25,832 research outputs found
Fingerprints of the Hierarchical Building up of the Structure on the Mass-Metallicity Relation
We study the mass-metallicity relation of galactic systems with stellar
masses larger than 10^9 Mo in Lambda-CDM scenarios by using chemical
hydrodynamical simulations. We find that this relation arises naturally as a
consequence of the formation of the structure in a hierarchical scenario. The
hierarchical building up of the structure determines a characteristic stellar
mass at M_c ~10^10.2 Moh^-1 which exhibits approximately solar metallicities
from z ~ 3 to z=0. This characteristic mass separates galactic systems in two
groups with massive ones forming most of their stars and metals at high
redshift. We find evolution in the zero point and slope of the mass-metallicity
relation driven mainly by the low mass systems which exhibit the larger
variations in the chemical properties. Although stellar mass and circular
velocity are directly related, the correlation between circular velocity and
metallicity shows a larger evolution with redshift making this relation more
appropriate to confront models and observations. The dispersion found in both
relations is a function of the stellar mass and reflects the different
dynamical history of evolution of the systems.Comment: 4 pages, 4 figures. Accepted MNRAS Letter
Clues for the origin of the fundamental metallicity relations. I: The hierarchical building up of the structure
We analyse the evolutionary history of galaxies formed in a hierarchical
scenario consistent with the concordance -CDM model focusing on the
study of the relation between their chemical and dynamical properties. Our
simulations consistently describe the formation of the structure and its
chemical enrichment within a cosmological context. Our results indicate that
the luminosity-metallicity (LZR) and the stellar mass-metallicity (MZR)
relations are naturally generated in a hierarchical scenario. Both relations
are found to evolve with redshift. In the case of the MZR, the estimated
evolution is weaker than that deduced from observational works by approximately
0.10 dex. We also determine a characteristic stellar mass, , which segregates the simulated galaxy population
into two distinctive groups and which remains unchanged since , with a
very weak evolution of its metallicity content. The value and role played by
is consistent with the characteristic mass estimated from the SDSS galaxy
survey by Kauffmann et al. (2004). Our findings suggest that systems with
stellar masses smaller than are responsible for the evolution of this
relation at least from . Larger systems are stellar dominated and
have formed more than 50 per cent of their stars at , showing very
weak evolution since this epoch. We also found bimodal metallicity and age
distributions from , which reflects the existence of two different
galaxy populations. Although SN feedback may affect the properties of galaxies
and help to shape the MZR, it is unlikely that it will significantly modify
since, from this stellar mass is found in systems with circular
velocities larger than 100 \kms.Comment: 17 pages, 13 figures. Minor changes to match accepted version.
Accepted October 3 MNRA
Milky Way type galaxies in a LCDM cosmology
We analyse a sample of 52,000 Milky Way (MW) type galaxies drawn from the
publicly available galaxy catalogue of the Millennium Simulation with the aim
of studying statistically the differences and similarities of their properties
in comparison to our Galaxy. Model galaxies are chosen to lie in haloes with
maximum circular velocities in the range 200-250 km/seg and to have
bulge-to-disk ratios similar to that of the Milky Way. We find that model MW
galaxies formed quietly through the accretion of cold gas and small satellite
systems. Only 12 per cent of our model galaxies experienced a major merger
during their lifetime. Most of the stars formed in situ, with only about 15 per
cent of the final mass gathered through accretion. Supernovae and AGN feedback
play an important role in the evolution of these systems. At high redshifts,
when the potential wells of the MW progenitors are shallower, winds driven by
supernovae explosions blow out a large fraction of the gas and metals. As the
systems grow in mass, SN feedback effects decrease and AGN feedback takes over,
playing a more important role in the regulation of the star formation activity
at lower redshifts. Although model Milky Way galaxies have been selected to lie
in a narrow range of maximum circular velocities, they nevertheless exhibit a
significant dispersion in the final stellar masses and metallicities. Our
analysis suggests that this dispersion results from the different accretion
histories of the parent dark matter haloes. Statically, we also find evidences
to support the Milky Way as a typical Sb/Sc galaxy in the same mass range,
providing a suitable benchmark to constrain numerical models of galaxy
formationComment: 10 pages, 7 figures, mne2.cls, MNRAS, replaced with accepted versio
Vibrations of free and embedded anisotropic elastic spheres: Application to low-frequency Raman scattering of silicon nanoparticles in silica
Vibrational mode frequencies and damping are calculated for an elastic sphere
embedded in an infinite, homogeneous, isotropic elastic medium. Anisotropic
elasticity of the sphere significantly shifts the frequencies in comparison to
simplified calculations that assume isotropy. New low frequency Raman light
scattering data are presented for silicon spheres grown in a SiO2 glass matrix.
Principal features of the Raman spectrum are not correctly described by a
simple model of the nanoparticle as a free, isotropic sphere, but require both
matrix effects and the anisotropy of the silicon to be taken into account.
Libration, not vibration, is the dominant mechanism
An Imperative for Change: Bridging Special and Language Learning Education to Ensure a Free and Appropriate Education in the Least Restrictive Environment for ELLs with Disabilities in Massachusetts
English Language Learners (ELLs) are the fastest-growing group of school-age students in public schools across the nation, and in Massachusetts. In this state, even as the total student enrollment declines slightly, the number of ELLs grows steeply. They number 68,820 in the 2010-2011 school year, an increase of 9,662 from the year before.
The number of ELLs identified as also having a disability doubled in Massachusetts (a striking increase of 115.4%) from 2001-2002 to 2010-2011. The proportion of ELLs placed in Special Education has increased by 5 percentage points, from 9.8% to 14.8%. This time period coincides almost exactly with the implementation of Question 2 (a public referendum approved by voters in 2002), which changed the state’s primary Language Learning Education policy from Transitional Bilingual Education to Sheltered English immersion, a language-restrictive policy similar to those in California and Arizona.
Misrepresentation of language minorities in Special Education has been a major problem in education for many years in the United States, including Massachusetts
Superscaling in electron- and neutrino-nucleus scattering
The superscaling properties of electron scattering data are used to extract
model-independent predictions for neutrino-nucleus cross sections.Comment: Contibution to NuInt05, 4th international workshop on
neutrino-nucleus interaction in the few GeV region, Sept. 26 - 29 2005,
Okayama, Japa
Calculation of Field Quality in Fast-Ramping Superconducting Magnets
Fast-ramping superconducting (SC) accelerator magnets are the subject of R&D efforts at various laboratories. The simulation of field quality in fast-ramping magnets requires modifications of magnet design tools such as the CERN field computation program ROXIE. In this paper we present the efforts towards dynamic 2-D simulations of fast-ramping SC magnets. Models for persistent currents, inter-strand coupling currents, inter-filament coupling currents, and for eddy-currents in conducting coil-wedges are described and validated
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