544 research outputs found
Galaxy Modelling - II. Multi-Wavelength Faint Counts from a Semi-Analytic Model of Galaxy Formation
(Abridged) This paper predicts self-consistent faint galaxy counts from the
UV to the submm wavelength range. The STARDUST spectral energy distributions
described in Devriendt et al. (1999) are embedded within the explicit
cosmological framework of a simple semi-analytic model of galaxy formation and
evolution. We build a class of models which capture the luminosity budget of
the universe through faint galaxy counts and redshift distributions in the
whole wavelength range spanned by our spectra. In contrast with a rather stable
behaviour in the optical and even in the far-IR, the submm counts are
dramatically sensitive to variations in the cosmological parameters and changes
in the star formation history. Faint submm counts are more easily accommodated
within an open universe with a low value of , or a flat universe with
a non-zero cosmological constant. This study illustrates the implementation of
multi-wavelength spectra into a semi-analytic model. In spite of its
simplicity, it already provides fair fits of the current data of faint counts,
and a physically motivated way of interpolating and extrapolating these data to
other wavelengths and fainter flux levels.Comment: 13 pages, 10 figures, to appear in A&
The spectral appearance of primeval galaxies
The current and forthcoming observations of large samples of high-redshift
galaxies selected according to various photometric and spectroscopic criteria
can be interpreted in the context of galaxy formation, by means of models of
evolving spectral energy distributions (SEDs). We hereafter present STARDUST
which gives synthetic SEDs from the far UV to the submm wavelength range. These
SEDs are designed to be implemented into semi-analytic models of galaxy
formation.Comment: 10 pages, Latex, 8 postscript figures, to be published in the
Proceedings of the meeting ``Clustering at High Redshift'', ASP Conference
Serie
Galaxy Modelling -- I. Spectral Energy Distributions from Far-UV to Sub-mm Wavelengths
(abridged) We present STARDUST, a new self-consistent modelling of the
spectral energy distributions (SEDs) of galaxies from far-UV to radio
wavelengths. In order to derive the SEDs in this broad spectral range, we first
couple spectrophotometric and (closed-box) chemical evolutions to account for
metallicity effects on the spectra of synthetic stellar populations. We then
use a phenomenological fit for the metal-dependent extinction curve and a
simple geometric distribution of the dust to compute the optical depth of
galaxies and the corresponding obscuration curve. This enables us to calculate
the fraction of stellar light reprocessed in the infrared range. In a final
step, we define a dust model with various components and we fix the weights of
these components in order to reproduce the IRAS correlation of IR colours with
total IR luminosities. This allows us to compute far-IR SEDs that
phenomenologically mimic observed trends. We are able to predict the spectral
evolution of galaxies in a broad wavelength range, and we can reproduce the
observed SEDs of local spirals, starbursts, luminous infrared galaxies (LIRGs)
and ultra luminous infrared galaxies (ULIRGs). This modelling is so far kept as
simple as possible and depends on a small number of free parameters, namely the
initial mass function (IMF), star formation rate (SFR) time scale, gas density,
and galaxy age, as well as on more refined assumptions on dust properties and
the presence (or absence) of gas inflows/outflows.Comment: 20 pages, 23 figures, Accepted for publication in Astronomy and
Astrophysics Main Journa
The FIR/submm window on galaxy formation
Our view on the deep universe has been so far biased towards optically bright
galaxies. Now, the measurement of the Cosmic Infrared Background in FIRAS and
DIRBE residuals, and the observations of FIR/submm sources by the ISOPHOT and
SCUBA instruments begin unveiling the ``optically dark side'' of galaxy
formation. Though the origin of dust heating is still unsolved, it appears very
likely that a large fraction of the FIR/submm emission is due to
heavily-extinguished star formation. Consequently, the level of the CIRB
implies that about 2/3 of galaxy/star formation in the universe is hidden by
dust shrouds. In this review, we introduce a new modeling of galaxy formation
and evolution that provides us with specific predictions in FIR/submm
wavebands. These predictions are compared with the current status of the
observations. Finally, the capabilities of current and forthcoming instruments
for all-sky and deep surveys of FIR/submm sources are briefly described.Comment: 10 pages, Latex, 5 postscript figures, to appear in ``The Birth of
Galaxies'', 1999, B. Guiderdoni, F.R. Bouchet, T.X. Thuan & J. Tran Thanh Van
(eds), Editions Frontiere
Contribution of Galaxies to the Background Hydrogen-Ionizing Flux
We estimate the evolution of the contribution of galaxies to the cosmic
background flux at by means of a semi-analytic model of galaxy
formation and evolution. Such a modelling has been quite successful in
reproducing the optical properties of galaxies. We assume hereafter the
high-redshift damped Lyman- (DLA) systems to be the progenitors of
present day galaxies, and we design a series of models which are consistent
with the evolution of cosmic comoving emissivities in the available near
infrared (NIR), optical, ultraviolet (UV), and far infrared (FIR) bands along
with the evolution of the neutral hydrogen content and average metallicity of
damped Lyman- systems (DLA). We use these models to compute the
galactic contribution to the Lyman-limit emissivity and background flux for . We take into account the absorption of Lyman-limit photons by
HI and dust in the interstellar medium (ISM) of the galaxies. We find that the
background Lyman-limit flux due to galaxies might dominate (or be comparable
to) the contribution from quasars at almost all redshifts if the absorption by
HI in the ISM is neglected. The ISM HI absorption results in a severe
diminishing of this flux--by almost three orders of magnitude at high redshifts
to between one and two orders at . Though the resulting galaxy flux
is completely negligible at high redshifts, it is comparable to the quasar flux
at .Comment: 14 pages, 5 figures, requires mn.sty, accepted for publication in
MNRA
LeMoMaF: Lensed Mock Map Facility
We present the Lensed Mock Map Facility (LeMoMaF), a tool designed to perform
mock weak lensing measurements on numerically simulated chunks of the universe.
Coupling N-body simulations to a semi-analytical model of galaxy formation,
LeMoMaF can create realistic lensed images and mock catalogues of galaxies, at
wavelengths ranging from the UV to the submm. To demonstrate the power of such
a tool we compute predictions of the source-lens clustering effect on the
convergence statistics, and quantify the impact of weak lensing on galaxy
counts in two different filters. We find that the source-lens clustering effect
skews the probability density function of the convergence towards low values,
with an intensity which strongly depends on the redshift distribution of
galaxies. On the other hand, the degree of enhancement or depletion in galaxy
counts due to weak lensing is independent of the source-lens clustering effect.
We discuss the impact on the two-points shear statistics to be measured by
future missions like SNAP and LSST. The source-lens clustering effect would
bias the estimation of sigma_8 from two point statistics by 2% -5%. We conclude
that accurate photometric redshifts for individual galaxies are necessary in
order to quantify and isolate the source-lens clustering effect.Comment: 14 pages, 11 figures, submitted to MNRA
Top-Down Fragmentation of a Warm Dark Matter Filament
We present the first high-resolution n-body simulations of the fragmentation
of dark matter filaments. Such fragmentation occurs in top-down scenarios of
structure formation, when the dark matter is warm instead of cold. In a
previous paper (Knebe et al. 2002, hereafter Paper I), we showed that WDM
differs from the standard Cold Dark Matter (CDM) mainly in the formation
history and large-scale distribution of low-mass haloes, which form later and
tend to be more clustered in WDM than in CDM universes, tracing more closely
the filamentary structures of the cosmic web. Therefore, we focus our
computational effort in this paper on one particular filament extracted from a
WDM cosmological simulation and compare in detail its evolution to that of the
same CDM filament. We find that the mass distribution of the halos forming via
fragmentation within the filament is broadly peaked around a Jeans mass of a
few 10^9 Msun, corresponding to a gravitational instability of smooth regions
with an overdensity contrast around 10 at these redshifts. Our results confirm
that WDM filaments fragment and form gravitationally bound haloes in a top-down
fashion, whereas CDM filaments are built bottom-up, thus demonstrating the
impact of the nature of the dark matter on dwarf galaxy properties.Comment: 7 pages, 7 figures, replaced with MNRAS accepted version (minor
revisions
Reionization history constraints from neural network based predictions of high-redshift quasar continua
Observations of the early Universe suggest that reionization was complete by
, however, the exact history of this process is still unknown. One
method for measuring the evolution of the neutral fraction throughout this
epoch is via observing the Ly damping wings of high-redshift quasars.
In order to constrain the neutral fraction from quasar observations, one needs
an accurate model of the quasar spectrum around Ly, after the spectrum
has been processed by its host galaxy but before it is altered by absorption
and damping in the intervening IGM. In this paper, we present a novel machine
learning approach, using artificial neural networks, to reconstruct quasar
continua around Ly. Our QSANNdRA algorithm improves the error in this
reconstruction compared to the state-of-the-art PCA-based model in the
literature by 14.2% on average, and provides an improvement of 6.1% on average
when compared to an extension thereof. In comparison with the extended PCA
model, QSANNdRA further achieves an improvement of 22.1% and 16.8% when
evaluated on low-redshift quasars most similar to the two high-redshift quasars
under consideration, ULAS J1120+0641 at and ULAS J1342+0928 at
, respectively. Using our more accurate reconstructions of these two
quasars, we estimate the neutral fraction of the IGM using a homogeneous
reionization model and find at
and at . Our
results are consistent with the literature and favour a rapid end to
reionization
On the Observed Diversity of Star Formation Efficiencies in Giant Molecular Clouds
Observations find a median star formation efficiency per free-fall time in
Milky Way Giant Molecular Clouds (GMCs) on the order of with dispersions of . The origin of this scatter in
is still debated and difficult to reproduce with analytical
models. We track the formation, evolution and destruction of GMCs in a
hydrodynamical simulation of a Milky Way-like galaxy and by deriving cloud
properties in an observationally motivated way, measure the distribution of
star formation efficiencies which are in excellent agreement with observations.
We find no significant link between and any measured global
property of GMCs (e.g. gas mass, velocity dispersion). Instead, a wide range of
efficiencies exist in the entire parameter space. From the cloud evolutionary
tracks, we find that each cloud follow a \emph{unique} evolutionary path which
gives rise to wide diversity in all properties. We argue that it is this
diversity in cloud properties, above all else, that results in the dispersion
of .Comment: 10 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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