410 research outputs found
The two regimes of the cosmic sSFR evolution are due to spheroids and discs
This paper aims at explaining the two phases in the observed specific star
formation rate (sSFR), namely the high (>3/Gyr) values at z>2 and the smooth
decrease since z=2. In order to do this, we compare to observations the
specific star formation rate evolution predicted by well calibrated models of
chemical evolution for elliptical and spiral galaxies, using the additional
constraints on the mean stellar ages of these galaxies (at a given mass). We
can conclude that the two phases of the sSFR evolution across cosmic time are
due to different populations of galaxies. At z>2 the contribution comes from
spheroids: the progenitors of present-day massive ellipticals (which feature
the highest sSFR) as well as halos and bulges in spirals (which contribute with
average and lower-than-average sSFR). In each single galaxy the sSFR decreases
rapidly and the star formation stops in <1 Gyr. However the combination of
different generations of ellipticals in formation might result in an apparent
lack of strong evolution of the sSFR (averaged over a population) at high
redshift. The z<2 decrease is due to the slow evolution of the gas fraction in
discs, modulated by the gas accretion history and regulated by the Schmidt law.
The Milky Way makes no exception to this behaviour.Comment: 8 pages, 5 figures, MNRAS accepte
The dust content of QSO hosts at high redshift
Infrared observations of high-z quasar (QSO) hosts indicate the presence of
large masses of dust in the early universe. When combined with other
observables, such as neutral gas masses and star formation rates, the dust
content of z~6 QSO hosts may help constraining their star formation history. We
have collected a database of 58 sources from the literature discovered by
various surveys and observed in the FIR. We have interpreted the available data
by means of chemical evolution models for forming proto-spheroids,
investigating the role of the major parameters regulating star formation and
dust production. For a few systems, given the derived small dynamical masses,
the observed dust content can be explained only assuming a top-heavy initial
mass function, an enhanced star formation efficiency and an increased rate of
dust accretion. However, the possibility that, for some systems, the dynamical
mass has been underestimated cannot be excluded. If this were the case, the
dust mass can be accounted for by standard model assumptions. We provide
predictions regarding the abundance of the descendants of QSO hosts; albeit
rare, such systems should be present and detectable by future deep surveys such
as Euclid already at z>4.Comment: 22 pages, 8 figures, MNRAS, accepte
Feedback from massive stars and gas expulsion from proto-globular clusters
© 2015. The American Astronomical Society. All rights reserved. Globular clusters (GCs) are considerably more complex structures than previously thought, harboring at least two stellar generations that present clearly distinct chemical abundances. Scenarios explaining the abundance patterns in GCs mostly assume that originally the clusters had to be much more massive than today, and that the second generation of stars originates from the gas shed by stars of the first generation (FG). The lack of metallicity spread in most GCs further requires that the supernova-enriched gas ejected by the FG is completely lost within ∼30 Myr, a hypothesis never tested by means of three-dimensional hydrodynamic simulations. In this paper, we use 3D hydrodynamic simulations including stellar feedback from winds and supernovae, radiative cooling and self-gravity to study whether a realistic distribution of OB associations in a massive proto-GC of initial mass M tot ∼ 10 7 M o is sufficient to expel its entire gas content. Our numerical experiment shows that the coherence of different associations plays a fundamental role: as the bubbles interact, distort, and merge, they carve narrow tunnels that reach deeper and deeper toward the innermost cluster regions, and through which the gas is able to escape. Our results indicate that after 3 Myr, the feedback from stellar winds is responsible for the removal of ∼40% of the pristine gas, and that after 14 Myr, 99% of the initial gas mass has been removed
Early chemical enrichment of the universe and the role of very massive pop III stars
In this paper the role of very massive pop III stars in the chemical
enrichment of the early universe is discussed. We first compare our predictions
with the abundance ratios measured in the high redshift Lyman-alpha forest to
check whether they are compatible with the values predicted by assuming that
the early universe was enriched by massive pop III stars. We conclude that to
explain the observed C/Si ratio in the intergalactic medium, a contribution
from pop II stars to carbon enrichment is necessary, already at redshift z=5.
We then evaluate the number of Pair-Instability Supernovae (SN_(gamma gamma))
required to enrich the universe to the critical metallicity Z_cr, i.e. the
metallicity value which causes the transition from a very massive star regime
(m > 100 M_sun) to a lower mass regime, similar to the one characteristic of
the present time (m < 100 M_sun). It is found that between 110 and 115
SN_(gamma gamma) are sufficient to chemically enrich a cubic megaparsec of the
intergalactic medium at high redshift for a variety of initial mass functions.
The number of ionizing photons provided by these SN_(gamma gamma) and also by
the pop III stars ending as black holes was computed and we conclude that there
are not enough photons to reionize the universe, being down by at least a
factor of ~ 3. Finally, we calculate the abundance ratios generated by pop III
stars and compare it with the ones observed in low metallicity Damped
Lyman-alpha systems (DLAs). We suggest that pop III stars alone cannot be
responsible for the abundance ratios in these objects and that intermediate
mass pop II stars must have played an important role especially in enriching
DLAs in nitrogen.Comment: 8 pages, 2 figures, MNRAS, accepted for publication - typos correcte
The [?/Fe] ratios of very metal-poor stars within the integrated galactic initial mass function theory
The aim of this paper is to quantify the amplitude of the predicted plateau in [α/Fe] ratios associated with the most metal-poor stars of a galaxy. We assume that the initial mass function (IMF) in galaxies is steeper if the star formation rate (SFR) is low – as per the integrated galactic initial mass function (IGIMF) theory. A variant of the theory, in which the IGIMF depends upon the metallicity of the parent galaxy, is also considered. The IGIMF theory predicts low [α/Fe] plateaus in dwarf galaxies, characterized by small SFRs. The [α/Fe] plateau is up to 0.7 dex lower than the corresponding plateau of the Milky Way. For a universal IMF one should expect instead that the [α/Fe] plateau is the same for all the galaxies, irrespective of their masses or SFRs. Assuming a strong dependence of the IMF on the metallicity of the parent galaxy, dwarf galaxies can show values of the [α/Fe] plateau similar to those of the Milky Way, and almost independent of the SFR. The [Mg/Fe] ratios of the most metal-poor stars in dwarf galaxies satellites of the Milky Way can be reproduced either if we consider metallicity-dependent IMFs or if the early SFRs of these galaxies were larger than we presently think. Present and future observations of dwarf galaxies can help disentangle between these different IGIMF formulations
The cosmic dust rate across the Universe
We investigate the evolution of interstellar dust in the Universe by means of chemical evolution models of galaxies of different morphological types, reproducing the main observed features of present-day galaxies. We adopt the most updated prescriptions for dust production from supernovae and asymptotic giant branch stars as well as for dust accretion and destruction processes. Then, we study the cosmic dust rate in the framework of three different cosmological scenarios for galaxy formation: (i) a pure luminosity scenario, (ii) a number density evolution scenario, as suggested by the classical hierarchical clustering scenario and (iii) an alternative scenario, in which both spirals and ellipticals are allowed to evolve in number on an observationally motivated basis. Our results give predictions about the evolution of the dust content in different galaxies as well as the cosmic dust rate as a function of redshift. Concerning the cosmic dust rate, the best scenario is the alternative one, which predicts a peak at 2 < z < 3 and reproduces the cosmic star formation rate. We compute the evolution of the comoving dust density parameter \u3a9dust and find agreement with data for z < 0.5 in the framework of DE and alternative scenarios. Finally, the evolution of the average cosmic metallicity is presented and it shows a quite fast increase in each scenario, reaching the solar value at the present time, although most of the heavy elements are incorporated into solid grains, and therefore not observable in the gas phase
Spectral catalogue of bright gamma-ray bursts detected with the BeppoSAX/GRBM
The emission process responsible for the so-called "prompt" emission of
gamma-ray bursts is still unknown. A number of empirical models fitting the
typical spectrum still lack a satisfactory interpretation. A few GRB spectral
catalogues derived from past and present experiments are known in the
literature and allow to tackle the issue of spectral properties of gamma-ray
bursts on a statistical ground. We extracted and studied the time-integrated
photon spectra of the 200 brightest GRBs observed with the Gamma-Ray Burst
Monitor which flew aboard the BeppoSAX mission (1996-2002) to provide an
independent statistical characterisation of GRB spectra. The spectra were fit
with three models: a simple power-law, a cut-off power law or a Band function.
The typical photon spectrum of a bright GRB consists of a low-energy index
around 1.0 and a peak energy of the nuFnu spectrum E_p~240 keV in agreement
with previous results on a sample of bright CGRO/BATSE bursts. Spectra of ~35%
of GRBs can be fit with a power-law with a photon index around 2, indicative of
peak energies either close to or outside the GRBM energy boundaries. We confirm
the correlation between E_p and fluence, with a logarithmic dispersion of 0.13
around the power-law with index 0.21+-0.06. The low-energy and peak energy
distributions are not yet explained in the current literature. The capability
of measuring time-resolved spectra over a broadband energy range, ensuring
precise measurements of parameters such as E_p, will be crucial for future
experiments (abridged).Comment: 28 pages, 20 figures, 3 tables, accepted to A&
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