616 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
Collision-induced galaxy formation: semi-analytical model and multi-wavelength predictions
A semi-analytic model is proposed that couples the Press-Schechter formalism
for the number of galaxies with a prescription for galaxy-galaxy interactions
that enables to follow the evolution of galaxy morphologies along the Hubble
sequence. Within this framework, we calculate the chemo-spectrophotometric
evolution of galaxies to obtain spectral energy distributions. We find that
such an approach is very successful in reproducing the statistical properties
of galaxies as well as their time evolution. We are able to make predictions as
a function of galaxy type: for clarity, we restrict ourselves to two categories
of galaxies: early and late types that are identified with ellipticals and
disks. In our model, irregulars are simply an early stage of galaxy formation.
In particular, we obtain good matches for the galaxy counts and redshift
distributions of sources from UV to submm wavelengths. We also reproduce the
observed cosmic star formation history and the diffuse background radiation,
and make predictions as to the epoch and wavelength at which the dust-shrouded
star formation of spheroids begins to dominate over the star formation that
occurs more quiescently in disks. A new prediction of our model is a rise in
the FIR luminosity density with increasing redshift, peaking at about , and with a ratio to the local luminosity density about 10 times higher than that in the blue
(B-band) which peaks near .Comment: Minor changes, replaced to match accepted MNRAS versio
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
Cloud Dispersal in Turbulent Flows
Cold clouds embedded in warm media are very common objects in astrophysics.
Their disruption timescale depends strongly on the dynamical configuration. We
discuss the evolution of an initially homogeneous cold cloud embedded in warm
turbulent gas. Within a couple of dynamical timescales, the filling factor of
the cold gas within the original cloud radius drops below 50%. Turbulent
diffusivities estimated from the time evolution of radial filling factor
profiles are not constant with time. Cold and warm gas are bodily transported
by turbulence and mixed. This is only mildly indicated by column density maps.
The radiation field within the cloud, however, increases by several orders of
magnitudes due to the mixing, with possible consequences for cloud chemistry
and evolution within a few dynamical timescales.Comment: 11 pages, 12 figures, accepted by MNRA
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
Magnetized Non-linear Thin Shell Instability: Numerical Studies in 2D
We revisit the analysis of the Non-linear Thin Shell Instability (NTSI)
numerically, including magnetic fields. The magnetic tension force is expected
to work against the main driver of the NTSI -- namely transverse momentum
transport. However, depending on the field strength and orientation, the
instability may grow. For fields aligned with the inflow, we find that the NTSI
is suppressed only when the Alfv\'en speed surpasses the (supersonic)
velocities generated along the collision interface. Even for fields
perpendicular to the inflow, which are the most effective at preventing the
NTSI from developing, internal structures form within the expanding slab
interface, probably leading to fragmentation in the presence of self-gravity or
thermal instabilities. High Reynolds numbers result in local turbulence within
the perturbed slab, which in turn triggers reconnection and dissipation of the
excess magnetic flux. We find that when the magnetic field is initially aligned
with the flow, there exists a (weak) correlation between field strength and gas
density. However, for transverse fields, this correlation essentially vanishes.
In light of these results, our general conclusion is that instabilities are
unlikely to be erased unless the magnetic energy in clouds is much larger than
the turbulent energy. Finally, while our study is motivated by the scenario of
molecular cloud formation in colliding flows, our results span a larger range
of applicability, from supernovae shells to colliding stellar winds.Comment: 12 pages, 17 figures, some of them at low resolution. Submitted to
ApJ, comments welcom
Massive spheroids can form in single minor mergers
Accepted for publication in MNRAS, 12 pages, 6 figuresUnderstanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of ‘ex situ’ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (M ∗ > 10 11M) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low ex situ mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11–0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong in situ star formation. While this channel produces a minority (∼5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the most massive spheroids need not involve major mergers – or any significant merging at all – contrary to what is classically believed.Peer reviewedFinal Accepted Versio
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
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