91 research outputs found
A Macro-econometric Model for Ireland
The Bankâs Macro-Econometric model has recently been revised. This paper outlines the context within which the model was initially built and the reasons for the revision and re-estimation. Compilation of the data used was a key component of the revision and this is described. The general structure of the model is outlined. Key equations are described and estimation issues noted. A discussion on simulating the model is provided along with results from sample simulations. The paper concludes with a discussion of how future work on the model might evolve.
The impact of the supersonic baryon-dark matter velocity difference on the z~20 21cm background
Recently, Tseliakhovich and Hirata (2010) showed that during the cosmic Dark
Ages the baryons were typically moving supersonically with respect to the dark
matter with a spatially variable Mach number. Such supersonic motion may source
shocks that heat the Universe. This motion may also suppress star formation in
the first halos. Even a small amount of coupling of the 21cm signal to this
motion has the potential to vastly enhance the 21cm brightness temperature
fluctuations at 15<z<40 as well as to imprint acoustic oscillations in this
signal. We present estimates for the size of this coupling, which we calibrate
with a suite of cosmological simulations. Our simulations, discussed in detail
in a companion paper, are initialized to self-consistently account for gas
pressure and the dark matter-baryon relative velocity, v_bc (in contrast to
prior simulations). We find that the supersonic velocity difference
dramatically suppresses structure formation at 10-100 comoving kpc scales, it
sources shocks throughout the Universe, and it impacts the accretion of gas
onto the first star-forming minihalos (even for halo masses as large as ~10^7
Msun). However, we find that the v_bc-sourced temperature fluctuations can
contribute only as much as ~10% of the fluctuations in the 21cm signal. We do
find that v_bc could source an O(1) component in the power spectrum of the 21cm
signal via the X-ray (but not ultraviolet) backgrounds produced once the first
stars formed. In a scenario in which ~10^6 Msun minihalos reheated the Universe
via their X-ray backgrounds, we find that the pre-reionization 21cm signal
would be larger than previously anticipated and exhibit significant acoustic
features. We show that structure formation shocks are unable to heat the
Universe sufficiently to erase a strong 21cm absorption trough at z ~ 20 that
is found in most models of the sky-averaged 21cm intensity.Comment: 17 pages, 11 figures, accepted to ApJ; for movies see
http://astro.berkeley.edu/~mmcquinn/firstligh
Evidence of Gunn-Peterson damping wings in high-z quasar spectra: strengthening the case for incomplete reionization
The spectra of several high-redshift (z>6) quasars have shown evidence for a
Gunn-Peterson (GP) damping wing, indicating a substantial mean neutral hydrogen
fraction (x_HI > 0.03) in the z ~ 6 intergalactic medium (IGM). However,
previous analyses assumed that the IGM was uniformly ionized outside of the
quasar's HII region. Here we relax this assumption and model patchy
reionization scenarios for a range of IGM and quasar parameters. We quantify
the impact of these differences on the inferred x_HI, by fitting the spectra of
three quasars: SDSS J1148+5251 (z=6.419), J1030+0524 (z=6.308), and J1623+3112
(z=6.247). We find that the best-fit values of x_HI in the patchy models agree
well with the uniform case. More importantly, we confirm that the observed
spectra favor the presence of a GP damping wing, with peak likelihoods
decreasing by factors of > few - 10 when the spectra are modeled without a
damping wing. We also find that the Ly alpha absorption spectra, by themselves,
cannot distinguish the damping wing in a relatively neutral IGM from a damping
wing in a highly ionized IGM, caused either by an isolated neutral patch, or by
a damped Ly alpha absorber (DLA). However, neutral patches in a highly ionized
universe (x_HI < 0.01), and DLAs with the large required column densities (N_HI
> few x 10^{20} cm^{-2}) are both rare. As a result, when we include reasonable
prior probabilities for the line of sight (LOS) to intercept either a neutral
patch or a DLA at the required distance of ~ 40-60 comoving Mpc away from the
quasar, we find strong lower limits on the neutral fraction in the IGM, x_HI >
0.1 (at 95% confidence). This strengthens earlier claims that a substantial
global fraction of hydrogen in the z~6 IGM is in neutral form.Comment: 18 pages, 7 figures, version accepted for publication in the MNRA
High Redshift Metals I.: The Decline of C IV at z > 5.3
We present the results from our search for C IV in the intergalactic medium
at redshifts z=5.3-6.0. We have observed four z~6 QSOs with Keck/NIRSPEC in
echelle mode. The data are the most sensitive yet taken to search for C IV at
these redshifts, being 50% complete at column densities log(N_{CIV})=13.4. We
find no C IV systems in any of the four sightlines. Taking into account our
completeness, this translates into a decline in the number density of C IV
absorbers in the range 13.0 < log(N_{CIV}) < 15.0 of at least a factor 4.4 (95%
confidence) from z~2-4.5, where the number density is relatively constant. We
use our lack of detections to set limits on the slope and normalization of the
column density distribution at z=5.3-6.0. The rapid evolution of C IV at these
redshifts suggests that the decrease in the number density may largely be due
to ionization effects, in which case many of the metals in the z~4.5 IGM could
already be in place at z~5.3, but in a lower ionization state. The lack of weak
systems in our data, combined with the presence of strong C IV absorbers along
at least one other sightline, further suggests that there may be large-scale
variations in the enrichment and/or ionization state of the z~6 IGM, or that C
IV absorbers at these redshifts are associated with rare, UV-bright
star-forming galaxies.Comment: 11 pages, 8 figures, submitted to Ap
Properties of Nearby Starburst Galaxies Based on their Diffuse Gamma-ray Emission
The physical relationship between the far-infrared and radio fluxes of star
forming galaxies has yet to be definitively determined. The favored
interpretation, the "calorimeter model," requires that supernova generated
cosmic ray (CR) electrons cool rapidly via synchrotron radiation. However, this
cooling should steepen their radio spectra beyond what is observed, and so
enhanced ionization losses at low energies from high gas densities are also
required. Further, evaluating the minimum energy magnetic field strength with
the traditional scaling of the synchrotron flux may underestimate the true
value in massive starbursts if their magnetic energy density is comparable to
the hydrostatic pressure of their disks. Gamma-ray spectra of starburst
galaxies, combined with radio data, provide a less ambiguous estimate of these
physical properties in starburst nuclei. While the radio flux is most sensitive
to the magnetic field, the GeV gamma-ray spectrum normalization depends
primarily on gas density. To this end, spectra above 100 MeV were constructed
for two nearby starburst galaxies, NGC 253 and M82, using Fermi data. Their
nuclear radio and far-infrared spectra from the literature are compared to new
models of the steady-state CR distributions expected from starburst galaxies.
Models with high magnetic fields, favoring galaxy calorimetry, are overall
better fits to the observations. These solutions also imply relatively high
densities and CR ionization rates, consistent with molecular cloud studies.Comment: Accepted to Ap
MARĂA DE MESA Y LĂPEZ [Material grĂĄfico]
ĂLBUM FAMILIAR CASA DE COLĂNCopia digital. Madrid : Ministerio de EducaciĂłn, Cultura y Deporte. SubdirecciĂłn General de CoordinaciĂłn Bibliotecaria, 201
Multicomponent and Variable Velocity Galactic Outflow in Cosmological Hydrodynamic Simulations
We develop a new ``Multicomponent and Variable Velocity'' (MVV) galactic
outflow model for cosmological smoothed particle hydrodynamic (SPH)
simulations. The MVV wind model reflects the fact that the wind material can
arise from different phases in the interstellar medium (ISM), and the
mass-loading factor in the MVV model is a function of galaxy stellar mass. We
find that the simulation with the MVV outflow has the following
characteristics: (i) the intergalactic medium (IGM) is hardly heated up, and
the mean IGM temperature is almost the same as in the no-wind run; (ii) it has
lower cosmic star formation rates (SFRs) compared to the no-wind run, but
higher SFRs than the constant velocity wind run; (iii) it roughly agrees with
the observed IGM metallicity, and roughly follows the observed evolution of
Omega(Civ); (iv) the lower mass galaxies have larger mass-loading factors, and
the low-mass end of galaxy stellar mass function is flatter than in the
previous simulations. Therefore, the MVV outflow model mildly alleviates the
problem of too steep galaxy stellar mass function seen in the previous SPH
simulations. In summary, the new MVV outflow model shows reasonable agreement
with observations,
and gives better results than the constant velocity wind model.Comment: 16 pages, 12 figures, and 1 table, accepted for publication in MNRAS.
A full resolution version is available at
http://www.physics.unlv.edu/~jhchoi/astro-ph/vwind.pd
The low density and magnetization of a massive galaxy halo exposed by a fast radio burst
Present-day galaxies are surrounded by cool and enriched halo gas extending
to hundreds of kiloparsecs. This halo gas is thought to be the dominant
reservoir of material available to fuel future star formation, but direct
constraints on its mass and physical properties have been difficult to obtain.
We report the detection of a fast radio burst (FRB 181112) with arcsecond
precision, which passes through the halo of a foreground galaxy. Analysis of
the burst shows the halo gas has low net magnetization and turbulence. Our
results imply predominantly diffuse gas in massive galactic halos, even those
hosting active supermassive black holes, contrary to some previous results.Comment: Published in Science on 2019 September 26; Main (3 figures; 1 Table)
+ Supp (12 figures; 7 Tables
How old are the stars in the halo of NGC 5128 (Centaurus A)?
NGC 5128 (Centaurus A) is, at the distance of just 3.8 Mpc, the nearest
easily observable giant elliptical galaxy. Therefore it is the best target to
investigate the early star formation history of an elliptical galaxy. Our aims
are to establish when the oldest stars formed in NGC 5128, and whether this
galaxy formed stars over a long period. We compare simulated colour-magnitude
diagrams with the deep ACS/HST photometry. We find that that the observed
colour-magnitude diagram can be reproduced satisfactorily only by simulations
that have the bulk of the stars with ages in excess of ~10 Gyr, and that the
alpha-enhanced models fit the data much better than the solar scaled ones. Data
are not consistent with extended star formation over more than 3-4 Gyr. Two
burst models, with 70-80% of the stars formed 12+/-1 Gyr ago and with 20-30%
younger contribution with 2-4 Gyr old stars provide the best agreement with the
data. The old component spans the whole metallicity range of the models
(Z=0.0001-0.04), while for the young component the best fitting models indicate
higher minimum metallicity (~1/10 - 1/4 Z_sun). The bulk of the halo stars in
NGC5128 must have formed at redshift z>=2 and the chemical enrichment was very
fast, reaching solar or even twice-solar metallicity already for the ~11-12 Gyr
old population. The minor young component, adding ~20-30% of the stars to the
halo, and contributing less than 10% of the mass, may have resulted from a
later star formation event ~2-4 Gyr ago. (abridged)Comment: 36 pages, 14 figures, accepted for publication in A&
Exploring the Universe with Metal-Poor Stars
The early chemical evolution of the Galaxy and the Universe is vital to our
understanding of a host of astrophysical phenomena. Since the most metal-poor
Galactic stars (with metallicities down to [Fe/H]\sim-5.5) are relics from the
high-redshift Universe, they probe the chemical and dynamical conditions of the
Milky Way and the origin and evolution of the elements through nucleosynthesis.
They also provide constraints on the nature of the first stars, their
associated supernovae and initial mass function, and early star and galaxy
formation. The Milky Way's dwarf satellites contain a large fraction (~30%) of
the known most metal-poor stars that have chemical abundances that closely
resemble those of equivalent halo stars. This suggests that chemical evolution
may be universal, at least at early times, and that it is driven by massive,
energetic SNe. Some of these surviving, ultra-faint systems may show the
signature of just one such PopIII star; they may even be surviving first
galaxies. Early analogs of the surviving dwarfs may thus have played an
important role in the assembly of the old Galactic halo whose formation can now
be studied with stellar chemistry. Following the cosmic evolution of small
halos in simulations of structure formation enables tracing the cosmological
origin of the most metal-poor stars in the halo and dwarf galaxies. Together
with future observations and additional modeling, many of these issues,
including the reionization history of the Milky Way, may be constrained this
way. The chapter concludes with an outlook about upcoming observational
challenges and ways forward is to use metal-poor stars to constrain theoretical
studies.Comment: 34 pages, 11 figures. Book chapter to appear in "The First Galaxies -
Theoretical Predictions and Observational Clues", 2012 by Springer, eds. V.
Bromm, B. Mobasher, T. Wiklin
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