3,224 research outputs found
On the formation of dwarf galaxies and stellar halos
Using analytic arguments and a suite of very high resolution (10^3 Msun per
particle) cosmological hydro-dynamical simulations, we argue that high
redshift, z ~ 10, M ~ 10^8 Msun halos, form the smallest `baryonic building
block' (BBB) for galaxy formation. These halos are just massive enough to
efficiently form stars through atomic line cooling and to hold onto their gas
in the presence of supernovae winds and reionisation. These combined effects,
in particular that of the supernovae feedback, create a sharp transition: over
the mass range 3-10x10^7 Msun, the BBBs drop two orders ofmagnitude in stellar
mass. Below ~2x10^7 Msun, galaxies will be dark with almost no stars and no
gas. Above this scale is the smallest unit of galaxy formation: the BBB.
A small fraction (~100) of these gas rich BBBs fall in to a galaxy the size
of the Milky Way. Ten percent of these survive to become the observed LG dwarf
galaxies at the present epoch. Those in-falling halos on benign orbits which
keep them far away from the Milky Way or Andromeda manage to retain their gas
and slowly form stars - these become the smallest dwarf irregular galax ies;
those on more severe orbits lose their gas faster than they can form stars and
become the dwarf spheroidals. The remaining 90% of the BBBs will be accreted.
We show that this gives a metallicity and total stellar mass consistent with
the Milky Way old stellar halo (abridged).Comment: 15 pages, 7 figures, one figure added to match accepted version. Some
typos fixed. MNRAS in pres
The Effects of Direct Instruction in Writing on English Speakers and English Language Learners with Disabilities
Many students struggle on writing tasks with little success because writing is a complex task. Students with learning disabilities (LD) and students who are served in English to Speakers of Other Language (ESOL) programs generally perform at lower rates on writing tasks than their English-only speaking peers without disabilities. Several researchers indicate that students with disabilities may be able to improve their performance on writing tasks through the implementation of Direct Instruction writing programs. The purpose of the current study was to demonstrate the relationship between the implementation of an accelerated Direct Instruction program and student writing performance with students who have LD in written expression, and who are either English-only speaking or native Spanish-speaking. Specifically, using a multiple-probe across participants single-subject research design, two groups of students received instruction using the Expressive Writing program. One group of three students were concurrently served in both special education for learning disabilities in the area of written expression and in a program for students who were English Language Learners (ELL), and another group of three were native English speakers who had learning disabilities in written expression. Students were divided into two separate groups to determine the effect of an abbreviated instructional sequence on both groups of students as the language background differences between the two groups did not allow them to be examined as one distinct group. The effects of instruction were measured by analyzing the number of correct word sequences, the number of words, and the types of errors when students were given three-minute writing probes. Additionally, performance on the Test of Written Language (3rd edition) (TOWL-3) and a classroom generalization measure were examined. Results indicated that when only half of the total lessons were presented to the students in both groups, the number of correct word sequences and the total number of words written increased on within-program writing probes, the TOWL-3, and on a generalization measure. An implication of these results is that adolescent students with writing deficits may be able improve their basic writing skills using half of the total program. Such a finding is important because students at this level who have not yet acquired these skills must quickly acquire them to be able to develop the more sophisticated skills required of students at their grade level. Limitations included the lack of the in-program placement test being proctored at the end of the intervention and the use of a nonconcurrent baseline with the second group of students
The effect of neutrinos on the matter distribution as probed by the Intergalactic Medium
We present a suite of full hydrodynamical cosmological simulations that
quantitatively address the impact of neutrinos on the (mildly non-linear)
spatial distribution of matter and in particular on the neutral hydrogen
distribution in the Intergalactic Medium (IGM), which is responsible for the
intervening Lyman-alpha absorption in quasar spectra. The free-streaming of
neutrinos results in a (non-linear) scale-dependent suppression of power
spectrum of the total matter distribution at scales probed by Lyman-alpha
forest data which is larger than the linear theory prediction by about 25% and
strongly redshift dependent. By extracting a set of realistic mock quasar
spectra, we quantify the effect of neutrinos on the flux probability
distribution function and flux power spectrum. The differences in the matter
power spectra translate into a ~2.5% (5%) difference in the flux power spectrum
for neutrino masses with Sigma m_{\nu} = 0.3 eV (0.6 eV). This rather small
effect is difficult to detect from present Lyman-alpha forest data and nearly
perfectly degenerate with the overall amplitude of the matter power spectrum as
characterised by sigma_8. If the results of the numerical simulations are
normalized to have the same sigma_8 in the initial conditions, then neutrinos
produce a smaller suppression in the flux power of about 3% (5%) for Sigma
m_{\nu} = 0.6 eV (2
sigma C.L.), comparable to constraints obtained from the cosmic microwave
background data or other large scale structure probes.Comment: 38 pages, 21 figures. One section and references added. JCAP in pres
Cosmography beyond standard candles and rulers
We perform a cosmographic analysis using several cosmological observables
such as the luminosity distance moduli, the volume distance, the angular
diameter distance and the Hubble parameter. These quantities are determined
using different data sets: Supernovae type Ia and Gamma Ray Bursts, the
Baryonic Acoustic Oscillations, the cosmic microwave background power spectrum
and the Hubble parameter as measured from surveys of galaxies. This data set
allows to put constraints on the cosmographic expansion with unprecedented
precision. We also present forecasts for the coefficients of the kinematic
expansion using future but realistic data sets: constraints on the coefficients
of the expansions are likely to improve by a factor ten with the upcoming large
scale structure probes. Finally, we derive the set of the cosmographic
parameters for several cosmological models (including CDM) and compare
them with our best fit set. While distance measurements are unable to
discriminate among these models, we show that the inclusion of the Hubble data
set leads to strong constraints on the lowest order coefficients and in
particular it is incompatible with CDM at 3- confidence level.
We discuss the reliability of this determination and suggest further
observations which might be of crucial importance for the viability of
cosmographic tests in the next future.Comment: 15 pages, 2 figures, 2 tables, Accepted for publication in PR
Expansion and Collapse in the Cosmic Web
We study the kinematics of the gaseous cosmic web at high redshift with Lyman
alpha forest absorption in multiple QSO sightlines. Using a simple analytic
model and a cosmological hydrodynamic simulation we constrain the underlying
three-dimensional distribution of velocities from the observed line-of-sight
distribution of velocity shear across the plane of the sky. The distribution is
found to be in good agreement with the intergalactic medium (IGM) undergoing
large scale motions dominated by the Hubble flow. Modeling the Lyman alpha
clouds analytically and with a hydrodynamics simulation, the average expansion
velocity of the gaseous structures causing the Lyman alpha forest in the lower
redshift (z = 2) sample appears about 20 percent lower than the local Hubble
expansion velocity. We interpret this as tentative evidence for some clouds
undergoing gravitational collapse. However, the distribution of velocities is
highly skewed, and the majority of clouds at redshifts from 2 to 3.8 expand
typically about 5 - 20 percent faster than the Hubble flow. This behavior is
explained if most absorbers in the column density range typically detectable
are expanding filaments that stretch and drain into more massive nodes. We find
no evidence for the observed distribution of velocity shear being significantly
influenced by processes other than Hubble expansion and gravitational
instability, like galactic winds. To avoid overly disturbing the IGM, winds may
be old and/or limp by the time we observe them in the Lyman alpha forest, or
they may occupy only an insignificant volume fraction of the IGM. (abridged)Comment: 63 pages, 26 figures, AAS Latex; ApJ, in pres
Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest
We compare the improved measurement of the Lya forest flux probability
distribution at 1.7<z<3.2 presented by Kim et al. (2007) to a large set of
hydrodynamical simulations of the Lya forest with different cosmological
parameters and thermal histories. The simulations are in good agreement with
the observational data if the temperature-density relation for the low density
intergalactic medium (IGM), T=T_0 Delta^{gamma-1}, is either close to
isothermal or inverted (gamma<1). Our results suggest that the voids in the IGM
may be significantly hotter and the thermal state of the low density IGM may be
substantially more complex than is usually assumed at these redshifts. We
discuss radiative transfer effects which alter the spectral shape of ionising
radiation during the epoch of HeII reionisation as a possible physical
mechanism for achieving an inverted temperature-density relation at z~3.Comment: 16 pages, 6 figures, accepted for publication in MNRAS following
minor revision. The accepted version includes an expanded discussion of the
flux power spectru
On the formation of dwarf galaxies and stellar haloes
Using analytic arguments and a suite of very high resolution (âŒ103âMâ per particle) cosmological hydrodynamical simulations, we argue that high-redshift, z⌠10, M⌠108âMâ haloes, form the smallest âbaryonic building block' (BBB) for galaxy formation. These haloes are just massive enough to efficiently form stars through atomic line cooling and to hold on to their gas in the presence of supernova (SN) winds and reionization. These combined effects, in particular that of the SN feedback, create a sharp transition: over the mass range 3-10 Ă 107âMâ, the BBBs drop two orders of magnitude in stellar mass. Below âŒ2 Ă 107âMâ, galaxies will be dark with almost no stars and no gas. Above this scale is the smallest unit of galaxy formation: the BBB. We show that the BBBs have stellar distributions which are spheroidal, of low rotational velocity, old and metal poor: they resemble the dwarf spheroidal galaxies (dSphs) of the Local Group (LG). Unlike the LG dSphs, however, they contain significant gas fractions. We connect these high-redshift BBBs to the smallest dwarf galaxies observed at z= 0 using linear theory. A small fraction (âŒ100) of these gas-rich BBBs at high redshift fall in to a galaxy the size of the Milky Way (MW). We suggest that 10 per cent of these survive to become the observed LG dwarf galaxies at the present epoch. This is consistent with recent numerical estimates. Those infalling haloes on benign orbits which keep them far away from the MW or Andromeda manage to retain their gas and slowly form stars - these become the smallest dwarf irregular galaxies; those on more severe orbits lose their gas faster than they can form stars and become the dwarf spheroidals. The remaining 90 per cent of the BBBs will be accreted. We show that this gives a metallicity and total stellar mass consistent with the MW old stellar hal
The sizes of mini-voids in the local universe: an argument in favor of a warm dark matter model?
Using high-resolution simulations within the Cold and Warm Dark Matter models
we study the evolution of small scale structure in the Local Volume, a sphere
of 8 Mpc radius around the Local Group. We compare the observed spectrum of
mini-voids in the Local Volume with the spectrum of mini-voids determined from
the simulations. We show that the \LWDM model can easily explain both the
observed spectrum of mini-voids and the presence of low-mass galaxies observed
in the Local Volume, provided that all haloes with circular velocities greater
than 20 km/s host galaxies. On the contrary within the LCDM model the
distribution of the simulated mini-voids reflects the observed one if haloes
with maximal circular velocities larger than 35 km/s host galaxies. This
assumption is in contradiction with observations of galaxies with circular
velocities as low as 20 km/s in our Local Universe. A potential problem of the
LWDM model could be the late formation of the haloes in which the gas can be
efficiently photo-evaporated. Thus star formation is suppressed and low-mass
haloes might not host any galaxy at all.Comment: 13 pages, 10 figures, version 2, subsection 3.1 added, accepted to
MNRA
Braneworld inflation from an effective field theory after WMAP three-year data
In light of the results from the WMAP three-year sky survey, we study an
inflationary model based on a single-field polynomial potential, with up to
quartic terms in the inflaton field. Our analysis is performed in the context
of the Randall-Sundrum II braneworld theory, and we consider both the
high-energy and low-energy (i.e. the standard cosmology case) limits of the
theory. We examine the parameter space of the model, which leads to both
large-field and small-field inflationary type solutions. We conclude that small
field inflation, for a potential with a negative mass square term, is in
general favored by current bounds on the tensor-to-scalar perturbation ratio
rs.Comment: 11 pages, 5 figures; references updated and a few comments added;
final version to appear in Phys. Rev.
Pressure Support vs. Thermal Broadening in the Lyman-alpha Forest I: Effects of the Equation of State on Longitudinal Structure
In the low density intergalactic medium (IGM) that gives rise to the
Lyman-alpha forest, gas temperature and density are tightly correlated. The
velocity scale of thermal broadening and the Hubble flow across the gas Jeans
scale are of similar magnitude (Hlambda_J ~ sigma_th). To separate the effects
of gas pressure support and thermal broadening on the Lya forest, we compare
spectra extracted from two smoothed particle hydrodynamics (SPH) simulations
evolved with different photoionization heating rates (and thus different Jeans
scales), imposing different temperature-density relations on the evolved
particle distributions. The turnover scales in the flux power spectrum and flux
autocorrelation function are determined mainly by thermal broadening rather
than pressure. However, the insensitivity to pressure arises partly from a
cancellation effect with a sloped temperature-density relation (T ~ rho^{0.6}
in our simulations): the high density peaks in the colder, lower pressure
simulation are less smoothed by pressure support than in the hotter simulation,
and it is this higher density gas that experiences the strongest thermal
broadening. Changes in thermal broadening and pressure support have comparably
important effects on the flux probability distribution (PDF), which responds
directly to the gas overdensity distribution rather than the scale on which it
is smooth. Tests on a lower resolution simulation show that our statistical
results are converged even at this lower resolution. While thermal broadening
generally dominates the longitudinal structure in the Lya forest, we show in
Paper II that pressure support determines the transverse coherence of the
forest observed towards close quasar pairs. [ABRIDGED]Comment: 12 figures, MNRAS in pres
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