed spatial distribution of UCDs which can be
tested in more detail with larger data sets.Comment: 8 pages, 8 figures, final version accepted for publication in MNRA
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