617 research outputs found
Hierarchical Model for the Evolution of Cloud Complexes
The structure of cloud complexes appears to be well described by a "tree
structure" representation when the image is partitioned into "clouds". In this
representation, the parent-child relationships are assigned according to
containment. Based on this picture, a hierarchical model for the evolution of
Cloud Complexes, including star formation, is constructed, that follows the
mass evolution of each sub-structure by computing its mass exchange
(evaporation or condensation) with its parent and children, which depends on
the radiation density at the interphase. For the set of parameters used as a
reference model, the system produces IMFs with a maximum at too high mass (~2
M_sun) and the characteristic times for evolution seem too long. We show that
these properties can be improved by adjusting model parameters. However, the
emphasis here is to illustrate some general properties of this nonlinear model
for the star formation process. Notwithstanding the simplifications involved,
the model reveals an essential feature that will likely remain if additional
physical processes are included. That is: the detailed behavior of the system
is very sensitive to variations on the initial and external conditions,
suggesting that a "universal" IMF is very unlikely. When an ensemble of IMFs
corresponding to a variety of initial or external conditions is examined, the
slope of the IMF at high masses shows variations comparable to the range
derived from observational data. (Abridged)Comment: Latex, 29 pages, 13 figures, accepted for publication in Ap
Damped Lyman Alpha Systems at z<1.65: The Expanded SDSS HST Sample
We present results of our HST Cycle 11 Survey for low-redshift (z<1.65) DLAs
in the UV spectra of quasars selected from the SDSS Early Data Release. These
quasars have strong intervening MgII-FeII systems which are known signatures of
high column density neutral gas. In total, UV observations of Ly-alpha
absorption in 197 MgII systems with z<1.65 and rest equivalent width (REW)
W2796 \ge 0.3A have now been obtained. The main results are: (1) 36(+/- 6)% of
systems with W2796 \ge 0.5 A and FeII W2600 \ge 0.5 A are DLAs. This increases
to 42(+/- 7)% for systems with W2796/W2600 0.1 A. (2) The
mean N(HI) of MgII systems with 0.3 A \le W2796 < 0.6 A is a factor of ~36
lower than that of systems with W2796 \ge 0.6 A. (3) The DLA incidence per unit
redshift is consistent with no evolution for z <~ 2 (Omega_L=0.7, Omega_M =
0.3), but exhibits significant evolution for z >~ 2. (4) Omega_{DLA} is
constant for 0.5<z<5.0 to within the uncertainties. This is larger than
Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution
does not change significantly with redshift. However, the low redshift
distribution is marginally flatter due to the higher fraction of high N(HI)
systems in our sample. (6) Finally, using the precision of MgII survey
statistics, we find that there may be evidence of a decreasing Omega_{DLA} from
z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very
high columns of neutral gas might be missed by DLA surveys because of their
very small cross sections, and therefore, that Omega_{DLA} might not include
the bulk of the neutral gas mass in the Universe. (Abridged)Comment: Accepted for publication in ApJ. 22 pages, 22 figure
RecomendaciĂłn de objetos de aprendizaje con base en opiniones escritas por usuarios
Actualmente la cantidad de recursos educativos disponibles en Internet ha crecido hasta límites insospechados, dando lugar al problema de sobrecarga de información. La tarea de buscar recursos que sean relevantes para los usuarios se ha convertido en una tarea tediosa. Con el fin de facilitarles la tarea a los usuarios y presentarles solo los recursos que ellos necesitan se implementan los sistemas recomendadores en el dominio de e-learning. Los métodos actuales de recomendación necesitan que los usuarios valoren los objetos de aprendizaje, muchos usuarios son reacios a valorarlo de forma explícita y el éxito de las recomendaciones depende de la cantidad de valoraciones obtenidas. Es común que en lugar de completar formularios con valoraciones de dichos objetos, muchos usuarios prefieran usar el lenguaje natural y expresar sus opiniones sobre ellos en forma de texto libre, similar a una conversación con un amigo. En este artículo se presenta un mecanismo que formaliza el proceso de selección y la recuperación de las opiniones textuales sobre objetos de aprendizaje y la utilización de esas opiniones para la recomendación de recursos educativos. Los resultados obtenidos demuestran que los usuarios se sienten más satisfechos con recomendaciones soportadas con base en el juicio de otras personas
String amplitudes in arbitrary dimensions
We calculate gravitational dressed tachyon correlators in non critcal
dimensions. The 2D gravity part of our theory is constrained to constant
curvature. Then scaling dimensions of gravitational dressed vertex operators
are equal to their bare conformal dimensions. Considering the model as d+2
dimensional critical string we calculate poles of generalized Shapiro-Virasoro
amplitudes.Comment: 14 page
Opinion Dynamics of Learning Agents: Does Seeking Consensus Lead to Disagreement?
We study opinion dynamics in a population of interacting adaptive agents
voting on a set of complex multidimensional issues. We consider agents which
can classify issues into for or against. The agents arrive at the opinions
about each issue in question using an adaptive algorithm. Adaptation comes from
learning and the information for the learning process comes from interacting
with other neighboring agents and trying to change the internal state in order
to concur with their opinions. The change in the internal state is driven by
the information contained in the issue and in the opinion of the other agent.
We present results in a simple yet rich context where each agent uses a Boolean
Perceptron to state its opinion. If there is no internal clock, so the update
occurs with asynchronously exchanged information among pairs of agents, then
the typical case, if the number of issues is kept small, is the evolution into
a society thorn by the emergence of factions with extreme opposite beliefs.
This occurs even when seeking consensus with agents with opposite opinions. The
curious result is that it is learning from those that hold the same opinions
that drives the emergence of factions. This results follows from the fact that
factions are prevented by not learning at all from those agents that hold the
same opinion. If the number of issues is large, the dynamics becomes trapped
and the society does not evolve into factions and a distribution of moderate
opinions is observed. We also study the less realistic, but technically simpler
synchronous case showing that global consensus is a fixed point. However, the
approach to this consensus is glassy in the limit of large societies if agents
adapt even in the case of agreement.Comment: 16 pages, 10 figures, revised versio
Triggers for displaced decays of long-lived neutral particles in the ATLAS detector
A set of three dedicated triggers designed to detect long-lived neutral particles decaying throughout the ATLAS detector to a pair of hadronic jets is described. The efficiencies of the triggers for selecting displaced decays as a function of the decay position are presented for simulated events. The effect of pile-up interactions on the trigger efficiencies and the dependence of the trigger rate on instantaneous luminosity during the 2012 data-taking period at the LHC are discussedFil: Aad, G.. Albert Ludwigs UniversitĂ€t; AlemaniaFil: Abajyan, T.. Universitaet Bonn; AlemaniaFil: Abbott, B.. University of Oklahoma; Estados UnidosFil: Abdallah, J.. Universitat AutĂČnoma de Barcelona; EspañaFil: Abdel Khalek, S.. Universite Paris Sud; FranciaFil: Alconada Verzini, MarĂa Josefina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de FĂsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FĂsica La Plata; ArgentinaFil: Alonso, Francisco. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de FĂsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FĂsica La Plata; ArgentinaFil: Anduaga, Xabier Sebastian. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de FĂsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FĂsica La Plata; ArgentinaFil: Dova, Maria Teresa. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de FĂsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FĂsica La Plata; ArgentinaFil: GonzĂĄlez Silva, MarĂa Laura. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Monticelli, Fernando Gabriel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de FĂsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FĂsica La Plata; ArgentinaFil: Otero y Garzon, Gustavo Javier. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Piegaia, Ricardo Nestor. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Romeo, Gaston Leonardo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Tripiana, Martin Fernando. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - La Plata. Instituto de FĂsica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de FĂsica La Plata; ArgentinaFil: Zhuang, X.. Ludwig Maximilians Universitat; AlemaniaFil: Zhuravlov, V.. Max-Planck Institut fĂŒr Physik; AlemaniaFil: Zieminska, D.. Indiana University; Estados UnidosFil: Zimin, N. I.. Joint Institute for Nuclear Research; RusiaFil: Zimmermann, R.. Universitaet Bonn; AlemaniaFil: Zimmermann, S.. Universitaet Bonn; AlemaniaFil: Zimmermann, S.. Albert Ludwigs UniversitĂ€t; AlemaniaFil: Ziolkowski, M.. UniversitĂ€t Siegen; AlemaniaFil: Zitoun, R.. UniversitĂ© de Savoie; FranciaFil: ĆœivkoviÄ, L.. Columbia University; Estados UnidosFil: Zmouchko, V. V.. State Research Center Institute for High Energy Physics; RusiaFil: Zobernig, G.. University of Wisconsin; Estados UnidosFil: Zoccoli, A.. UniversitĂ di Bologna; ItaliaFil: zur Nedden, M.. Humboldt University; AlemaniaFil: Zutshi, V.. Northern Illinois University; Estados Unido
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Identification of a 6-cM Minimal Deletion at 11q23.1â23.2 and Exclusion of PPP2R1B Gene as a Deletion Target in Cervical Cancer
Previous functional and deletion mapping studies on cervical cancer (CC) have implicated one or more tumor suppressor genes (TSGs) on chromosome 11 at q13 and q22â24 regions. Of these, the 11q22â24 region exhibits frequent allelic deletions in a variety of solid tumor types, sug- gesting the presence of critical genes for tumor suppression in this region. However, the precise region of deletion on 11q is not clearly defined in CC. In an attempt to accurately map the deleted region, we performed an extensive loss of heterozygosity (LOH) mapping in 58 tumors using 25 polymorphic loci on both the short and long arms. The pattern of LOH identified three sites of deletions, two on 11p (p15.11âp15.3 and p12â13), and one on 11q (q23.1âq23.2). The 11q23.1âq23.2 exhibited highest fre- quency (60.6%) of deletions, suggesting that this could be the site of a candidate TSG in CC. The minimal deletion at 11q23.1â23.2 was re- stricted to a 6-cM region between 123.5 and 129.5 cM genetic distance on chromosome 11, identifying the site of a potential TSG important in the pathogenesis of CC. At least five known genes and 28 UniGene clusters were mapped to the present commonly deleted region. In addition, we have excluded a previously known TSG PPP2R1B at 11q23 as a deletion target in CC. The definition of the minimal deletion and the availability of expressed sequence resources should facilitate the identification of the candidate TSG
Fluxes of atmospheric muons underwater depending on the small-x gluon density
The prompt muon contribution to the deep-sea atmospheric muon flux can serve
as a tool for probing into the small-x feature of the gluon density inside of a
nucleon, if the muon energy threshold could be lifted to 100 TeV. The prompt
muon flux underwater is calculated taking into consideration predictions of
recent charm production models in which the small-x behaviour of the gluon
distribution is probed. We discuss the possibility of distinguishing the PQCD
models of the charm production differing in the small-x exponent of the gluon
distribution, in measurements of the muon flux at energies 10-100 TeV with
neutrino telescopes.Comment: 9 pages, 4 eps figures, uses iopart.st
The Case for a Low Extragalactic Gamma-ray Background
Measurements of the diffuse extragalactic gamma-ray background (EGRB) are
complicated by a strong Galactic foreground. Estimates of the EGRB flux and
spectrum, obtained by modeling the Galactic emission, have produced a variety
of (sometimes conflicting) results. The latest analysis of the EGRET data found
an isotropic flux I_x=1.45+-0.05 above 100 MeV, in units of 10^-5 s^-1 cm^-2
sr^-1. We analyze the EGRET data in search for robust constraints on the EGRB
flux, finding the gamma-ray sky strongly dominated by Galactic foreground even
at high latitudes, with no conclusive evidence for an additional isotropic
component. The gamma-ray intensity measured towards the Galactic poles is
similar to or lower than previous estimates of I_x. The high latitude profile
of the gamma-ray data is disk-like for 40<|b[deg]|<70, and even steeper for
|b|>70; overall it exhibits strong Galactic features and is well fit by a
simple Galactic model. Based on the |b|>40 data we find that I_x<0.5 at a 99%
confidence level, with evidence for a much lower flux. We show that
correlations with Galactic tracers, previously used to identify the Galactic
foreground and estimate I_x, are not satisfactory; the results depend on the
tracers used and on the part of the sky examined, because the Galactic emission
is not linear in the Galactic tracers and exhibits spectral variations across
the sky. The low EGRB flux favored by our analysis places stringent limits on
extragalactic scenarios involving gamma-ray emission, such as radiation from
blazars, intergalactic shocks and production of ultra-high energy cosmic rays
and neutrinos. We suggest methods by which future gamma-ray missions such as
GLAST and AGILE could indirectly identify the EGRB.Comment: Accepted for publication in JCAP. Increased sizes of polar regions
examined, and added discussion of spectral data. Results unchange
GMRT mini-survey to search for 21-cm absorption in Quasar-Galaxy Pairs at z~0.1
We present the results from our 21-cm absorption survey of a sample of 5
quasar-galaxy pairs (QGPs), with the redshift of the galaxies in the range
0.03<zg<0.18, selected from the SDSS. The HI 21-cm absorption was searched
towards the 9 sight lines with impact parameters ranging from 10 to 55 kpc
using GMRT. 21-cm absorption was detected only in one case i.e. towards the
Quasar (zq=2.625 SDSS J124157.54+633241.6)-galaxy (zg=0.143 SDSS
J124157.26+633237.6) pair with the impact parameter 11 kpc. The quasar sight
line in this case pierces through the stellar disk of a galaxy having near
solar metallicity (i.e (O/H)+12=8.7) and star formation rate uncorrected for
dust attenuation of 0.1 M_odot/yr. The quasar spectrum reddened by the
foreground galaxy is well fitted with the Milky Way extinction curve (with an
Av of 0.44) and the estimated HI column density is similar to the value
obtained from 21-cm absorption assuming spin temperature of 100K. Combining our
sample with the z<0.1 data available in the literature, we find the
detectability of 21-cm absorption with integrated optical depth greater than
0.1 km\s to be 50% for the impact parameter less than 20 kpc. Using the surface
brightness profiles and relationship between the optical size and extent of the
HI disk known for nearby galaxies, we conclude that in most of the cases of
21-cm absorption non-detection, the sight lines may not be passing through the
HI gas. We also find that in comparison to the absorption systems associated
with these QGPs, z<1 DLAs with 21-cm absorption detections have lower CaII
equivalent widths despite having higher 21-cm optical depths and smaller impact
parameters. This suggests that the current sample of DLAs may be a biased
population that avoids sight lines through dusty star-forming galaxies. A
systematic survey of QGPs is needed to confirm these findings and understand
the nature of 21-cm absorbers.Comment: 17 pages, 5 tables, 19 figures, accepted for publication in MNRAS
(abstract abridged
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