3,939 research outputs found
Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization
The spectra of the first galaxies and quasars in the Universe should be
strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by
neutral hydrogen (HI) in the intervening intergalactic medium. However, the
Lyman-alpha line photons emitted by these sources are not eliminated but rather
scatter until they redshift out of resonance and escape due to the Hubble
expansion of the surrounding intergalactic HI. We calculate the resulting
brightness distribution and the spectral shape of the diffuse Lyman-alpha line
emission around high redshift sources, before the intergalactic medium was
reionized. Typically, the Lyman-alpha photons emitted by a source at z=10
scatter over a characteristic angular radius of order 15 arcseconds around the
source and compose a line which is broadened and redshifted by about a thousand
km/s relative to the source. The scattered photons are highly polarized.
Detection of the diffuse Lyman-alpha halos around high redshift sources would
provide a unique tool for probing the neutral intergalactic medium before the
epoch of reionization. On sufficiently large scales where the Hubble flow is
smooth and the gas is neutral, the Lyman-alpha brightness distribution can be
used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3
corrected; new section added on the detectability of Lyman alpha halos;
conclusions update
Tracking Cluster Debris (TraCD) â I. Dissolution of clusters and searching for the solar cradle
The capability to reconstruct dissolved stellar systems in dynamical and chemical space is a key factor in improving our understanding of the evolution of the Milky Way. Here we concentrate on the dynamical aspect and given that a significant portion of the stars in the Milky Way have been born in stellar associations or clusters that have lived a few Myr up to several Gyr, we further restrict our attention to the evolution of star clusters. We have carried out our simulations in two steps: (1) we create a simulation of dissolution and mixing processes which yields a close fit to the present-day Milky Way dynamics and (2) we have evolved three sets of stellar clusters with masses of 400, 1000 and 15 000 Mâ to dissolution. The birth location of these sets was 4, 6, 8 and 10 kpc for the 400 and 1000 Mâ clusters and 4, 6, 8, 10 and 12 kpc for the 15 000 Mâ. We have focused our efforts on studying the state of the escapers from these clusters after 4.5 Gyr of evolution with particular attention to stars that reach the solar annulus, i.e. 7.5 †Rgc †8.5 kpc. We give results for solar twins and siblings over a wide range of radii and cluster masses for two dissolution mechanisms. From kinematics alone, we conclude that the Sun was âŒ50âperâcent more likely to have been born near its current Galactocentric radius, rather than have migrated (radially) âŒ2 kpc since birth. We conclude our analysis by calculating magnitudes and colours of our single stars for comparison with the samples that the Gaia, Gaia-ESO and GALAH-AAO surveys will obtain. In terms of reconstructing dissolved star clusters, we find that on short time-scales we cannot rely on kinematic evolution alone and thus it will be necessary to extend our study to include information on chemical space
Models of dust around Europa and Ganymede
We use numerical models, supported by our laboratory data, to predict the
dust densities of ejecta outflux at any altitude within the Hill spheres of
Europa and Ganymede. The ejecta are created by micrometeoroid bombardment and
five different dust populations are investigated as sources of dust around the
moons. The impacting dust flux (influx) causes the ejection of a certain amount
of surface material (outflux). The outflux populates the space around the
moons, where a part of the ejecta escapes and the rest falls back to the
surface. These models were validated against existing Galileo DDS (Dust
Detector System) data collected during Europa and Ganymede flybys.
Uncertainties of the input parameters and their effects on the model outcome
are also included. The results of this model are important for future missions
to Europa and Ganymede, such as JUICE (JUpiter ICy moon Explorer), recently
selected as ESA's next large space mission to be launched in 2022
The X-ray spectra of the first galaxies: 21cm signatures
The cosmological 21cm signal is a physics-rich probe of the early Universe,
encoding information about both the ionization and the thermal history of the
intergalactic medium (IGM). The latter is likely governed by X-rays from
star-formation processes inside very high redshift (z > 15) galaxies. Due to
the strong dependence of the mean free path on the photon energy, the X-ray SED
can have a significant impact on the interferometric signal from the cosmic
dawn. Recent Chandra observations of nearby, star-forming galaxies show that
their SEDs are more complicated than is usually assumed in 21cm studies. In
particular, these galaxies have ubiquitous, sub-keV thermal emission from the
hot interstellar medium (ISM), which generally dominates the soft X-ray
luminosity (with energies < 1 keV, sufficiently low to significantly interact
with the IGM). Using illustrative soft and hard SEDs, we show that the IGM
temperature fluctuations in the early Universe would be substantially increased
if the X-ray spectra of the first galaxies were dominated by the hot ISM,
compared with X-ray binaries with harder spectra. The associated large-scale
power of the 21cm signal would be higher by roughly a factor of three. More
generally, we show that the peak in the redshift evolution of the large-scale
(k = 0.2 1/Mpc) 21cm power is a robust probe of the soft-band SED of the first
galaxies, and importantly, is not degenerate with their bolometric
luminosities. On the other hand, the redshift of the peak constrains the X-ray
luminosity and halo masses which host the first galaxies.Comment: 9 pages, 7 figures. Accepted for publication on MNRA
The Escape Fraction of Ionizing Radiation from Galaxies
The escape of ionizing radiation from galaxies plays a critical role in the
evolution of gas in galaxies, and the heating and ionization history of the
intergalactic medium. We present semi-analytic calculations of the escape
fraction of ionizing radiation for both hydrogen and helium from galaxies
ranging from primordial systems to disk-type galaxies that are not heavily
dust-obscured. We consider variations in the galaxy density profile, source
type, location, and spectrum, and gas overdensity/distribution factors. For
sufficiently hard first-light sources, the helium ionization fronts closely
track or advance beyond that of hydrogen. Key new results in this work include
calculations of the escape fractions for He I and He II ionizing radiation, and
the impact of partial ionization from X-rays from early AGN or stellar clusters
on the escape fractions from galaxy halos. When factoring in
frequency-dependent effects, we find that X-rays play an important role in
boosting the escape fractions for both hydrogen and helium, but especially for
He II. We briefly discuss the implications of these results for recent
observations of the He II reionization epoch at low redshifts, as well as the
UV data and emission-line signatures from early galaxies anticipated from
future satellite missions.Comment: 43 pages, 9 figures, accepted in ApJ, comments welcom
Investigating electron interacting dark matter
Some extensions of the Standard Model provide Dark Matter candidate particles
which can have a dominant coupling with the lepton sector of the ordinary
matter. Thus, such Dark Matter candidate particles () can be directly
detected only through their interaction with electrons in the detectors of a
suitable experiment, while they are lost by experiments based on the rejection
of the electromagnetic component of the experimental counting rate. These
candidates can also offer a possible source of the 511 keV photons observed
from the galactic bulge. In this paper this scenario is investigated. Some
theoretical arguments are developed and related phenomenological aspects are
discussed. Allowed intervals and regions for the characteristic
phenomenological parameters of the considered model and of the possible
mediator of the interaction are also derived considering the DAMA/NaI data.Comment: 16 pages, 6 figures. Accepted for publication in PRD. One typo
correcte
The Birth of a Galaxy - III. Propelling reionisation with the faintest galaxies
Starlight from galaxies plays a pivotal role throughout the process of cosmic
reionisation. We present the statistics of dwarf galaxy properties at z > 7 in
haloes with masses up to 10^9 solar masses, using a cosmological radiation
hydrodynamics simulation that follows their buildup starting with their
Population III progenitors. We find that metal-enriched star formation is not
restricted to atomic cooling ( K) haloes, but can occur
in haloes down to masses ~10^6 solar masses, especially in neutral regions.
Even though these smallest galaxies only host up to 10^4 solar masses of stars,
they provide nearly 30 per cent of the ionising photon budget. We find that the
galaxy luminosity function flattens above M_UV ~ -12 with a number density that
is unchanged at z < 10. The fraction of ionising radiation escaping into the
intergalactic medium is inversely dependent on halo mass, decreasing from 50 to
5 per cent in the mass range . Using our galaxy
statistics in a semi-analytic reionisation model, we find a Thomson scattering
optical depth consistent with the latest Planck results, while still being
consistent with the UV emissivity constraints provided by Ly forest
observations at z = 4-6.Comment: 21 pages, 15 figures, 4 tables. Accepted in MNRA
Negative feedback effects on star formation history and cosmic reionization
After considering the effects of negative feedback on the process of star
formation, we explore the relationship between star formation process and the
associated feedback, by investigating how the mechanical feedback from
supernovae(SNe) and radiative feedback from luminous objects regulate the star
formation rate and therefore affect the cosmic reionization.Based on our
present knowledge of the negative feedback theory and some numerical
simulations, we construct an analytic model in the framework of the Lambda cold
dark matter model. In certain parameter regions, our model can explain some
observational results properly. In large halos(T_vir>10000 K), both mechanical
and radiative feedback have a similar behavior: the relative strength of
negative feedback reduces as the redshift decreases. In contrast, in small
halos (T_vir<10000 K$) that are thought to breed the first stars at early time,
the radiative feedback gets stronger when the redshift decreases. And the star
formation rate in these small halos depends very weakly on the star-formation
efficiency. Our results show that the radiative feedback is important for the
early generation stars. It can suppress the star formation rate considerably.
But the mechanical feedback from the SNe explosions is not able to affect the
early star formation significantly. The early star formation in small-halo
objects is likely to be self-regulated. The radiative and mechanical feedback
dominates the star formation rate of the PopII/I stars all along. The feedback
from first generation stars is very strong and should not be neglected.
However, their effects on the cosmic reionization are not significant, which
results in a small contribution to the optical depth of Thomson scattering.Comment: 12 pages,6 figure
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