66 research outputs found
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
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
The elliptical colour-magnitude relation as a discriminant between the monolithic and merger paradigms: the importance of progenitor bias
The colour-magnitude relation (CMR) of cluster ellipticals has been widely
used to constrain their star formation histories (SFHs) and to discriminate
between the monolithic and merger paradigms of elliptical galaxy formation. We
investigate the elliptical CMR predicted in the merger paradigm by using a LCDM
hierarchical merger model. We first highlight sections of the literature which
indicate that the traditional use of fixed apertures to derive colours gives a
distorted view of the CMR due to the presence of colour gradients in galaxies.
Fixed aperture observations make the CMR steeper and tighter than it really is.
We then show that the star formation history (SFH) of cluster ellipticals
predicted by the model is quasi-monolithic, with over 95 percent of the total
stellar mass formed before a redshift of 1. The quasi-monolithic SFH produces a
predicted CMR that agrees well at all redshifts with its observed counterpart
once the fixed aperture effect is removed. More importantly, we present
arguments to show that the elliptical-only CMR can be used to constrain the
SFHs of present-day cluster ellipticals only if we believe a priori in the
monolithic collapse model. It is not a meaningful tool for constraining the SFH
in the merger paradigm, because a progressively larger fraction of the
progenitor set of present-day cluster ellipticals is contained in late-type
star forming systems at higher redshift, which cannot be ignored when deriving
the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between
the two competing theories of elliptical galaxy evolution.Comment: replaced with accepted versio
Cooling, Gravity and Geometry: Flow-driven Massive Core Formation
We study numerically the formation of molecular clouds in large-scale
colliding flows including self-gravity. The models emphasize the competition
between the effects of gravity on global and local scales in an isolated cloud.
Global gravity builds up large-scale filaments, while local gravity --
triggered by a combination of strong thermal and dynamical instabilities --
causes cores to form. The dynamical instabilities give rise to a local focusing
of the colliding flows, facilitating the rapid formation of massive
protostellar cores of a few 100 M. The forming clouds do not reach an
equilibrium state, though the motions within the clouds appear comparable to
``virial''. The self-similar core mass distributions derived from models with
and without self-gravity indicate that the core mass distribution is set very
early on during the cloud formation process, predominantly by a combination of
thermal and dynamical instabilities rather than by self-gravity.Comment: 13 pages, 12 figures, accepted by Ap
GALICS III: Predicted properties for Lyman Break Galaxies at redshift 3
This paper illustrates how mock observational samples of high-redshift
galaxies with sophisticated selection criteria can be extracted from the
predictions of GALICS, a hybrid model of hierarchical galaxy formation that
couples the outputs of large cosmological simulations and semi-analytic recipes
to describe dark matter collapse and the physics of baryons respectively. As an
example of this method, we focus on the properties of Lyman Break Galaxies at
redshift 3. With the MOMAF software package described in a companion paper, we
generate a mock observational sample with selection criteria as similar as
possible to those implied in the actual observations of z = 3 LBGs by Steidel
et al.(1995). Our model predictions are in good agreement with the observed
number density and 2D correlation function. We investigate the optical/IR
luminosity budget as well as several other physical properties of LBGs and find
them to be in general agreement with observed values. Looking into the future
of these LBGs we predict that 75% of them end up as massive ellipticals today,
even though only 35% of all our local ellipticals are predicted to have a LBG
progenitor. In spite of some shortcomings, this new 'mock observation' method
clearly represents a necessary first step toward a more accurate comparison
between hierarchical models of galaxy formation and real observational surveys.Comment: 19 pages, 15 figures, submitted to MNRAS. Full resolution figures at
http://galics.iap.fr
Source-lens clustering effects on the skewness of the lensing convergence
The correlation between source galaxies and lensing potentials causes a
systematic effect on measurements of cosmic shear statistics, known as the
source-lens clustering (SLC) effect. The SLC effect on the skewness of lensing
convergence, , is examined using a nonlinear semi-analytic approach and is
checked against numerical simulations. The semi-analytic calculations have been
performed in a wide variety of generic models for the redshift distribution of
source galaxies and power-law models for the bias parameter between the galaxy
and dark matter distributions. The semi-analytic predictions are tested
successfully against numerical simulations. We find the relative amplitude of
the SLC effect on to be of the order of five to forty per cent. It
depends significantly on the redshift distribution of sources and on the way
the bias parameter evolves. We discuss possible measurement strategies to
minimize the SLC effects.Comment: 14 pages, 14 figures, accepted for publication in MNRA
Formation of Structure in Molecular Clouds: A Case Study
Molecular clouds (MCs) are highly structured and ``turbulent''. Colliding gas
streams of atomic hydrogen have been suggested as a possible source of MCs,
imprinting the filamentary structure as a consequence of dynamical and thermal
instabilities. We present a 2D numerical analysis of MC formation via
converging HI flows. Even with modest flow speeds and completely uniform
inflows, non-linear density perturbations as possible precursors of MCs arise.
Thus, we suggest that MCs are inevitably formed with substantial structure,
e.g., strong density and velocity fluctuations, which provide the initial
conditions for subsequent gravitational collapse and star formation in a
variety of galactic and extragalactic environments.Comment: 4 pages, 5 figures, resubmitted to ApJ
MoMaF : The Mock Map Facility
We present the Mock Map Facility, a powerful tool to generate mock catalogues
or images from semi-analytically post-processed snapshots of cosmological
N-body simulations. The paper describes in detail an efficient technique to
create such mocks from the GALICS semi-analytic model, providing the reader
with an accurate quantification of the artifacts it introduces at every step.
We show that replication effects introduce a negative bias on the clustering
signal -- typically peaking at less than 10 percent around the correlation
length. We also thoroughly discuss how the clustering signal is affected by
finite volume effects, and show that it vanishes at scales larger than about a
tenth of the simulation box size. For the purpose of analysing our method, we
show that number counts and redshift distributions obtained with GALICS and
MOMAF compare well to K-band observations and to the 2dFGRS. Given finite
volume effects, we also show that the model can reproduce the APM angular
correlation function. The MOMAF results discussed here are made publicly
available to the astronomical community through a public database. Moreover, a
user-friendly Web interface (http://galics.iap.fr) allows any user to recover
her/his own favourite galaxy samples through simple SQL queries. The
flexibility of this tool should permit a variety of uses ranging from extensive
comparisons between real observations and those predicted by hierarchical
models of galaxy formation, to the preparation of observing strategies for deep
surveys and tests of data processing pipelines.Comment: 19 pages, 15 Figs, significantly modified version now accepted for
publication in MNRAS. High-resolution version available at
http://galics.cosmologie.fr/papers/momaf.ps.g
Merger Histories in Warm Dark Matter Structure Formation Scenario
Observations on galactic scales seem to be in contradiction with recent high
resolution N-body simulations. This so-called cold dark matter (CDM) crisis has
been addressed in several ways, ranging from a change in fundamental physics by
introducing self-interacting cold dark matter particles to a tuning of complex
astrophysical processes such as global and/or local feedback. All these efforts
attempt to soften density profiles and reduce the abundance of satellites in
simulated galaxy halos. In this paper, we explore a somewhat different approach
which consists of filtering the dark matter power spectrum on small scales,
thereby altering the formation history of low mass objects. The physical
motivation for damping these fluctuations lies in the possibility that the dark
matter particles have a different nature i.e. are warm (WDM) rather than cold.
We show that this leads to some interesting new results in terms of the merger
history and large-scale distribution of low mass halos, as compared to the
standard CDM scenario. However, WDM does not appear to be the ultimate
solution, in the sense that it is not able to fully solve the CDM crisis, even
though one of the main drawbacks, namely the abundance of satellites, can be
remedied. Indeed, the cuspiness of the halo profiles still persists, at all
redshifts, and for all halos and sub-halos that we investigated. Despite the
persistence of the cuspiness problem of DM halos, WDM seems to be still worth
taking seriously, as it alleviates the problems of overabundant sub-structures
in galactic halos and possibly the lack of angular momentum of simulated disk
galaxies. WDM also lessens the need to invoke strong feedback to solve these
problems, and may provide a natural explanation of the clustering properties
and ages of dwarfs.Comment: 11 pages, 17 figures, MNRAS submitted, high-res figures can be found
at http://www-thphys.physics.ox.ac.uk/users/AlexanderKnebe/publications.html,
replaced with accepted version (warmon masses corrected!
The Horizon Run 5 Cosmological Hydrodynamic Simulation: Probing Galaxy Formation from Kilo- to Giga-parsec Scales
Horizon Run 5 (HR5) is a cosmological hydrodynamical simulation which captures the properties of the Universe on a Gpc scale while achieving a resolution of 1kpc. Inside the simulation box we zoom-in on a high-resolution cuboid region with a volume of 1049×114×114cMpc3.The sub-grid physics chosen to model galaxy formation includes radiative heating/cooling, UV background, star formation, supernova feedback, chemical evolution tracking the enrichment of oxygen and iron, the growth of supermassive black holes and feedback from active galactic nuclei (AGN) in the form of a dual jet-heating mode. For this simulation we implemented a hybrid MPI-OMP version of RAMSES, specifically targeted for modern many-core many thread parallel architectures. In addition to the traditional simulation snapshots, light-cone data was generated on the fly. For the post-processing, we extended the Friends-of-Friend (FoF) algorithm and developed a new galaxy finder PGalF to analyse the outputs of HR5. The simulation successfully reproduces observations, such as the cosmic star formation history and connectivity of galaxy distribution, We identify cosmological structures at a wide range of scales, from filaments with a length of several cMpc, to voids with a radius of ~100 cMpc. The simulation also indicates that hydrodynamical effects on small scales impact galaxy clustering up to very large scales near and beyond the baryonic acoustic oscillation (BAO) scale. Hence, caution should be taken when using that scale as a cosmic standard ruler: one needs to carefully understand the corresponding biases. The simulation is expected to be an invaluable asset for the interpretation of upcoming deep surveys of the Universe
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