70 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
Topology and Dark Energy: Testing Gravity in Voids
Modified gravity has garnered interest as a backstop against dark matter and
dark energy (DE). As one possible modification, the graviton can become
massive, which introduces a new scalar field - here with a Galileon-type
symmetry. The field can lead to a nontrivial equation of state (EOS) of DE
which is density-and-scale-dependent. Tension between Type Ia supernovae and
Planck could be reduced. In voids the scalar field dramatically alters the EOS
of DE, induces a soon-observable gravitational slip between the two metric
potentials, and develops a topological defect (domain wall) due to a nontrivial
vacuum structure for the field.Comment: Revised version, added detail, conclusions unchanged, matches PRL
published version in content. 4 pages, 2 figure
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
The Chemical Signature of a Relic Star Cluster in the Sextans Dwarf Spheroidal Galaxy - Implications for Near-Field Cosmology
We present tentative evidence for the existence of a dissolved star cluster
at [Fe/H]=-2.7 in the Sextans dwarf spheroidal galaxy. We use the technique of
chemical tagging to identify stars that are highly clustered in a
multi-dimensional chemical abundance space (C-space). In a sample of six stars,
three, possibly four stars are identified as potential cluster stars. The
initial stellar mass of the parent cluster is estimated from two independent
observations to M*,init=1.9^{+1.5}_{-0.9} (1.6^{+1.2}_{-0.8}) x 10^5 M_sol,
assuming a Salpeter (Kroupa) initial mass function (IMF). If corroborated by
follow-up spectroscopy, this star cluster is the most metal-poor system
identified to date. Chemical signatures of remnant clusters in dwarf galaxies
like Sextans provide us with a very powerful probe to the high-redshift
Universe. From available observational data, we argue that the average star
cluster mass in the majority of the newly discovered ultra-faint dwarf galaxies
was notably lower than it is in the Galaxy today and possibly lower than in the
more luminous, classical dwarf spheroidal galaxies. Furthermore, the mean
cumulative metallicity function of the dwarf spheroidals falls below that of
the ultra-faints, which increases with increasing metallicity as predicted from
our stochastic chemical evolution model. These two findings, together with a
possible difference in the ratio suggest that the ultra-faint dwarf
galaxy population, or a significant fraction thereof, and the dwarf spheroidal
population, were formed in different environments and would thus be distinct in
origin.Comment: 18 pages, 8 figures, new Fig. 5, some clarifications, references
added, accepted for publication in the Ap
Hierarchical models of high redshift galaxies with thermally pulsing asymptotic giant branch stars: comparison with observations
In a recent paper we presented the first semi-analytic model of galaxy
formation in which the Thermally-Pulsing Asymptotic Giant Branch phase of
stellar evolution has been fully implemented. Here we address the comparison
with observations, and show how the TP-AGB recipe affects the performance of
the model in reproducing the colours and near-IR luminosities of high-redshift
galaxies. We find that the semi-analytic model with the TP-AGB better matches
the colour-magnitude and colour-colour relations at z ~ 2, both for
nearly-passive and for star-forming galaxies. The model with TP-AGB produces
star-forming galaxies with red V-K colours, thus revising the unique
interpretation of high-redshift red objects as 'red & dead'. We also show that
without the TP-AGB the semi-analytic model fails at reproducing the observed
colours, a situation that cannot be corrected by dust reddening. We also
explore the effect of nebular emission on the predicted colour-magnitude
relation of star-forming galaxies, to conclude that it does not play a
significant role in reddening their colours, at least in the range of
star-formation rates covered by the model. Finally, the rest-frame K-band
luminosity function at z ~ 2.5 is more luminous by almost 1 magnitude. This
indicates that the AGN feedback recipe that is adopted to regulate the
high-mass end of the luminosity function should be sophisticated to take the
effect of the stellar populations into account at high redshifts.Comment: 10 pages, 8 figures; effects of nebular emission included; accepted
for publication on MNRA
- …