61 research outputs found
Relative merits of different types of rest-frame optical observations to constrain galaxy physical parameters
We present a new approach to constrain galaxy physical parameters from the
combined interpretation of stellar and nebular emission in wide ranges of
observations. This approach relies on the Bayesian analysis of any type of
galaxy spectral energy distribution using a comprehensive library of synthetic
spectra assembled using state-of-the-art models of star formation and chemical
enrichment histories, stellar population synthesis, nebular emission and
attenuation by dust. We focus on the constraints set by 5-band photometry and
low- and medium-resolution spectroscopy at optical rest wavelengths on a set of
physical parameters characterizing the stars and the interstellar medium. Since
these parameters cannot be known a priori for any galaxy sample, we assess the
accuracy to which they can be retrieved by simulating `pseudo-observations'
using models with known parameters. Assuming that these models are good
approximations of true galaxies, we find that the combined analysis of stellar
and nebular emission in low-resolution galaxy spectra provides valuable
constraints on all physical parameters. At higher resolution, the analysis of
the combined stellar and nebular emission in 12,660 SDSS star-forming galaxies
using our approach yields likelihood distributions of stellar mass, gas-phase
oxygen abundance, optical depth of the dust and specific star formation rate
similar to those obtained in previous separate analyses of the stellar and
nebular emission at the original (twice higher) SDSS spectral resolution. We
show that the constraints derived on galaxy physical parameters from these
different types of observations depend sensitively on signal-to-noise ratio.
Our approach can be extended to the analysis of any type of observation across
the wavelength range covered by spectral evolution models. [abridged]Comment: 24 pages, 19 figures, accepted for publication in MNRAS.
Full-resolution version available from
ftp://ftp.iap.fr/pub/from_users/pacifici/paper_pacifici_hr.pd
The hierarchical formation of the brightest cluster galaxies
We use semi-analytic techniques to study the formation and evolution of
brightest cluster galaxies (BCGs). We show the extreme hierarchical nature of
these objects and discuss the limits of simple ways to capture their evolution.
In a model where cooling flows are suppressed at late times by AGN activity,
the stars of BCGs are formed very early (50 per cent at z~5, 80 per cent at
z~3) and in many small galaxies. The high star formation rates in these high-z
progenitors are fuelled by rapid cooling, not by merger-triggered starbursts.
We find that model BCGs assemble surprisingly late: half their final mass is
typically locked-up in a single galaxy after z~0.5. Because most of the
galaxies accreted onto BCGs have little gas content and red colours, late
mergers do not change the apparent age of BCGs. It is this accumulation of a
large number of old stellar populations -- driven mainly by the merging history
of the dark matter halo itself -- that yields the observed homogeneity of BCG
properties. In the second part of the paper, we discuss the evolution of BCGs
to high redshifts, from both observational and theoretical viewpoints. We show
that our model BCGs are in qualitative agreement with high-z observations. We
discuss the hierarchical link between high-z BCGs and their local
counter-parts. We show that high-z BCGs belong to the same population as the
massive end of local BCG progenitors, although they are not in general the same
galaxies. Similarly, high-z BCGs end-up as massive galaxies in the local
Universe, although only a fraction of them are actually BCGs of massive
clusters.Comment: 13 pages, 17 figures, MNRAS accepted versio
Building a control sample for galaxy pairs
Several observational works have attempted to isolate the effects of galaxy interactions by comparing galaxies in pairs with isolated galaxies. However, different authors have proposed different ways to build these so-called control samples (CS). By using mock galaxy catalogues of the Sloan Digital Sky Survey Data Release 4 buildup from the Millennium Simulation, we explore how the way of building a CS might introduce biases which could affect the interpretation of results. We make use of the fact that the physics of interactions are not included in the semi-analytic model, to infer that any difference between the mock control and pair samples can be ascribed to selection biases. Thus, we find that galaxies in pairs artificially tend to be older and more bulge dominated, and to have less cold gas and different metallicities than their isolated counterparts. Also because of a biased selection, galaxies in pairs tend to live in higher density environments and in haloes of larger masses. We find that imposing constraints on redshift, stellar masses and local densities diminishes the selection biases by ≈70 per cent. Based on these findings, we suggest observers how to build a unique and unbiased CS in order to reveal the effect of galaxy interactions.Facultad de Ciencias Astronómicas y Geofísica
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
A Detailed Study of Feedback from a Massive Star
We present numerical simulations of a 15 solar mass star in a suite of
idealised environments in order to quantify the amount of energy transmitted to
the interstellar medium (ISM). We include models of stellar winds, UV
photoionisation and the subsequent supernova based on theoretical models and
observations of stellar evolution. The system is simulated in 3D using
RAMSES-RT, an Adaptive Mesh Refinement Radiation Hydrodynamics code. We find
that stellar winds have a negligible impact on the system owing to their
relatively low luminosity compared to the other processes. The main impact of
photoionisation is to reduce the density of the medium into which the supernova
explodes, reducing the rate of radiative cooling of the subsequent supernova.
Finally, we present a grid of models quantifying the energy and momentum of the
system that can be used to motivate simulations of feedback in the ISM unable
to fully resolve the processes discussed in this work.Comment: 19 pages, 12 figures, accepted by MNRA
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
A Census of the LyC Photons that Form the UV Background During Reionization
We present a new, on-the-fly photon flux and absorption tracer algorithm
designed to directly measure the contribution of different source populations
to the metagalactic UV background and to the ionisation fraction of gas in the
Universe. We use a suite of multifrequency radiation hydrodynamics simulations
that are carefully calibrated to reproduce a realistic reionization history and
galaxy properties at , to disentangle the contribution of photons
emitted by different mass haloes and by stars with different metallicities and
ages to the UV background during reionization. While at very early cosmic times
low mass, metal poor haloes provide most of the LyC photons, their contribution
decreases steadily with time. At it is the photons emitted by massive
systems () and by the
metal enriched stars () that provide the
largest contribution to the ionising UV background. We demonstrate that there
are large variations in the escape fraction depending on the source, with the
escape fraction being highest () for photons emitted by the
oldest stars that penetrate into the IGM via low opacity channels carved by the
ionising photons and supernova from younger stars. Before HII regions begin to
overlap, the photoionisation rate strongly fluctuates between different,
isolated HII bubbles, depending on the embedded ionising source, which we
suggest may result in spatial variations in the properties of dwarf galaxies
Testing SALT Approximations with Numerical Radiation Transfer Code Part 1: Validity and Applicability
Absorption line spectroscopy offers one of the best opportunities to
constrain the properties of galactic outflows and the environment of the
circumgalactic medium. Extracting physical information from line profiles is
difficult, however, for the physics governing the underlying radiation transfer
is complicated and depends on many different parameters. Idealized analytical
models are necessary to constrain the large parameter spaces efficiently, but
are typically plagued by model degeneracy and systematic errors. Comparison
tests with idealized numerical radiation transfer codes offer an excellent
opportunity to confront both of these issues. In this paper, we present a
detailed comparison between SALT, an analytical radiation transfer model for
predicting UV spectra of galactic outflows, with the numerical radiation
transfer software, RASCAS. Our analysis has lead to upgrades to both models
including an improved derivation of SALT and a customizable adaptive mesh
refinement routine for RASCAS. We explore how well SALT, when paired with a
Monte Carlo fitting procedure, can recover flow parameters from non-turbulent
and turbulent flows. When the velocity and density gradients are excluded, we
find that flow parameters are well recovered from high resolution (20
) data and moderately well from medium resolution (100
) data without turbulence at a S/N = 10, while derived quantities
(e.g., mass outflow rates, column density, etc.) are well recovered at all
resolutions. In the turbulent case, biased errors emerge in the recovery of
individual parameters, but derived quantities are still well recovered
Building a control sample for galaxy pairs
Several observational works have attempted to isolate the effects of galaxy interactions by comparing galaxies in pairs with isolated galaxies. However, different authors have proposed different ways to build these so-called control samples (CS). By using mock galaxy catalogues of the Sloan Digital Sky Survey Data Release 4 buildup from the Millennium Simulation, we explore how the way of building a CS might introduce biases which could affect the interpretation of results. We make use of the fact that the physics of interactions are not included in the semi-analytic model, to infer that any difference between the mock control and pair samples can be ascribed to selection biases. Thus, we find that galaxies in pairs artificially tend to be older and more bulge dominated, and to have less cold gas and different metallicities than their isolated counterparts. Also because of a biased selection, galaxies in pairs tend to live in higher density environments and in haloes of larger masses. We find that imposing constraints on redshift, stellar masses and local densities diminishes the selection biases by ≈70 per cent. Based on these findings, we suggest observers how to build a unique and unbiased CS in order to reveal the effect of galaxy interactions.Facultad de Ciencias Astronómicas y Geofísica
Simulating the diversity of shapes of the Lyman- line
The Ly line is a powerful probe of distant galaxies, which contains
information about inflowing/outflowing gas through which Ly photons
scatter. To develop our understanding of this probe, we post-process a zoom-in
radiation-hydrodynamics simulation of a low-mass ()
galaxy to construct 22500 mock spectra in 300 directions from to 4.
Remarkably, we show that one galaxy can reproduce the variety of a large sample
of spectroscopically observed Ly line profiles. While most mock spectra
exhibit double-peak profiles with a dominant red peak, their shapes cover a
large parameter space in terms of peak velocities, peak separation and flux
ratio. This diversity originates from radiative transfer effects at ISM and CGM
scales, and depends on galaxy inclination and evolutionary phase. Red-dominated
lines preferentially arise in face-on directions during post-starburst outflows
and are bright. Conversely, accretion phases usually yield symmetric double
peaks in the edge-on direction and are fainter. While resonant scattering
effects at are responsible for the broadening and
velocity shift of the red peak, the extended CGM acts as a screen and impacts
the observed peak separation. The ability of simulations to reproduce observed
Ly profiles and link their properties with galaxy physical parameters
offers new perspectives to use Ly to constrain the mechanisms that
regulate galaxy formation and evolution. Notably, our study implies that deeper
Ly surveys may unveil a new population of blue-dominated lines tracing
inflowing gas.Comment: Accepted for publication in MNRA
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