2,226 research outputs found
The Two Phases of Galaxy Formation
Cosmological simulations of galaxy formation appear to show a two-phase
character with a rapid early phase at z>2 during which in-situ stars are formed
within the galaxy from infalling cold gas followed by an extended phase since
z<3 during which ex-situ stars are primarily accreted. In the latter phase
massive systems grow considerably in mass and radius by accretion of smaller
satellite stellar systems formed at quite early times (z>3) outside of the
virial radius of the forming central galaxy. These tentative conclusions are
obtained from high resolution re-simulations of 39 individual galaxies in a
full cosmological context with present-day virial halo masses ranging from 7e11
M_sun h^-1 < M_vir < 2.7e13 M_sun h^-1 and central galaxy masses between 4.5e10
M_sun h^-1 < M_* < 3.6e11 M_sun h^-1. The simulations include the effects of a
uniform UV background, radiative cooling, star formation and energetic feedback
from SNII. The importance of stellar accretion increases with galaxy mass and
towards lower redshift. In our simulations lower mass galaxies (M_* > 1.7e11 M_sun h^-1) assembly is dominated by accretion and
merging with about 80 per cent of the stars added by the present-day. In
general the simulated galaxies approximately double their mass since z=1. For
massive systems this mass growth is not accompanied by significant star
formation. The majority of the in-situ created stars is formed at z>2,
primarily out of cold gas flows. We recover the observational result of
archaeological downsizing, where the most massive galaxies harbor the oldest
stars. We find that this is not in contradiction with hierarchical structure
formation. Most stars in the massive galaxies are formed early on in smaller
structures, the galaxies themselves are assembled late.Comment: 13 pages, 13 figures, accepted for publication in Ap
Latent Space Model for Multi-Modal Social Data
With the emergence of social networking services, researchers enjoy the
increasing availability of large-scale heterogenous datasets capturing online
user interactions and behaviors. Traditional analysis of techno-social systems
data has focused mainly on describing either the dynamics of social
interactions, or the attributes and behaviors of the users. However,
overwhelming empirical evidence suggests that the two dimensions affect one
another, and therefore they should be jointly modeled and analyzed in a
multi-modal framework. The benefits of such an approach include the ability to
build better predictive models, leveraging social network information as well
as user behavioral signals. To this purpose, here we propose the Constrained
Latent Space Model (CLSM), a generalized framework that combines Mixed
Membership Stochastic Blockmodels (MMSB) and Latent Dirichlet Allocation (LDA)
incorporating a constraint that forces the latent space to concurrently
describe the multiple data modalities. We derive an efficient inference
algorithm based on Variational Expectation Maximization that has a
computational cost linear in the size of the network, thus making it feasible
to analyze massive social datasets. We validate the proposed framework on two
problems: prediction of social interactions from user attributes and behaviors,
and behavior prediction exploiting network information. We perform experiments
with a variety of multi-modal social systems, spanning location-based social
networks (Gowalla), social media services (Instagram, Orkut), e-commerce and
review sites (Amazon, Ciao), and finally citation networks (Cora). The results
indicate significant improvement in prediction accuracy over state of the art
methods, and demonstrate the flexibility of the proposed approach for
addressing a variety of different learning problems commonly occurring with
multi-modal social data.Comment: 12 pages, 7 figures, 2 table
Life interrupted and life regained? Coping with stroke at a young age
Stroke is a leading cause of disability across the developed world, affecting an increasing number of younger people. In this article, we seek to understand the experience of stroke as a disabling life situation among young people and the strategies that they use to recover and cope. Directed content analysis was conducted from interviews with 17 community-dwelling stroke survivors aged 55 years and younger across the United Kingdom. The sample was drawn from a larger maximum variation sample of stroke survivors. Using the sociological concepts of biographical disruption and biographical repair as a guide, excerpts from the interviews pertaining to aspects of the patients’ life that were interrupted, in addition to how they coped with the changes, were selected and analysed. All individuals described an ‘‘altered sense of self,’’ a theme that included loss of identity, family disruption, and/or loss of valued activities. Individuals sought to adapt their sense of self by seeking external support, by restoring normality, and/or through positive reflection. Despite the adapted self that emerged, most individuals continued to experience impairments. While young stroke survivors adapt to their illness over time, they continue to experience impairments and disruptions in their personal and work lives.Aholistic model of rehabilitation that helps individuals regain the capacity for everyday activities related to work, family life, and leisure can begin to address the emotional ramifications of diseases such as stroke, restore wellness, and work towards minimizing the burden felt by family caregivers and children
A Network and Repository for Online Laboratory based on Ontology
Our propose is to build a network of virtual laboratories and also use it as a global repository of online laboratory’s and experiences. This set of virtual and online laboratories can be “stored” in an “virtual closet”, and also the system will allow us to build new experiences and online laboratories, and store them is this “virtual closet”. With the drawing of this new standard we pretend define methods for storing and retrieving learning objects for remote laboratories. The objective of this standard is also define methods for linking learning objects to design and implement smart learning environments for remote online laboratories. The objects defined by this standard are, for example, interfaces for devices connected to user computers over computers networks and the devices themselves. They are also learning scenarios or collaboration tools for communications necessary to conduct an activity of practical online laboratory work, they will allow to design and implement mechanisms that make smart learning environment formed by the ad hoc aggregation of learning objects taking always into account the pedagogical context for their use. This will allow to easy design and implement the pedagogically driven remote laboratory environment and experiments as also is learning environments. The experiences and laboratories are build using the parts and separate components that we have in a separate “virtual closet” with parts, components, and already build experiences. To build this complex network we need to find a system that supports effectively this structure. This probably will be a enormous database of v-labs and independent elements, where will be possible sometimes to “recycle” some of the elements. For this structure we propose an Ontology because it allows to “re-use” the same element several times in many experiences, and provide a very detailed description of each kind of element through is classes and sub-classes.Com o apoio RAADRI
Theory of High-Tc Superconductivity: Accurate Predictions of Tc
The superconducting transition temperatures of high-Tc compounds based on
copper, iron, ruthenium and certain organic molecules are discovered to be
dependent on bond lengths, ionic valences, and Coulomb coupling between
electronic bands in adjacent, spatially separated layers [1]. Optimal
transition temperature, denoted as T_c0, is given by the universal expression
; is the spacing between interacting
charges within the layers, \zeta is the distance between interacting layers and
\Lambda is a universal constant, equal to about twice the reduced electron
Compton wavelength (suggesting that Compton scattering plays a role in
pairing). Non-optimum compounds in which sample degradation is evident
typically exhibit Tc < T_c0. For the 31+ optimum compounds tested, the
theoretical and experimental T_c0 agree statistically to within +/- 1.4 K. The
elemental high Tc building block comprises two adjacent and spatially separated
charge layers; the factor e^2/\zeta arises from Coulomb forces between them.
The theoretical charge structure representing a room-temperature superconductor
is also presented.Comment: 7 pages 5 references, 6 figures 1 tabl
Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies and the epoch of reionization
We present the final results from our ultra-deep spectroscopic campaign with
FORS2 at the ESO/VLT for the confirmation of z~7 "z--band dropout" candidates
selected from our VLT/Hawk-I imaging survey over three independent fields. In
particular we report on two newly discovered galaxies at redshift ~6.7 in the
NTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs
of the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present
the results of ultra-deep observations of a sample of i-dropout galaxies, from
which we set a solid upper limit on the fraction of interlopers. Out of the 20
z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is
systematically below the expectations drawn on the basis of lower redshift
observations: in particular there is a significant lack of objects with
intermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for
the fraction of Ly-alpha emission in LBGs that is constantly increasing from
z~3 to z~6 is most probably reversed from z~6 to z~7.
Explaining the observed rapid change in the LAE fraction among the drop-out
population with reionization requires a fast evolution of the neutral fraction
of hydrogen in the Universe. Assuming that the Universe is completely ionized
at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we
find that our data require a change of the neutral hydrogen fraction of the
order Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape
fraction does not increase dramatically over the same redshift interval.Comment: Submitted to Ap
A Comparison of Polarization Observables in Electron Scattering from the Proton and Deuteron
Recoil proton polarization observables were measured for both the p(,e) and d(,en reactions at two values of Q using a newly commissioned proton
Focal Plane Polarimeter at the M.I.T.-Bates Linear Accelerator Center. The
hydrogen and deuterium spin-dependent observables and
, the induced polarization and the form factor ratio
were measured under identical kinematics. The deuterium and
hydrogen results are in good agreement with each other and with the plane-wave
impulse approximation (PWIA).Comment: 9 pages, 1 figure; accepted by Phys. Rev. Let
- …