36 research outputs found
Monolithic or hierarchical star formation? A new statistical analysis
We consider an analytic model of cosmic star formation which incorporates
supernova feedback, gas accretion and enriched outflows, reproducing the
history of cosmic star formation, metallicity, supernovae type II rates and the
fraction of baryons allocated to structures. We present a new statistical
treatment of the available observational data on the star formation rate and
metallicity that accounts for the presence of possible systematics. We then
employ a Bayesian Markov Chain Monte Carlo method to compare the predictions of
our model with observations and derive constraints on the 7 free parameters of
the model. We find that the dust correction scheme one chooses to adopt for the
star formation data is critical in determining which scenario is favoured
between a hierarchical star formation model, where star formation is prolonged
by accretion, infall and merging, and a monolithic scenario, where star
formation is rapid and efficient. We distinguish between these modes by
defining a characteristic minimum mass, M > 10^{11} M_solar, in our fiducial
model, for early type galaxies where star formation occurs efficiently. Our
results indicate that the hierarchical star formation model can achieve better
agreement with the data, but that this requires a high efficiency of
supernova-driven outflows. In a monolithic model, our analysis points to the
need for a mechanism that drives metal-poor winds, perhaps in the form of
supermassive black hole-induced outflows. Furthermore, the relative absence of
star formation beyond z ~ 5 in the monolithic scenario requires an alternative
mechanism to dwarf galaxies for reionizing the universe at z ~ 11, as required
by observations of the microwave background. While the monolithic scenario is
less favoured in terms of its quality-of-fit, it cannot yet be excluded.Comment: Expanded discussion on the role of mergers and on reionization in the
monolithic scenario, refs added, main results unchanged. Matches version to
appear in MNRA
Monolithic or hierarchical star formation? A new statistical analysis
We consider an analytic model of cosmic star formation which incorporates
supernova feedback, gas accretion and enriched outflows, reproducing the
history of cosmic star formation, metallicity, supernovae type II rates and the
fraction of baryons allocated to structures. We present a new statistical
treatment of the available observational data on the star formation rate and
metallicity that accounts for the presence of possible systematics. We then
employ a Bayesian Markov Chain Monte Carlo method to compare the predictions of
our model with observations and derive constraints on the 7 free parameters of
the model. We find that the dust correction scheme one chooses to adopt for the
star formation data is critical in determining which scenario is favoured
between a hierarchical star formation model, where star formation is prolonged
by accretion, infall and merging, and a monolithic scenario, where star
formation is rapid and efficient. We distinguish between these modes by
defining a characteristic minimum mass, M > 10^{11} M_solar, in our fiducial
model, for early type galaxies where star formation occurs efficiently. Our
results indicate that the hierarchical star formation model can achieve better
agreement with the data, but that this requires a high efficiency of
supernova-driven outflows. In a monolithic model, our analysis points to the
need for a mechanism that drives metal-poor winds, perhaps in the form of
supermassive black hole-induced outflows. Furthermore, the relative absence of
star formation beyond z ~ 5 in the monolithic scenario requires an alternative
mechanism to dwarf galaxies for reionizing the universe at z ~ 11, as required
by observations of the microwave background. While the monolithic scenario is
less favoured in terms of its quality-of-fit, it cannot yet be excluded.Comment: Expanded discussion on the role of mergers and on reionization in the
monolithic scenario, refs added, main results unchanged. Matches version to
appear in MNRA
The Infrared Luminosity of Galaxy Clusters
The aim of this study is to quantify the infrared luminosity of clusters as a
function of redshift and compare this with the X-ray luminosity. This can
potentially constrain the origin of the infrared emission to be intracluster
dust and/or dust heated by star formation in the cluster galaxies. We perform a
statistical analysis of a large sample of galaxy clusters selected from
existing databases and catalogues.We coadd the infrared IRAS and X-ray RASS
images in the direction of the selected clusters within successive redshift
intervals up to z = 1. We find that the total infrared luminosity is very high
and on average 20 times higher than the X-ray luminosity. If all the infrared
luminosity is to be attributed to emission from diffuse intracluster dust, then
the IR to X-ray ratio implies a dust-to-gas mass abundance of 5e-4. However,
the infrared luminosity shows a strong enhancement for 0.1 < z < 1, which
cannot be attributed to cluster selection effects. We show that this
enhancement is compatible with a star formation rate in the member galaxies
that is typical of the central Mpc of the Coma cluster at z = 0 and evolves
with the redshift as (1+z)^5. It is likely that most of the infrared luminosity
that we measure is generated by the ongoing star formation in the member
galaxies. From theoretical predictions calibrated on extinction measurements
(dust mass abundance equal to 1e-5), we expect only a minor contribution, of a
few percent, from intracluster dust.Comment: 9 pages, 7 figures, accepted july 31st 2008 for publication in
Astronomy and Astrophysics, language improved for this versio
The effective stability parameter for two-component galactic discs: Is 1/Q ~ 1/Q_stars + 1/Q_gas ?
The Wang-Silk approximation, 1/Q ~ 1/Q_stars + 1/Q_gas, is frequently used
for estimating the effective Q parameter in two-component discs of stars and
gas. Here we analyse this approximation in detail, and show how its accuracy
depends on the radial velocity dispersions and Toomre parameters of the two
components. We then propose a much more accurate but still simple approximation
for the effective Q parameter, which further takes into account the stabilizing
effect of disc thickness. Our effective Q parameter is a natural generalization
of Toomre's Q, and as such can be used in a wide variety of contexts, e.g. for
predicting star formation thresholds in galaxies or for measuring the stability
level of galactic discs at low and high redshifts.Comment: MNRAS, in pres
Tidal disruption of satellite galaxies in a semi-analytic model of galaxy formation
We introduce a new physical recipe into the De Lucia and Blaizot version of the Munich semi-analytic model built upon the Millennium dark matter simulation: the tidal stripping of stellar material from satellite galaxies during mergers. To test the significance of the new physical process we apply a Monte Carlo Markov Chain parameter estimation technique constraining the model with the -band luminosity function, colours and the black hole-bulge mass relation. The differences in parameter correlations, and in the allowed regions in likelihood space, reveal the impact of the new physics on the basic ingredients of the model, such as the star-formation laws, feedback recipes and the black hole growth model. With satellite disruption in place, we get a model likelihood four times higher than in the original model, indicating that the new process seems to be favoured by observations. This is achieved mainly due to a reduction in black hole growth that produces a better agreement between the properties of central black holes and host galaxies. Compared to the best-fit model without disruption, the new model removes the excess of dwarf galaxies in the original recipe with a more modest supernova heating. The new model is now consistent with the three observational data sets used to constrain it, while significantly improving the agreement with observations for the distribution of metals in stars. Moreover, the model now follows the build up of intra-cluster light
The effect of thermally pulsating asymptotic giant branch stars on the evolution of the rest-frame near-infrared galaxy luminosity function
We address the fundamental question of matching the rest-frame K-band
luminosity function (LF) of galaxies over the Hubble time using semi-analytic
models, after modification of the stellar population modelling. We include the
Maraston evolutionary synthesis models, that feature a higher contribution by
the Thermally Pulsating - Asymptotic Giant Branch (TP-AGB) stellar phase, into
three different semi-analytic models, namely the De Lucia and Blaizot version
of the Munich model, MORGANA and the Menci model. We leave all other input
physics and parameters unchanged. We find that the modification of the stellar
population emission can solve the mismatch between models and the observed
rest-frame K-band luminosity from the brightest galaxies derived from UKIDSS
data at high redshift. For all explored semi-analytic models this holds at the
redshifts - between 2 and 3 - where the discrepancy was recently pointed out.
The reason for the success is that at these cosmic epochs the model galaxies
have the right age (~1 Gyr) to contain a well-developed TP-AGB phase which
makes them redder without the need of changing their mass or age. At the same
time, the known overestimation of the faint end is enhanced in the K-band when
including the TP-AGB contribution. At lower redshifts (z<2) some of the
explored models deviate from the data. This is due to too short merging
timescales and inefficient 'radio-mode' AGN feedback. Our results show that a
strong evolution in mass predicted by hierarchical models is compatible with no
evolution on the bright-end of the K-band LF from z=3 to the local universe.
This means that, at high redshifts and contrary to what is commonly accepted,
K-band emission is not necessarily a good tracer of galaxy mass.Comment: 10 pages, 5 figures, accepted by MNRA
A Bayesian approach to the semi-analytic model of galaxy formation: methodology
We believe that a wide range of physical processes conspire to shape the
observed galaxy population but we remain unsure of their detailed interactions.
The semi-analytic model (SAM) of galaxy formation uses multi-dimensional
parameterisations of the physical processes of galaxy formation and provides a
tool to constrain these underlying physical interactions. Because of the high
dimensionality, the parametric problem of galaxy formation may be profitably
tackled with a Bayesian-inference based approach, which allows one to constrain
theory with data in a statistically rigorous way. In this paper we develop a
SAM in the framework of Bayesian inference. We show that, with a parallel
implementation of an advanced Markov-Chain Monte-Carlo algorithm, it is now
possible to rigorously sample the posterior distribution of the
high-dimensional parameter space of typical SAMs. As an example, we
characterise galaxy formation in the current CDM cosmology using the
stellar mass function of galaxies as an observational constraint. We find that
the posterior probability distribution is both topologically complex and
degenerate in some important model parameters, suggesting that thorough
explorations of the parameter space are needed to understand the models. We
also demonstrate that because of the model degeneracy, adopting a narrow prior
strongly restricts the model. Therefore, the inferences based on SAMs are
conditional to the model adopted. Using synthetic data to mimic systematic
errors in the stellar mass function, we demonstrate that an accurate
observational error model is essential to meaningful inference.Comment: revised version to match published article published in MNRA
Influence of Population III stars on cosmic chemical evolution
New observations from the Hubble ultra deep field suggest that the star
formation rate at z>7 drops off faster than previously thought. Using a newly
determined star formation rate for the normal mode of Population II/I stars
(PopII/I), including this new constraint, we compute the Thomson scattering
optical depth and find a result that is marginally consistent with WMAP5
results. We also reconsider the role of Population III stars (PopIII) in light
of cosmological and stellar evolution constraints. While this input may be
needed for reionization, we show that it is essential in order to account for
cosmic chemical evolution in the early Universe. We investigate the
consequences of PopIII stars on the local metallicity distribution function of
the Galactic halo (from the recent Hamburg/ESO survey of metal-poor stars) and
on the evolution of abundances with metallicity (based on the ESO large program
on very metal-poor stars), with special emphasis on carbon-enhanced metal-poor
stars. Our most important results show that the nucleosynthetic yields of
PopIII stars lead to abundance patterns in agreement with those observed in
extremely metal-poor stars. In this chemical approach to cosmic evolution,
PopIII stars prove to be a compulsory ingredient, and extremely metal-poor
stars are inevitably born at high redshift. (Abridged)Comment: 11 pages, 7 figures, MNRAS in pres
The impact of dark matter cusps and cores on the satellite galaxy population around spiral galaxies
(Abridged) We use N-body simulations to study the effects that a divergent
(i.e. "cuspy") dark matter (DM) profile introduces on the tidal evolution of
dwarf spheroidal galaxies (dSphs). Our models assume cosmologically-motivated
initial conditions where dSphs are DM-dominated systems on eccentric orbits
about a host galaxy composed of a dark halo and a baryonic disc. We find that
the resilience of dSphs to tidal stripping is extremely sensitive to the halo
cuspiness; whereas dwarfs with a cored profile can be easily destroyed by the
host disc, those with cusps always retain a bound remnant. For a given halo
profile the evolution of the structural parameters as driven by tides is
controlled solely by the total amount of mass lost. This information is used to
construct a semi-analytic code that simulates the hierarchical build-up of
spiral galaxies assuming different halo profiles and disc masses. We find that
tidal encounters with discs tend to decrease the average mass of satellites at
all galactocentric radii. Interestingly, satellites accreted before
re-ionization (z>6), which may be singled out by anomalous metallicity
patterns, survive only if haloes are cuspy. We show that the size-mass relation
established from Milky Way (MW) dwarfs strongly supports the presence of cusps
in the majority of these systems, as cored models systematically underestimate
the masses of the known Ultra-Faint dSphs. Our models also indicate that a
massive M31 disc may explain why many of its dSphs fall below the size-mass
relationship derived from MW dSphs. We use our models to constrain the mass
threshold below which star formation is suppressed in DM haloes, finding that
luminous satellites must be accreted with masses above 10^8--10^9 M_sol in
order to explain the size-mass relation observed in MW dwarfs.Comment: 17 pages, 14 figures, MNRAS accepted after minor revisio