44 research outputs found
Lyman-\alpha{} Emitters in the context of hierarchical galaxy formation: predictions for VLT/MUSE surveys
The VLT Multi Unit Spectroscopic Explorer (MUSE) integral-field spectrograph
can detect Ly\alpha{} emitters (LAE) in the redshift range in a homogeneous way. Ongoing MUSE surveys will notably probe
faint Ly\alpha{} sources that are usually missed by current narrow-band
surveys. We provide quantitative predictions for a typical wedding-cake
observing strategy with MUSE based on mock catalogs generated with a
semi-analytic model of galaxy formation coupled to numerical Ly\alpha{}
radiation transfer models in gas outflows. We expect 1500 bright LAEs
( erg s cm) in a typical
Shallow Field (SF) survey carried over 100 arcmin, and
2,000 sources as faint as erg s cm in a Medium-Deep
Field (MDF) survey over 10 arcmin. In a typical Deep Field (DF) survey of 1
arcmin, we predict that 500 extremely faint LAEs (
erg s cm) will be found. Our
results suggest that faint Ly\alpha{} sources contribute significantly to the
cosmic Ly\alpha{} luminosity and SFR budget. While the host halos of bright
LAEs at z 3 and 6 have descendants with median masses of and respectively, the faintest sources
detectable by MUSE at these redshifts are predicted to reside in halos which
evolve into typical sub- and galaxy halos at z = 0. We expect
typical DF and MDF surveys to uncover the building blocks of Milky Way-like
objects, even probing the bulk of the stellar mass content of LAEs located in
their progenitor halos at z 3.Comment: 18 pages, 13 figures, accepted for publication in 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
Towards a new modelling of gas flows in a semi-analytical model of galaxy formation and evolution
We present an extended version of the semi-analytical model, GalICS. Like its
predecessor, eGalICS applies a post-treatment of the baryonic physics on
pre-computed dark-matter merger trees extracted from an N-body simulation. We
review all the mechanisms that affect, at any given time, the formation and
evolution of a galaxy in its host dark-matter halo. We mainly focus on the gas
cycle from the smooth cosmological accretion to feedback processes. To follow
this cycle with high accuracy we introduce some novel prescriptions: i) a
smooth baryonic accretion with two phases: a cold mode and a hot mode built on
the continuous dark-matter accretion. In parallel to this smooth accretion, we
implement the standard photoionisation modelling to reduce the input gas flow
on the smallest structures. ii) a complete monitoring of the hot gas phase. We
compute the evolution of the core density, the mean temperature and the
instantaneous escape fraction of the hot atmosphere by considering that the hot
gas is in hydrostatic equilibrium in the dark-matter potential well, and by
applying a principle of conservation of energy on the treatment of gas
accretion, supernovae and super massive black hole feedback iii) a new
treatment for disc instabilities based on the formation, the migration and the
disruption of giant clumps. The migration of such clumps in gas-rich galaxies
allows to form pseudo-bulges. The different processes in the gas cycle act on
different time scales, and we thus build an adaptive time-step scheme to solve
the evolution equations. The model presented here is compared in detail to the
observations of stellar-mass functions, star formation rates, and luminosity
functions, in a companion paper
Galaxy stellar mass assembly: the difficulty matching observations and semi-analytical predictions
Semi-analytical models (SAMs) are currently the best way to understand the
formation of galaxies within the cosmic dark-matter structures. While they
fairly well reproduce the local stellar mass functions, correlation functions
and luminosity functions, they fail to match observations at high redshift (z >
3) in most cases, particularly in the low-mass range. The inconsistency between
models and observations indicates that the history of gas accretion in
galaxies, within their host dark-matter halo, and the transformation of gas
into stars, are not well followed. Hereafter, we briefly present a new version
of the GalICS semi-analytical model. We explore the impacts of classical
mechanisms, such as supernova feedback or photoionization, on the evolution of
the stellar mass assembly. Even with a strong efficiency, these two processes
cannot explain the observed stellar mass function and star formation rate
distribution and some other relations. We thus introduce an ad-hoc modification
of the standard paradigm, based on the presence of a \textit{no-star-forming}
gas component, and a concentration of the star-forming gas in galaxy discs. The
main idea behind the existence of the no-star-forming gas reservoir is that
only a fraction of the total gas mass in a galaxy is available to form stars.
The reservoir generates a delay between the accretion of the gas and the star
formation process. This new model is in much better agreement with the
observations of the stellar mass function in the low-mass range than the
previous models, and agrees quite well with a large set of observations,
including the redshift evolution of the specific star formation rate. However,
it predicts a large fraction of no-star-forming baryonic gas, potentially
larger than observed, even if its nature has still to be examined in the
context of the missing baryon problem
Accretion, feedback and galaxy bimodality: a comparison of the GalICS semi-analytic model and cosmological SPH simulations
We compare the galaxy population of an SPH simulation to those predicted by
the GalICS semi-analytic model and a stripped down version without supernova
and AGN feedback. The SPH simulation and the no-feedback GalICS model make
similar predictions for the baryonic mass functions of galaxies and for the
dependence of these mass functions on environment and redshift. The two methods
also make similar predictions for the galaxy content of dark matter haloes as a
function of halo mass and for the gas accretion history of galaxies. Both the
SPH and no-feedback GalICS models predict a bimodal galaxy population at z=0.
The "red'' sequence of gas poor, old galaxies is populated mainly by satellite
systems while, contrary to observations, the central galaxies of massive haloes
lie on the "blue'' star-forming sequence as a result of continuing hot gas
accretion at late times. Furthermore, both models overpredict the observed
baryonic mass function, especially at the high mass end. In the full GalICS
model, supernova-driven outflows reduce the masses of low and intermediate mass
galaxies by about a factor of two. AGN feedback suppresses gas cooling in large
haloes, producing a sharp cut-off in the baryonic mass function and moving the
central galaxies of these massive haloes to the red sequence. Our results imply
that the observational failings of the SPH simulation and the no-feedback
GalICS model are a consequence of missing input physics rather than
computational inaccuracies, that truncating gas accretion by satellite galaxies
automatically produces a bimodal galaxy distribution with a red sequence, but
that explaining the red colours of the most massive galaxies requires a
mechanism like AGN feedback that suppresses the accretion onto central galaxies
in large haloes.Comment: 17 pages, 11 figures, submitted to MNRA
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
Search and analysis of giant radio galaxies with associated nuclei (SAGAN) -- I : New sample and multi-wavelength studies
We present the first results of a project called SAGAN, which is dedicated
solely to the studies of relatively rare megaparsec-scale radio galaxies in the
Universe, called giant radio galaxies (GRGs). We have identified 162 new GRGs
primarily from the NVSS with sizes ranging from ~0.71 Mpc to 2.82 Mpc in the
redshift range of ~0.03 - 0.95, of which 23 are hosted by quasars (giant radio
quasars, GRQs). As part of the project SAGAN, we have created a database of all
known GRGs, the GRG catalogue, from the literature (including our new sample);
it includes 820 sources. For the first time, we present the multi-wavelength
properties of the largest sample of GRGs. Our results establish that the
distributions of the radio spectral index and the black hole mass of GRGs do
not differ from the corresponding distributions of normal-sized radio galaxies
(RGs). However, GRGs have a lower Eddington ratio (ER) than RGs. Using the
mid-infrared data, we classified GRGs in terms of their accretion mode: either
a high-power radiatively efficient high-excitation state, or a radiatively
inefficient low-excitation state. We find that GRGs in high-excitation state
statistically have larger sizes, stronger radio power, jet kinetic power, and
higher ER than those in low-excitation state. Our analysis reveals a strong
correlation between the ER and the scaled jet kinetic power, which suggests a
disc-jet coupling. Our environmental study reveals that ~10% of all GRGs may
reside at the centres of galaxy clusters, in a denser galactic environment,
while the majority appears to reside in a sparse environment. We find that the
probability of BCG as a GRG is quite low. We present new results for GRGs that
range from black hole mass to large-scale environment properties. We discuss
their formation and growth scenarios, highlighting the key physical factors
that cause them to reach their gigantic size. Abridged.Comment: Accepted for publication in Astronomy & Astrophysics. 14 figures, 7
tables and 7 montages. Comments are welcome. "SAGAN Project website
https://sites.google.com/site/anantasakyatta/sagan