458 research outputs found
The metallicity of galactic winds
The abundance evolution of galaxies depends critically on the balance between the mixing of metals in their interstellar medium (ISM), the inflow of new gas and the outflow of enriched gas. We study these processes in gas columns perpendicular to a galactic disc using sub-parsec resolution simulations that track stellar ejecta with the flash code. We model a simplified ISM stirred and enriched by supernovae and their progenitors. We vary the density distribution of the gas column and integrate our results over an exponential disc to predict wind and ISM enrichment properties for disc galaxies. We find that winds from more massive galaxies are hotter and more highly enriched, in stark contrast to that which is often assumed in galaxy formation models. We use these findings in a simple model of galactic enrichment evolution, in which the metallicity of forming galaxies is the result of accretion of nearly pristine gas and outflow of enriched gas along an equilibrium sequence. We compare these predictions to the observed massâmetallicity relation, and demonstrate how the galaxy's gas fraction is a key controlling parameter. This explains the observed flattening of the massâmetallicity relation at higher stellar masses
Tidal Disruption of Protoclusters in Giant Molecular Clouds
We study the collapse of protoclusters within a giant molecular cloud (GMC)
to determine the conditions under which collapse is significantly disrupted.
Motivated by observations of star forming regions which exhibit flattened cloud
structures, this study considers collapsing protoclusters with disk geometries.
The collapse of a 10^3 Msun protocluster initially a distance of 2-10 pc from a
10^3 - 10^6 Msun point mass is numerically calculated. Simulations with zero
initial relative velocity between the two are completed as well as simulations
with relative velocities consistent with those observed in GMCs. The results
allow us to define the conditions under which it is safe to assume protocluster
collapse proceeds as if in isolation. For instance, we find the collapse of a
10^3 Msun protocluster will be significantly disrupted if it is within 2-4 pc
of a 10^4 Msun point mass. Thus, the collapse of a 10^3 Msun protocluster can
be considered to proceed as if in isolation if it is more than ~ 4 pc away from
a 10^4 Msun compact object. In addition, in no portion of the sampled parameter
space does the gravitational interaction between the protocluster disk and the
massive particle significantly disperse the disk into the background GMC. We
discuss the distribution of clusters of young stellar objects within the
Perseus and Mon R2 star forming regions, which are consistent with the results
of our simulations and the limitations of our results in gas dominated regions
such as the Orion cloud.Comment: 12 pages, 6 figures, Accepted for publication in Ap
Impact of the application of redecision methods in executive coaching workshops on psychological wellbeing : a quantitative evaluation of effectiveness
Previous research has found that participants in redecision marathons experience increased personal growth and improvements in psychological well-being (McNeel, 1982; Noriega-Gayol, 1997; Widdowson & Rosseau, 2014). In this article, the authors conducted a quantitative analysis based on the use of the Ryff Scales of Psychological Wellbeing to determine whether participants (n=49) at an executive coaching redecision marathon would experience an increase in psychological well-being. The findings show statistically significant improvements in psychological well-being overall, and specifically within the sub-scales of autonomy, environmental mastery, personal growth and self-acceptance, suggesting that redecision- based workshops are effective for improving subjective psychological well-being
The Thermal Memory of Reionization History
The recent measurement by WMAP of a large electron scattering optical depth
tau_e = 0.17 +- 0.04 is consistent with a simple model of reionization in which
the intergalactic medium (IGM) is ionized at redshift z ~ 15, and remains
highly ionized thereafter. Here, we show that existing measurements of the IGM
temperature from the Lyman-alpha forest at z ~ 2 - 4 rule out this ``vanilla''
model. Under reasonable assumptions about the ionizing spectrum, as long as the
universe is reionized before z = 10, and remains highly ionized thereafter, the
IGM reaches an asymptotic thermal state which is too cold compared to
observations. To simultaneously satisfy the CMB and forest constraints, the
reionization history must be complex: reionization begins early at z >~ 15, but
there must have been significant (order unity) changes in fractions of neutral
hydrogen and/or helium at 6 < z < 10, and/or singly ionized helium at 4 < z <
10. We describe a physically motivated reionization model that satisfies all
current observations. We also explore the impact of a stochastic reionization
history and show that a late epoch of (HeII --> HeIII) reionization induces a
significant scatter in the IGM temperature, but the scatter diminishes with
time quickly. Finally, we provide an analytic formula for the thermal
asymptote, and discuss possible additional heating mechanisms that might evade
our constraints.Comment: 10 pages, submitted to ApJ, new references, additional discussion on
earlier work and partial HeII reionizatio
The Evolution of Optical Depth in the Ly-alpha Forest: Evidence Against Reionization at z~6
We examine the evolution of the IGM Ly-alpha optical depth distribution using
the transmitted flux probability distribution function (PDF) in a sample of 63
QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to two
theoretical optical depth distributions: a model distribution based on the
density distribution of Miralda-Escude et al. (2000) (MHR00), and a lognormal
distribution. We assume a uniform UV background and an isothermal IGM for the
MHR00 model, as has been done in previous works. Under these assumptions, the
MHR00 model produces poor fits to the observed flux PDFs at redshifts where the
optical depth distribution is well sampled, unless large continuum corrections
are applied. However, the lognormal optical depth distribution fits the data at
all redshifts with only minor continuum adjustments. We use a simple
parametrization for the evolution of the lognormal parameters to calculate the
expected mean transmitted flux at z > 5.4. The lognormal optical depth
distribution predicts the observed Ly-alpha and Ly-beta effective optical
depths at z > 5.7 while simultaneously fitting the mean transmitted flux down
to z = 1.6. If the evolution of the lognormal distribution at z < 5 reflects a
slowly-evolving density field, temperature, and UV background, then no sudden
change in the IGM at z ~ 6 due to late reionization appears necessary. We have
used the lognormal optical depth distribution without any assumption about the
underlying density field. If the MHR00 density distribution is correct, then a
non-uniform UV background and/or IGM temperature may be required to produce the
correct flux PDF. We find that an inverse temperature-density relation greatly
improves the PDF fits, but with a large scatter in the equation of state index.
[Abridged]Comment: 45 pages, 16 figures, submitted to Ap
Radio Afterglows of Gamma-Ray Bursts and Hypernovae at High Redshift, and their Potential for 21-cm Absorption Studies
We investigate the radio afterglows of gamma-ray bursts (GRBs) and hypernovae
(HNe) at high redshifts and quantify their detectability, as well as their
potential usefulness for 21 cm absorption line studies of the intergalactic
medium (IGM) and intervening structures. We examine several sets of source and
environment model parameters that are physically plausible at high redshifts.
The radio afterglows of GRBs would be detectable out to z ~ 30, while the
energetic HNe could be detectable out to z ~ 20 even by the current Very Large
Array (VLA). We find that the 21 cm absorption line due to the diffuse neutral
IGM is difficult to detect even by the proposed Square Kilometer Array (SKA),
except for highly energetic sources. We also find that the 21 cm line due to
collapsed gas clouds with high optical depth may be detected on rare occasions.Comment: 36 pages, 12 figures, 1 table, accepted for publication in Ap
Sample variance and Lyman α forest transmission statistics
We compare the observed probability distribution function (PDF) of the transmission in the H I Lyman α forest, measured from the Ultraviolet and Visual Echelle Spectrograph (UVES) âLarge Programmeâ sample at redshifts z = [2, 2.5, 3], to results from the GIMIC cosmological simulations. Our measured values for the mean transmission and its PDF are in good agreement with published results. Errors on statistics measured from high-resolution data are typically estimated using bootstrap or jackknife resampling techniques after splitting the spectra into chunks. We demonstrate that these methods tend to underestimate the sample variance unless the chunk size is much larger than is commonly the case. We therefore estimate the sample variance from the simulations. We conclude that observed and simulated transmission statistics are in good agreement; in particular, we do not require the temperatureâdensity relation to be âinvertedâ
Hydrogen Clouds before Reionization: a Lognormal Model Approach
We study the baryonic gas clouds (the IGM) in the universe before the
reionization with the lognormal model which is shown to be dynamcially
legitimate in describing the fluctuation evolution in quasilinear as well as
nonlinear regimes in recent years. The probability distribution function of the
mass field in the LN model is long tailed and so plays an important role in
rare events, such as the formation of the first generation of baryonic objects.
We calculate density and velocity distributions of the IGM at very high spatial
resolutions, and simulate the distributions at resolution of 0.15 kpc from z=7
to 15 in the LCDM cosmological model. We performed a statistics of the hydrogen
clouds including column densities, clumping factors, sizes, masses, and spatial
number density etc. One of our goals is to identify which hydrogen clouds are
going to collapse. By inspecting the mass density profile and the velocity
profile of clouds, we found that the velocity outflow significantly postpones
the collapsing process in less massive clouds, in spite of their masses are
larger than the Jeans mass. Consequently, only massive (> 10^5 M_sun) clouds
can form objects at higher redshift, and less massive (10^4-10^5) collapsed
objects are formed later. For example, although the mass fraction in clouds
with sizes larger than the Jeans length is already larger than 1 at z=15, there
is only a tiny fraction of mass (10^{-8}) in the clouds which are collapsed at
that time. If all the ionizing photons, and the 10^{-2} metallicity observed at
low redshift are produced by the first 1% mass of collapsed baryonic clouds,
the majority of those first generation objects would not happen until z=10.Comment: Paper in AAStex, 12 figure
The Reionization History at High Redshifts I: Physical Models and New Constraints from CMB Polarization
The recent discovery of a high optical depth tau to Thomson scattering from
the WMAP data implies that significant reionization took place at redshifts
z~15. This discovery has important implications for the sources of
reionization, and allows, for the first time, constraints to be placed on
physical reionization scenarios out to redshift z~20. Using a new suite of
semi-analytic reionization models, we show that the high value of tau requires
a surprisingly high efficiency epsilon of the first generation of UV sources
for injecting ionizing photons into the intergalactic medium. We find that no
simple reionization model can be consistent with the combination of the WMAP
result with data from the z<6.5 universe. Satisfying both constraints requires
either of the following: (i) H_2 molecules form efficiently at z~20, survive
feedback processes, and allow UV sources in halos with virial temperatures
below Tvir=10^4 K to contribute substantially to reionization, or (ii) the
efficiency epsilon in halos with Tvir>10^4K decreased by a factor of ~ 30
between (z~20) and (z~6). We discuss the relevant physical issues to produce
either scenario, and argue that both options are viable, and allowed by current
data. In detailed models of the reionization history, we find that the
evolution of the ionized fractions in the two scenarios have distinctive
features that Planck can distinguish at 3 sigma significance. At the high WMAP
value for tau, Planck will also be able to provide tight statistical
constraints on reionization model parameters, and elucidate much of the physics
at the end of the Dark Ages. The sources responsible for the high optical depth
discovered by WMAP should be directly detectable out to z~15 by the James Webb
Space Telescope.Comment: cosmetic changes to figures; text unchange
An accurate tool for the fast generation of dark matter halo catalogues
We present a new parallel implementation of the PINpointing Orbit Crossing-Collapsed HIerarchical Objects (PINOCCHIO) algorithm, a quick tool, based on Lagrangian Perturbation Theory, for the hierarchical build-up of dark matter (DM) haloes in cosmological volumes. To assess its ability to predict halo correlations on large scales, we compare its results with those of an N-body simulation of a 3 hâ1 Gpc box sampled with 20483 particles taken from the MICE suite, matching the same seeds for the initial conditions. Thanks to the Fastest Fourier Transforms in the West (FFTW) libraries and to the relatively simple design, the code shows very good scaling properties. The CPU time required by PINOCCHIO is a tiny fraction (âŒ1/2000) of that required by the MICE simulation. Varying some of PINOCCHIO numerical parameters allows one to produce a universal mass function that lies in the range allowed by published fits, although it underestimates the MICE mass function of Friends-of-Friends (FoF) haloes in the high-mass tail. We compare the matterâhalo and the haloâhalo power spectra with those of the MICE simulation and find that these two-point statistics are well recovered on large scales. In particular, when catalogues are matched in number density, agreement within 10âperâcent is achieved for the halo power spectrum. At scales k > 0.1âhâMpcâ1, the inaccuracy of the Zelâdovich approximation in locating halo positions causes an underestimate of the power spectrum that can be modelled as a Gaussian factor with a damping scale of d = 3âhâ1âMpc at z = 0, decreasing at higher redshift. Finally, a remarkable match is obtained for the reduced halo bispectrum, showing a good description of non-linear halo bias. Our results demonstrate the potential of PINOCCHIO as an accurate and flexible tool for generating large ensembles of mock galaxy surveys, with interesting applications for the analysis of large galaxy redshift surveys
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