65 research outputs found
Cosmological formation and chemical evolution of an elliptical galaxy
We aim at studying the effect of a cosmologically motivated gas infall law
for the formation of a massive elliptical galaxy in order to understand its
impact on the formation of the spheroids. We replace the empirical infall law
of the model by Pipino & Matteucci with a cosmologically derived infall law for
the formation of an elliptical galaxy. We constrast our predictions with
observations. We also compare the obtained results with those of Pipino &
Matteucci. We computed models with and without galactic winds: we found that
models without wind predict a too large current SNIa rate. In particular, the
cosmological model produces a current SNIa which is about ten times higher than
the observed values. Moreover models without wind predict a large current SNII
rate, too large even if compared with the recent GALEX data. The predicted SNII
rate for the model with wind, on the other hand, is too low if compared with
the star formation histories given by GALEX. Last but not least, the mean value
for the [Mg/Fe] ratio in the dominant stellar population of the simulated
galaxy, as predicted by the cosmological model, is too low if compared to
observations. This is, a very important result indicating that the cosmological
infall law is in contrast with the chemical evolution. A cosmologically derived
infall law for an elliptical galaxy cannot reproduce all the chemical
constraints given by the observations. The problem resides in the fact that the
cosmologically derived infall law implies a slow gas accretion with consequent
star formation rate active for a long period. In this situation low [Mg/Fe]
ratios are produced for the dominant stellar population in a typical
elliptical, at variance with observations.Comment: 8 pages, 6 figures, accepted for publication by A&
The origin of abundance gradients in the Milky Way: the predictions of different models
We aim at studying the abundance gradients along the Galactic disk and their
dependence upon several parameters: a threshold in the surface gas density
regulating star formation, the star formation efficiency, the timescale for the
formation of the thin disk and the total surface mass density of the stellar
halo. We test a model which considers a cosmological infall law. This law does
not predict an inside-out disk formation, but it allows to well fit the
properties of the solar vicinity. We study several cases. We find that to
reproduce at the same time the abundance, star formation rate and surface gas
density gradients along the Galactic disk it is necessary to assume an
inside-out formation for the disk. The threshold in the gas density is not
necessary and the same effect could be reached by assuming a variable star
formation efficiency. A cosmologically derived infall law with an inside-out
process for the disk formation and a variable star formation efficiency can
indeed well reproduce all the properties of the disk. However, the cosmological
model presented here does not have sufficient resolution to capture the
requested inside-out formation for the disk.Comment: 13 pages, 17 figures and 2 tables. Accepted for publication in
Astronomy & Astrophysic
Effects of radial flows on the chemical evolution of the Milky Way disk
The majority of chemical evolution models assume that the Galactic disk forms
by means of infall of gas and divide the disk into several independent rings
without exchange of matter between them. However, if gas infall is important,
radial gas flows should be taken into account as a dynamical consequence of
infall. The aim of this paper is to test the effect of radial gas flows on
detailed chemical evolution models (one-infall and two-infall) for the Milky
Way disk with different prescriptions for the infall law and star formation
rate. We found, that with a gas radial inflow of constant speed the metallicity
gradient tends to steepen. Taking into account a constant time scale for the
infall rate along the Galaxy disk and radial flows with a constant speed, we
obtained a too flat gradient, at variance with data, implying that an
inside-out formation and/or a variable gas flow speed are required. To
reproduce the observed gradients the gas flow should increase in modulus with
the galactocentric distance, both in the one-infall and two-infall models.
However, the inside-out disk formation coupled with a threshold in the gas
density (only in the two-infall model) for star formation and/or a variable
efficiency of star formation with galactocentric distance can also reproduce
the observed gradients without radial flows. We showed that the radial flows
can be the most important process in reproducing abundance gradients but only
with a variable gas speed. Finally, one should consider that uncertainties in
the data concerning gradients prevent us to draw firm conclusions. Future more
detailed data will help to ascertain whether the radial flows are a necessary
ingredient in the formation and evolution of the Galactic disk and disks in
general.Comment: Accepted by A&A; 11 pages, 16 figure
Effects of the integrated galactic IMF on the chemical evolution of the solar neighbourhood
The initial mass function determines the fraction of stars of different
intial mass born per stellar generation. In this paper, we test the effects of
the integrated galactic initial mass function (IGIMF) on the chemical evolution
of the solar neighbourhood. The IGIMF (Weidner & Kroupa 2005) is computed from
the combination of the stellar intial mass function (IMF), i.e. the mass
function of single star clusters, and the embedded cluster mass function, i.e.
a power law with index beta. By taking into account also the fact that the
maximum achievable stellar mass is a function of the total mass of the cluster,
the IGIMF becomes a time-varying IMF which depends on the star formation rate.
We applied this formalism to a chemical evolution model for the solar
neighbourhood and compared the results obtained by assuming three possible
values for beta with the results obtained by means of a standard, well-tested,
constant IMF. In general, a lower absolute value of beta implies a flatter
IGIMF, hence a larger number of massive stars and larger metal ejection rates.
This translates into higher type Ia and II supernova rates, higher mass
ejection rates from massive stars and a larger amount of gas available for star
formation, coupled with lower present-day stellar mass densities. (abridged) We
also discuss the importance of the present day stellar mass function (PDMF) in
providing a way to disentangle among various assumptions for beta. Our results
indicate that the model adopting the IGIMF computed with beta ~2 should be
considered the best since it allows us to reproduce the observed PDMF and to
account for most of the chemical evolution constraints considered in this work.Comment: 22 pages, 19 figure
Chemical gradients in the Milky Way from the RAVE data
Aims. We aim at measuring the chemical gradients of the elements Mg, Al, Si, and Fe along the Galactic radius to provide new constraints on the chemical evolution models of the Galaxy and Galaxy models such as the Besancon model. Thanks to the large number of stars of our RAVE sample we can study how the gradients vary as function of the distance from the Galactic plane.
Methods. We analysed three different samples selected from three independent datasets: a sample of 19 962 dwarf stars selected from the RAVE database, a sample of 10 616 dwarf stars selected from the Geneva-Copenhagen Survey (GCS) dataset, and a mock sample (equivalent to the RAVE sample) created by using the GALAXIA code, which is based on the Besancon model. The three samples were analysed by using the very same method for comparison purposes. We integrated the Galactic orbits and obtained the guiding radii (R-g) and the maximum distances from the Galactic plane reached by the stars along their orbits (Z(max)). We measured the chemical gradients as functions of R-g at different Z(max).
Results. We found that the chemical gradients of the RAVE and GCS samples are negative and show consistent trends, although they are not equal: at Z(max) < 0.4 kpc and 4.5 < R-g(kpc) < 9.5, the iron gradient for the RAVE sample is d[Fe/H]/dR(g) = -0.065 dex kpc(-1), whereas for the GCS sample it is d[Fe/H]/dR(g) = -0.043 dex kpc(-1) with internal errors of +/-0.002 and +/-0.004 dex kpc(-1), respectively. The gradients of the RAVE and GCS samples become flatter at larger Z(max). Conversely, the mock sample has a positive iron gradient of d[Fe/H]/dR(g) = +0.053 +/- 0.003 dex kpc(-1) at Z(max) < 0.4 kpc and remains positive at any Z(max). These positive and unrealistic values originate from the lack of correlation between metallicity and tangential velocity in the Besancon model. In addition, the low metallicity and asymmetric drift of the thick disc causes a shift of the stars towards lower R-g and metallicity which, together with the thin-disc stars with a higher metallicity and R-g, generates a fictitious positive gradient of the full sample. The flatter gradient at larger Z(max) found in the RAVE and the GCS samples may therefore be due to the superposition of thin-and thick-disc stars, which mimicks a flatter or positive gradient. This does not exclude the possibility that the thick disc has no chemical gradient. The discrepancies between the observational samples and the mock sample can be reduced by i) decreasing the density; ii) decreasing the vertical velocity; and iii) increasing the metallicity of the thick disc in the Besancon model
The Effect of Different Type Ia Supernova Progenitors on Galactic Chemical Evolution
Our aim is to show how different hypotheses about Type Ia supernova
progenitors can affect Galactic chemical evolution. We include different Type
Ia SN progenitor models, identified by their distribution of time delays, in a
very detailed chemical evolution model for the Milky Way which follows the
evolution of several chemical species. We test the single degenerate and the
double degenerate models for supernova Ia progenitors, as well as other more
empirical models based on differences in the time delay distributions. We find
that assuming the single degenerate or the double degenerate scenario produces
negligible differences in the predicted [O/Fe] vs. [Fe/H] relation. On the
other hand, assuming a percentage of prompt (exploding in the first 100 Myr)
Type Ia supernovae of 50%, or that the maximum Type Ia rate is reached after
3-4 Gyr from the beginning of star formation, as suggested by several authors,
produces more noticeable effects on the [O/Fe] trend. However, given the spread
still existing in the observational data no model can be firmly excluded on the
basis of only the [O/Fe] ratios. On the other hand, when the predictions of the
different models are compared with the G-dwarf metallicity distribution, the
scenarios with very few prompt Type Ia supernovae can be excluded. Models
including the single degenerate or double degenerate scenario with a percentage
of 10-13% of prompt Type Ia supernovae produce results in very good agreement
with the observations. A fraction of prompt Type Ia supernovae larger than 30%
worsens the agreement with observations and the same occurs if no prompt Type
Ia supernovae are allowed. In particular, two empirical models for the Type Ia
SN progenitors can be excluded: the one without prompt Type Ia supernovae and
the one assuming delay time distribution going like t^{-0.5}.Comment: Accepted by A&
New analytical solutions for chemical evolution models: : characterizing the population of star-forming and passive galaxies
E. Spitoni, V. Vincenzo, and F. Matteucci, 'New analytical solutions for chemical evolution models: characterizing the population of star-forming and passive galaxies', Astronomy & Astrophysics, Vol 599, first published online 20 February 2017, available at DOI: https://doi.org/10.1051/0004-6361/201629745. Reproduced with permission from Astronomy & Astrophysics, © 2017 ESO.Context. Analytical models of chemical evolution, including inflow and outflow of gas, are important tools for studying how the metal content in galaxies evolves as a function of time. Aims. We present new analytical solutions for the evolution of the gas mass, total mass, and metallicity of a galactic system when a decaying exponential infall rate of gas and galactic winds are assumed. We apply our model to characterize a sample of local star-forming and passive galaxies from the Sloan Digital Sky Survey data, with the aim of reproducing their observed mass-metallicity relation. Methods. We derived how the two populations of star-forming and passive galaxies differ in their particular distribution of ages, formation timescales, infall masses, and mass loading factors. Results. We find that the local passive galaxies are, on average, older and assembled on shorter typical timescales than the local star-forming galaxies; on the other hand, the star-forming galaxies with higher masses generally show older ages and longer typical formation timescales compared than star-forming galaxies with lower masses. The local starforming galaxies experience stronger galactic winds than the passive galaxy population. Exploring the effect of assuming different initial mass functions in our model, we show that to reproduce the observed mass-metallicity relation, stronger winds are requested if the initial mass function is top-heavy. Finally, our analytical models predict the assumed sample of local galaxies to lie on a tight surface in the 3D space defined by stellar metallicity, star formation rate, and stellar mass, in agreement with the well-known fundamental relation from adopting gas-phase metallicity. Conclusions. By using a new analytical model of chemical evolution, we characterize an ensemble of SDSS galaxies in terms of their infall timescales, infall masses, and mass loading factors. Local passive galaxies are, on average, older and assembled on shorter typical timescales than the local star-forming galaxies. Moreover, the local star-forming galaxies show stronger galactic winds than the passive galaxy population. Finally, we find that the fundamental relation between metallicity, mass, and star formation rate for these local galaxies is still valid when adopting the average galaxy stellar metallicity.Peer reviewe
Chemical evolution of local galaxies in a hierarchical model
We investigate the chemical properties of local galaxies within a
cosmological framework in the hierarchical picture of galaxy formation. To this
aim, we use a hierarchical semi-analytic model which includes the contribution
from (i) low and intermediate mass stars, (ii) type Ia Supernovae (SNe) and
(iii) massive stars. - Abridged - We compare our predictions with available
observations in the Milky Way (MW), in local dwarf galaxies and in local
ellipticals. For Milky-Way-like galaxies, we can successfully reproduce the
[O-Fe] vs [Fe/H] relation observed in disc stars and the stellar metallicity
distribution (SMD). For dwarf galaxies, the stellar metallicity vs mass
relation is reproduced by assuming that a substantial fraction of the heavy
elements is lost through metal-enhanced outflows and a type Ia SN realization
probability lower than the one of MW-like galaxies. - Abridged - In
ellipticals, the observations indicate higher [alpha/Fe] values in larger
galaxies. - Abridged - Our results computed with a standard Salpeter initial
mass function (IMF) indicate a flat [alpha/Fe]-sigma relation. However, we
suggest a possible solution to this problem and show how, by assuming a star
formation-dependent IMF with a slope x=1.35 in systems with star formation
rates < 100 M_sun/yr and slightly flatter (i.e. with x=1) in object with
stronger star formation, the observed correlation between [alpha/Fe] and sigma
can be accounted for on a large velocity dispersion range. Fundamental roles
are played also by interaction-triggered starbursts and AGN.Comment: MNRAS, accepted, 27 pages, 22 figure
A Stochastic Step Model of Replicative Senescence Explains ROS Production Rate in Ageing Cell Populations
Increases in cellular Reactive Oxygen Species (ROS) concentration with age have been observed repeatedly in mammalian tissues. Concomitant increases in the proportion of replicatively senescent cells in ageing mammalian tissues have also been observed. Populations of mitotic human fibroblasts cultured in vitro, undergoing transition from proliferation competence to replicative senescence are useful models of ageing human tissues. Similar exponential increases in ROS with age have been observed in this model system. Tracking individual cells in dividing populations is difficult, and so the vast majority of observations have been cross-sectional, at the population level, rather than longitudinal observations of individual cells
The cytoprotective drug amifostine modifies both expression and activity of the pro-angiogenic factor VEGF-A
Peer reviewedPublisher PD
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