522,793 research outputs found
Early massive clusters and the bouncing coupled dark energy
The abundance of the most massive objects in the Universe at different epochs
is a very sensitive probe of the cosmic background evolution and of the growth
history of density perturbations, and could provide a powerful tool to
distinguish between a cosmological constant and a dynamical dark energy field.
In particular, the recent detection of very massive clusters of galaxies at
high redshifts has attracted significant interest as a possible indication of a
failure of the standard LCDM model. Several attempts have been made in order to
explain such detections in the context of non-Gaussian scenarios or interacting
dark energy models, showing that both these alternative cosmologies predict an
enhanced number density of massive clusters at high redshifts, possibly
alleviating the tension. However, all the models proposed so far also
overpredict the abundance of massive clusters at the present epoch, and are
therefore in contrast with observational bounds on the low-redshift halo mass
function. In this paper we present for the first time a new class of
interacting dark energy models that simultaneously account for an enhanced
number density of massive clusters at high redshifts and for both the standard
cluster abundance at the present time and the standard power spectrum
normalization at CMB. The key feature of this new class of models is the
"bounce" of the dark energy scalar field on the cosmological constant barrier
at relatively recent epochs. We present the background and linear perturbations
evolution of the model, showing that the standard amplitude of density
perturbations is recovered both at CMB and at the present time, and we
demonstrate by means of large N-body simulations that our scenario predicts an
enhanced number of massive clusters at high redshifts without affecting the
present halo abundance. (Abridged)Comment: 11 pages, 6 figures, 2 tables. Minor changes, references added.
Accepted for publication in MNRA
Grain growth across protoplanetary discs: 10-micron silicate feature versus millimetre slope
Young stars are formed within dusty discs. The grains in the disc are
originally of the same size as interstellar dust. Models predict that these
grains will grow in size through coagulation. Observations of the silicate
features at micron wavelengths are consistent with growth to micron sizes
whereas the slope of the SED at longer wavelengths traces growth up to mm
sizes. We here look for a correlation between these two grain growth
indicators. A large sample of T-Tauri and Herbig-Ae/Be stars was observed with
the Spitzer Space Telescope at 5-13 micron; a subsample was observed at mm
wavelengths. We complement this subsample with data from the literature to
maximise the overlap between micron and mm observations and search for
correlations. Synthetic spectra are produced to determine which processes may
produce the dust evolution. Dust disc masses in the range <1 to 7 x 10^-4 MSun
are obtained. Most sources have a mm spectral slope consistent with grain
growth. There is a tentative correlation between the 10-micron silicate feature
and the mm slope of the SED. The observed sources seem to be grouped per
star-forming region in the micron-vs-mm diagram. The modelling results show
that the 10-micron feature becomes flatter and subsequently the mm slope
becomes shallower. Grain size distributions shallower than that of the ISM
and/or bright central stars are required to explain specific features. Settling
of larger grains towards the disc midplane affects the 10-micron feature, but
hardly the mm slope. The tentative correlation between the strength of the
10-micron feature and the mm slope suggests that the inner and outer disc
evolve simultaneously. Dust with a mass dominated by mm-sized grains is
required to explain the shallowest mm slopes. Other processes besides grain
growth may also be responsible for the removal of small grains.Comment: 23 pages, 14 figures, accepted by A&
Time-resolved infrared emission from radiation-driven central obscuring structures in Active Galactic Nuclei
The central engines of Seyfert galaxies are thought to be enshrouded by
geometrically thick gas and dust structures. In this article, we derive
observable properties for a self-consistent model of such toroidal gas and dust
distributions, where the geometrical thickness is achieved and maintained with
the help of X-ray heating and radiation pressure due to the central engine.
Spectral energy distributions (SEDs) and images are obtained with the help of
dust continuum radiative transfer calculations with RADMC-3D. For the first
time, we are able to present time-resolved SEDs and images for a physical model
of the central obscurer. Temporal changes are mostly visible at shorter
wavelengths, close to the combined peak of the dust opacity as well as the
central source spectrum and are caused by variations in the column densities of
the generated outflow. Due to the three-component morphology of the
hydrodynamical models -- a thin disc with high density filaments, a surrounding
fluffy component (the obscurer) and a low density outflow along the rotation
axis -- we find dramatic differences depending on wavelength: whereas the
mid-infrared images are dominated by the elongated appearance of the outflow
cone, the long wavelength emission is mainly given by the cold and dense disc
component. Overall, we find good agreement with observed characteristics,
especially for those models, which show clear outflow cones in combination with
a geometrically thick distribution of gas and dust, as well as a geometrically
thin, but high column density disc in the equatorial plane.Comment: 16 pages, 12 figures, accepted for publication in MNRA
BD+30 3639: The Infrared Spectrum During Post-AGB Stellar Evolution
We present a radiative-transfer calculation which reproduces the infrared
spectrum of the planetary nebula BD~+303639. We calculate the
transfer process through absorption and scattering in a spherical-symmetric
multi-grain dust shell. The emission of transiently heated particles is taken
into account, as well as polycyclic aromatic hydrocarbons. We obtain an
acceptable fit to most of the spectrum, including the PAH infrared bands. At
submillimetre wavelengths the observed emission is larger than the model
predicts, indicating that large dust conglomerates (``f{}luffy grains'') may be
needed as an additional constituent. The fit favours a distance of kpc, which implies that BD~+303639 has evolved from a massive
progenitor of several solar masses. A low dust-to-gas mass ratio is found in
the ionised region. The calculations yield an original mass-loss rate of
2\times10^{-4} \msolar \peryr on the Asymptotic Giant Branch. Using this
mass-loss rate, we calculate how the infrared spectrum has evolved during the
post-AGB evolution. We show in particular the evolution of the IRAS colours
during the preceding post-AGB evolution.Comment: accepted for publication in MNRAS. LaTeX, 15 pages, hardcopy and 8
figures available from [email protected] or [email protected]
The History of Galaxies and Galaxy Number Counts
(Abridged) A simple quantitative model is presented for the history of
galaxies to explain galaxy number counts, redshift distributions and some other
related observations. We first infer that irregular galaxies and the disks of
spiral galaxies are young, probably formed at from a
simultaneous consideration of colours and gas content under a moderate
assumption on the star formation history. Assuming that elliptical galaxies and
bulges of spiral galaxies, both called spheroids in the discussion, had formed
early in the universe, the resulting scenario is that spiral galaxies formed as
intergalactic gas accreting onto pre-existing bulges mostly at ;
irregular galaxies as seen today formed by aggregation of clouds at . Taking the formation epochs thus estimated into account, we construct
a model for the history of galaxies employing a stellar population synthesis
model. We assume that the number of galaxies does not change except that some
of them (irregulars) were newly born, and use a morphology-dependent local
luminosity function to constrain the number of galaxies. The predictions of the
model are compared with the observation of galaxy number counts and redshift
distributions for the , and colour bands. It is shown that young
irregular galaxies cause the steep slope of the -band counts. The fraction
of irregular galaxies increases with decreasing brightness: at mag, they
contribute as much as spiral galaxies. Thus, ``the faint blue galaxy problem''
is solved by invoking young galaxies. This interpretation is corroborated by a
comparison of our prediction with the morphologically-classified galaxy counts
in the band.Comment: 25 pages, LaTeX (aaspp4), 24 PostScript figures. Submitted to ApJ in
February 199
Grain growth and dust settling in a brown dwarf disk: Gemini/T-ReCS observations of CFHT-BD-Tau 4
We present accurate mid-infrared observations of the disk around the young,
bona-fide brown dwarf CFHT-BD-Tau 4. We report GEMINI/T-ReCS measurements in
the 7.9, 10.4 and 12.3 micron filters, from which we infer the presence of a
prominent, broad silicate emission feature. The shape of the silicate feature
is dominated by emission from 2 micron amorphous olivine grains. Such grains,
being an order of magnitude larger than those in the interstellar medium, are a
first proof of dust processing and grain growth in disks around brown dwarfs.
The object's spectral energy distribution is below the prediction of the
classical flared disk model but higher than that of the two-layer flat disk. A
good match can be achieved by using an intermediate disk model with strongly
reduced but non-zero flaring. Grain growth and dust settling processes provide
a natural explanation for this disk geometry and we argue that such
intermediate flaring might explain the observations of several other brown
dwarf disks as well.Comment: Accepted for publication in Astronomy & Astrophysics Letters, 4.5
pages with 1 figur
PAH emission from Herbig AeBe stars
We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained
with the Infrared Spectrograph on the Spitzer Space Telescope. All but one of
the Herbig stars show emission from polycyclic aromatic hydrocarbons (PAHs) and
seven of the spectra show PAH emission, but no silicate emission at 10 microns.
The central wavelengths of the 6.2, 7.7--8.2, and 11.3 micron emission features
decrease with stellar temperature, indicating that the PAHs are less
photo-processed in cooler radiation fields. The apparent low level of photo
processing in HAeBe stars, relative to other PAH emission sources, implies that
the PAHs are newly exposed to the UV-optical radiation fields from their host
stars. HAeBe stars show a variety of PAH emission intensities and ionization
fractions, but a narrow range of PAH spectral classifications based on
positions of major PAH feature centers. This may indicate that, regardless of
their locations relative to the stars, the PAH molecules are altered by the
same physical processes in the proto-planetary disks of intermediate-mass
stars. Analysis of the mid-IR spectral energy distributions indicates that our
sample likely includes both radially flared and more flattened/settled disk
systems, but we do not see the expected correlation of overall PAH emission
with disk geometry. We suggest that the strength of PAH emission from HAeBe
stars may depend not only on the degree of radial flaring, but also on the
abundance of PAHs in illuminated regions of the disks and possibly on the
vertical structure of the inner disk as well.Comment: 52 pages, 12 figure
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