398 research outputs found
Effects of asphericity and substructure on the determination of cluster mass with weak gravitational lensing
Weak gravitational lensing can be used to directly measure the mass along a
line-of-sight without any dependence on the dynamical state of the mass, and
thus can be used to measure the masses of clusters even if they are not
relaxed. One common technique used to measure cluster masses is fitting
azimuthally-averaged gravitational shear profiles with a spherical mass model.
In this paper we quantify how asphericity and projected substructure in
clusters can affect the virial mass and concentration measured with this
technique by simulating weak lensing observations on 30 independent
lines-of-sights through each of four high-resolution N-body cluster
simulations. We find that the variations in the measured virial mass and
concentration are of a size similar to the error expected in ideal weak lensing
observations and are correlated, but that the virial mass and concentration of
the mean shear profile agree well with that measured in three dimensional
models of the clusters. The dominant effect causing the variations is the
proximity of the line-of-sight to the major axis of the 3-D cluster mass
distribution, with projected substructure only causing minor perturbations in
the measured concentration. Finally we find that the best-fit "universal" CDM
models used to fit the shear profiles over-predict the surface density of the
clusters due to the cluster mass density falling off faster than the r^{-3}
model assumption.Comment: 10 pages, 10 figures, accepted by MNRA
The evolution of dwarf galaxies in the Coma supercluster
We employ spectroscopic and photometric data from SDSS DR7, in a 500 sq
degree region, to understand the evolution of dwarf (~M*+2<M_z<M*+4) galaxies
in the Coma supercluster (z=0.023). We show that in the Coma supercluster, the
red dwarfs are mostly concentrated in the dense cores of the Coma and Abell
1367 clusters, and in the galaxy groups embedded in the filament connecting
them. The post-starburst (k+A) dwarfs however are found in the infall regions
of the Coma and Abell 1367 clusters, and occasionally in galaxy groups embedded
along the filament, suggesting that strong velocity fields prevalent in the
vicinity of deep potential wells may be closely related to the mechanism(s)
leading to the post-starburst phase in dwarf galaxies. Moreover, the blue
colour of some k+A dwarfs in the Coma cluster, found within its virial radius,
suggests that the star formation in these galaxies was quenched very rapidly in
the last 500 Myr. More than 60% of all red dwarf galaxies in the supercluster
have 0-3 ang of H_\delta in absorption, which suggests that a major episode of
star formation occurred in a non-negligible fraction of these galaxies, ending
within the last Gyr, allowing them to move to the red sequence. The
distribution of the blue dwarf galaxies in the Coma supercluster is bimodal in
the EW(H_\alpha)-EW(H_\delta) plane, with one population having very high
emission in H_\alpha, and some emission in H_\delta. A sub-population of blue
dwarfs is coincident with the red dwarfs in the EW(H_\alpha)-EW(H_\delta)
plane, showing absorption in H_\delta and relatively lower emission in
H_\alpha. We suggest that a large fraction of the latter population are the
progenitors of the passive dwarf galaxies that are abundantly found in the
cores of low-redshift rich clusters such as Coma.Comment: 6 Pages, 5 Figures, Accepted for publication in MNRA
As redes sociais na internet e suas apropriações por jovens brasileiros e portugueses em idade escolar
O fenômeno das redes sociais on-line é marcante na atual fase da
internet 2.0, tendo crescido vertiginosamente, a partir do ano 2005, com
a adesão majoritária de jovens, que as acessam por computadores fixos e
móveis, em plataformas dos mais variados tipos. Por meio delas, trocam
mensagens e compartilham conteúdos os mais diversos. Diante desse
cenário, com base em duas pesquisas convergentes, pretende-se discutir
os usos e as apropriações das redes sociais on-line por jovens alunos do
ensino fundamental e médio e destacar os pontos mais pertinentes da
atual fase da internet nos contextos pessoal, familiar e escolar. Foram
aplicados questionários, inspirados em modelo de pesquisa elaborado na
Itália, a 404 alunos brasileiros de oito escolas no Rio de Janeiro e a 549
alunos portugueses de 11 escolas na região portuguesa de Castelo Branco.
Com essa rica empiria, verificou-se em que pontos os jovens se aproximam
dos ideais de uma nova subjetividade (o leitor imerso nas novas mÃdias) e
de um jovem naturalmente afeito aos suportes digitais (o nativo digital).The phenomenon of online social networks has been remarkable in the
current phase of internet 2.0 and has grown rapidly from the year 2005, with
the majority membership of young people, who access the social networks
from desktops and mobile platforms of all kinds. Using these platforms,
they exchange messages and share various contents. Given this scenario,
based on two converging researches, this paper discusses the uses and
appropriations of online social networks by students of elementary school
and high school. Also, it highlights the most relevant points in the current
phase of the internet in relation to personal, family and school contexts.
Inspired by a research model developed in Italy, 404 questionnaires were
applied to Brazilian students from eight schools in Rio de Janeiro and 549
Portuguese students from 11 schools in the Portuguese region of Castelo
Branco. With this comprehensive database, we could verify at which
points the youth approaches the ideals of a new subjectivity (the reader
immersed in new medias) and a youngster naturally used to digital media
(the digital native).info:eu-repo/semantics/publishedVersio
Velocity and spatial biases in CDM subhalo distributions
We present a statistical study of substructure within a sample of LCDM
clusters and galaxies simulated with up to 25 million particles. With thousands
of subhalos per object we can accurately measure their spatial clustering and
velocity distribution functions and compare these with observational data. The
substructure properties of galactic halos closely resembles those of galaxy
clusters with a small scatter in the mass and circular velocity functions. The
velocity distribution function is non-Maxwellian and flat topped with a
negative kurtosis of about -0.7. Within the virial radius the velocity bias
, increasing to b > 1.3
within the halo centers. Slow subhalos are much less common, due to physical
disruption by gravitational tides early in the merging history. This leads to a
spatially anti-biased subhalo distribution that is well fitted by a cored
isothermal. Observations of cluster galaxies do not show such biases which we
interpret as a limitation of pure dark matter simulations - we estimate that we
are missing half of the halo population which has been destroyed by physical
overmerging. High resolution hydrodynamical simulations are required to study
these issues further. If CDM is correct then the cluster galaxies must survive
the tidal field, perhaps due to baryonic inflow during elliptical galaxy
formation. Spirals can never exist near the cluster centers and the elliptical
galaxies there will have little remaining dark matter. This implies that the
morphology-density relation is set {\it before} the cluster forms, rather than
a subsequent transformation of disks to S0's by virtue of the cluster
environment.Comment: MNRAS accepted version. Due to an error in the initial conditions
these simulations have a lower sigma_8 than the published value, 0.7 instead
of 0.9. We thank Mike Kuhlen who helped us finding this mistake. See the
erratum at http://www-theorie.physik.unizh.ch/~diemand/suberr.pdf . Images
and movies available at http://www-theorie.physik.unizh.ch/~diemand/clusters
Galactic stellar haloes in the CDM model
We present six simulations of galactic stellar haloes formed by the tidal disruption of accreted dwarf galaxies in a fully cosmological setting. Our model is based on the Aquarius project, a suite of high-resolution N-body simulations of individual dark matter haloes. We tag subsets of particles in these simulations with stellar populations predicted by the galform semi-analytic model. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function (LF) and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate. We find that accreted stellar haloes are assembled between 1 < z < 7 from less than five significant progenitors. These progenitors are old, metal-rich satellites with stellar masses similar to the brightest Milky Way dwarf spheroidals (107–108 M⊙). In contrast to previous stellar halo simulations, we find that several of these major contributors survive as self-bound systems to the present day. Both the number of these significant progenitors and their infall times are inherently stochastic. This results in great diversity among our stellar haloes, which amplifies small differences between the formation histories of their dark halo hosts. The masses (∼ 108–109 M⊙) and density/surface-brightness profiles of the stellar haloes (from 10 to 100 kpc) are consistent with expectations from the Milky Way and M31. Each halo has a complex structure, consisting of well-mixed components, tidal streams, shells and other subcomponents. This structure is not adequately described by smooth models. The central regions (<10 kpc) of our haloes are highly prolate (c/a∼ 0.3), although we find one example of a massive accreted thick disc. Metallicity gradients in our haloes are typically significant only where the halo is built from a small number of satellites. We contrast the ages and metallicities of halo stars with surviving satellites, finding broad agreement with recent observations
Substructures in Cold Dark Matter Haloes
We analyse the properties of substructures within dark matter halos
(subhalos) using a set of high-resolution numerical simulations of the
formation of structure in a Lambda-CDM Universe. Our simulation set includes 11
high-resolution simulations of massive clusters as well as a region of mean
density, allowing us to study the spatial and mass distribution of
substructures down to a mass resolution limit of 10^9 h^(-1)Mo. We also
investigate how the properties of substructures vary as a function of the mass
of the `parent' halo in which they are located. We find that the substructure
mass function depends at most weakly on the mass of the parent halo and is well
described by a power-law. The radial number density profiles of substructures
are steeper in low mass halos than in high mass halos. More massive
substructures tend to avoid the centres of halos and are preferentially located
in the external regions of their parent halos. We also study the mass accretion
and merging histories of substructures, which we find to be largely independent
of environment. We find that a significant fraction of the substructures
residing in clusters at the present day were accreted at redshifts z < 1. This
implies that a significant fraction of present-day `passive' cluster galaxies
should have been still outside the cluster progenitor and more active at z~1.Comment: 13 pages, 15 figure. Accepted to MNRA
The MultiDark Database: Release of the Bolshoi and MultiDark Cosmological Simulations
We present the online MultiDark Database -- a Virtual Observatory-oriented,
relational database for hosting various cosmological simulations. The data is
accessible via an SQL (Structured Query Language) query interface, which also
allows users to directly pose scientific questions, as shown in a number of
examples in this paper. Further examples for the usage of the database are
given in its extensive online documentation (www.multidark.org). The database
is based on the same technology as the Millennium Database, a fact that will
greatly facilitate the usage of both suites of cosmological simulations. The
first release of the MultiDark Database hosts two 8.6 billion particle
cosmological N-body simulations: the Bolshoi (250/h Mpc simulation box, 1/h kpc
resolution) and MultiDark Run1 simulation (MDR1, or BigBolshoi, 1000/h Mpc
simulation box, 7/h kpc resolution). The extraction methods for halos/subhalos
from the raw simulation data, and how this data is structured in the database
are explained in this paper. With the first data release, users get full access
to halo/subhalo catalogs, various profiles of the halos at redshifts z=0-15,
and raw dark matter data for one time-step of the Bolshoi and four time-steps
of the MultiDark simulation. Later releases will also include galaxy mock
catalogs and additional merging trees for both simulations as well as new large
volume simulations with high resolution. This project is further proof of the
viability to store and present complex data using relational database
technology. We encourage other simulators to publish their results in a similar
manner.Comment: 28 pages, 9 figures, submitted to New Astronom
The impact of Early Dark Energy on non-linear structure formation
We study non-linear structure formation in high-resolution simulations of
Early Dark Energy (EDE) cosmologies and compare their evolution with the
standard LCDM model. Extensions of the spherical top-hat collapse model predict
that the virial overdensity and linear threshold density for collapse should be
modified in EDE model, yielding significant modifications in the expected halo
mass function. Here we present numerical simulations that directly test these
expectations. Interestingly, we find that the Sheth & Tormen formalism for
estimating the abundance of dark matter halos continues to work very well in
its standard form for the Early Dark Energy cosmologies, contrary to analytic
predictions. The residuals are even slightly smaller than for LCDM. We also
study the virial relationship between mass and dark matter velocity dispersion
in different dark energy cosmologies, finding excellent agreement with the
normalization for Lambda as calibrated by Evrard et al.(2008). The earlier
growth of structure in EDE models relative to LCDM produces large differences
in the mass functions at high redshift. This could be measured directly by
counting groups as a function of the line-of-sight velocity dispersion,
skirting the ambiguous problem of assigning a mass to the halo. Using dark
matter substructures as a proxy for member galaxies, we demonstrate that even
with 3-5 members sufficiently accurate measurements of the halo velocity
dispersion function are possible. Finally, we determine the concentration-mass
relationship for our EDE cosmologies. Consistent with the earlier formation
time, the EDE halos show higher concentrations at a given halo mass. We find
that the magnitude of the difference in concentration is well described by the
prescription of Eke et al.(2001) for estimating halo concentrations.Comment: 17 pages,17 figure
Supramolecular macrocycles reversibly assembled by Te ⋯ O chalcogen bonding
Organic molecules with heavy main-group elements frequently form supramolecular links to electron-rich centres. One particular case of such interactions is halogen bonding. Most studies of this phenomenon have been concerned with either dimers or infinitely extended structures (polymers and lattices) but well-defined cyclic structures remain elusive. Here we present oligomeric aggregates of heterocycles that are linked by chalcogen-centered interactions and behave as genuine macrocyclic species. The molecules of 3-methyl-5-phenyl-1,2-tellurazole 2-oxide assemble a variety of supramolecular aggregates that includes cyclic tetramers and hexamers, as well as a helical polymer. In all these aggregates, the building blocks are connected by Te(…)O-N bridges. Nuclear magnetic resonance spectroscopic experiments demonstrate that the two types of annular aggregates are persistent in solution. These self-assembled structures form coordination complexes with transition-metal ions, act as fullerene receptors and host small molecules in a crystal
From dwarf spheroidals to cDs: Simulating the galaxy population in a LCDM cosmology
We apply updated semi-analytic galaxy formation models simultaneously to the
stored halo/subhalo merger trees of the Millennium and Millennium-II
simulations. These differ by a factor of 125 in mass resolution, allowing
explicit testing of resolution effects on predicted galaxy properties. We have
revised the treatments of the transition between the rapid infall and cooling
flow regimes of gas accretion, of the sizes of bulges and of gaseous and
stellar disks, of supernova feedback, of the transition between central and
satellite status as galaxies fall into larger systems, and of gas and star
stripping once they become satellites. Plausible values of efficiency and
scaling parameters yield an excellent fit not only to the observed abundance of
low-redshift galaxies over 5 orders of magnitude in stellar mass and 9
magnitudes in luminosity, but also to the observed abundance of Milky Way
satellites. This suggests that reionisation effects may not be needed to solve
the "missing satellite" problem except, perhaps, for the faintest objects. The
same model matches the observed large-scale clustering of galaxies as a
function of stellar mass and colour. The fit remains excellent down to ~30kpc
for massive galaxies. For M* < 6 x 10^10Msun, however, the model overpredicts
clustering at scales below 1 Mpc, suggesting that the sigma_8 adopted in the
simulations (0.9) is too high. Galaxy distributions within rich clusters agree
between the simulations and match those observed, but only if galaxies without
dark matter subhalos (so-called orphans) are included. Our model predicts a
larger passive fraction among low-mass galaxies than is observed, as well as an
overabundance of ~10^10Msun galaxies beyond z~0.6, reflecting deficiencies in
the way star-formation rates are modelled.Comment: Accepted for publication in MNRAS. SQL databases containing the full
galaxy data at all redshifts and for both the Millennium and Millennium-II
simulations are publicly released at
http://www.mpa-garching.mpg.de/millenniu
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