arXiv subjects

Francesca Matteucci

Publications and source records attributed to Francesca Matteucci.

At least 73 records · Page 4Linked to original sources

arXiv2013

Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 AA absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log epsilon (Ba) < 0.7 dex (3 sigma). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 AA-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range.

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arXiv2012

The thick disk rotation-metallicity correlation as a fossil of an "inverse chemical gradient" in the early Galaxy

The thick disk rotation--metallicity correlation, \partial V_ϕ/\partial[Fe/H] =40÷50 km s^{-1}dex^{-1} represents an important signature of the formation processes of the galactic disk. We use nondissipative numerical simulations to follow the evolution of a Milky Way (MW)-like disk to verify if secular dynamical processes can account for this correlation in the old thick disk stellar population. We followed the evolution of an ancient disk population represented by 10 million particles whose chemical abundances were assigned by assuming a cosmologically plausible radial metallicity gradient with lower metallicity in the inner regions, as expected for the 10-Gyr-old MW. Essentially, inner disk stars move towards the outer regions and populate layers located at higher |z|. A rotation--metallicity correlation appears, which well resembles the behaviour observed in our Galaxy at a galactocentric distance between 8 kpc and 10 kpc. In particular,we measure a correlation of \partial V_ϕ/\partial[Fe/H]\simeq 60 km s^{-1}dex^{-1} for particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Our pure N-body models can account for the V_ϕvs. [Fe/H] correlation observed in the thick disk of our Galaxy, suggesting that processes internal to the disk such as heating and radial migration play a role in the formation of this old stellar component. In this scenario, the positive rotation-metallicity correlation of the old thick disk population would represent the relic signature of an ancient "inverse" chemical (radial) gradient in the inner Galaxy, which resulted from accretion of primordial gas.

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arXiv2012

Small-scale hero: massive-star enrichment in the Hercules dwarf spheroidal

Dwarf spheroidal galaxies are often conjectured to be the sites of the first stars. The best current contenders for finding the chemical imprints from the enrichment by those massive objects are the "ultrafaint dwarfs" (UFDs). Here we present evidence for remarkably low heavy element abundances in the metal poor Hercules UFD. Combined with other peculiar abundance patterns this indicates that Hercules was likely only influenced by very few, massive explosive events - thus bearing the traces of an early, localized chemical enrichment with only very little other contributions from other sources at later times.

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arXiv2011

Manganese evolution in Omega Centauri: a clue to the cluster formation mechanisms?

We model the evolution of manganese relative to iron in the progenitor system of the globular cluster Omega Centauri by means of a self-consistent chemical evolution model. We use stellar yields that already reproduce the measurements of [Mn/Fe] versus [Fe/H] in Galactic field disc and halo stars, in Galactic bulge stars and in the Sagittarius dwarf spheroidal galaxy. We compare our model predictions to the Mn abundances measured in a sample of 10 red giant members and 6 subgiant members of Omega Cen. The low values of [Mn/Fe] observed in a few, metal-rich stars of the sample cannot be explained in the framework of our standard, homogeneous chemical evolution model. Introducing cooling flows that selectively bring to the cluster core only the ejecta from specific categories of stars does not help to heal the disagreement with the observations. The capture of field stars does not offer a viable explanation either. The observed spread in the data and the lowest [Mn/Fe] values could, in principle, be understood if the system experienced inhomogeneous chemical evolution. Such an eventuality is qualitatively discussed in the present paper. However, more measurements of Mn in Omega Cen stars are needed to settle the issue of Mn evolution in this cluster.

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arXiv2011

Chemical Evolution of Dwarf Irregular and Blue Compact Galaxies

Dwarf irregular and blue compact galaxies are very interesting objects since they are relatively simple and unevolved. We present new models for the chemical evolution of these galaxies by assuming different regimes of star formation (bursting and continuous) and different kinds of galactic winds (normal and metal-enhanced). Our results show that in order to reproduce all the properties of these galaxies, including the spread in the chemical abundances, the star formation should have proceeded in bursts and the number of bursts should be not larger than 10 in each galaxy, and that metal-enhanced galactic winds are required. A metal-enhanced wind efficiency increasing with galactic mass can by itself reproduce the observed mass-metallicity relation although also an increasing efficiency of star formation and/or number and/or duration of bursts can equally well reproduce such a relation. Metal enhanced winds together with an increasing amount of star formation with galactic mass are required to explain most of the properties of these galaxies. Normal galactic winds, where all the gas is lost at the same rate, do not reproduce the features of these galaxies. We suggest that these galaxies should have suffered a different number of bursts varying from 2 to 10 and that the efficiency of metal-enhanced winds should have been not too high ($λ_{mw}\sim1$). We predict for these galaxies present time Type Ia SN rates from 0.00084 and 0.0023 per century. Finally, by comparing the abundance patterns of Damped Lyman-$α$ objects with our models we conclude that they are very likely the progenitors of the present day dwarf irregulars. (abridged)

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arXiv2011

The Role of the IGIMF in the chemical evolution of the solar neighbourhood

The integrated galactic initial mass function (IGIMF) is computed from the combination of the stellar initial mass function (IMF) and the embedded cluster mass function, described by a power law with index beta. The result of the combination is a time-varying IMF which depends on the star formation rate. We applied the IGIMF formalism to a chemical evolution model for the solar neighbourhood and compared the results obtained by assuming three possible values for beta with the ones 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, higher Type Ia and II supernova rates, higher mass ejection rates and higher [alpha/Fe] values at a given metallicity. Our suggested fiducial value for beta is 2, since with this value we can account for most of the local observables. We discuss our results in a broader perspective, with some implications regarding the possible universality of the IMF and the importance of the star formation threshold.

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arXiv2010

Galactic astroarchaeology: reconstructing the bulge history by means of the newest data

The chemical abundances measured in stars of the Galactic bulge offer an unique opportunity to test galaxy formation models as well as impose strong constraints on the history of star formation and stellar nucleosynthesis. The aims of this paper are to compare abundance predictions from a detailed chemical evolution model for the bulge with the newest data. Some of the predictions have already appeared on previous paper (O, Mg, Si, S and Ca) but some other predictions are new (Ba, Cr and Ti). We compute several chemical evolution models by adopting different initial mass functions for the Galactic bulge and then compare the results to new data including both giants and dwarf stars in the bulge. In this way we can impose strong constraints on the star formation history of the bulge. We find that in order to reproduce at best the metallicity distribution function one should assume a flat IMF for the bulge not steeper than the Salpeter one. The initial mass function derived for the solar vicinity provides instead a very poor fit to the data. The [el/Fe] vs. [Fe/H] relations in the bulge are well reproduced by a very intense star formation rate and a flat IMF as in the case of the stellar metallicity distribution. Our model predicts that the bulge formed very quickly with the majority of stars formed inside the first 0.5 Gyr. Our results strongly suggest that the new data, and in particular the MDF of the bulge, confirm what concluded before and in particular that the bulge formed very fast, from gas shed by the halo, and that the initial mass function was flatter than in the solar vicinity and in the disk, although not so flat as previously thought. Finally, our model can also reproduce the decrease of the [O/Mg] ratio for [Mg/H] > 0 in the bulge, which is confirmed by the new data and interpreted as due to mass loss in massive stars.

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arXiv2010

The chemical evolution of IC10

Dwarf irregular galaxies are relatively simple unevolved objects where it is easy to test models of galactic chemical evolution. We attempt to determine the star formation and gas accretion history of IC10, a local dwarf irregular for which abundance, gas, and mass determinations are available. We apply detailed chemical evolution models to predict the evolution of several chemical elements (He, O, N, S) and compared our predictions with the observational data. We consider additional constraints such as the present-time gas fraction, the star formation rate (SFR), and the total estimated mass of IC10. We assume a dark matter halo for this galaxy and study the development of a galactic wind. We consider different star formation regimes: bursting and continuous. We explore different wind situations: i) normal wind, where all the gas is lost at the same rate and ii) metal-enhanced wind, where metals produced by supernovae are preferentially lost. We study a case without wind. We vary the star formation efficiency (SFE), the wind efficiency, and the time scale of the gas infall, which are the most important parameters in our models. We find that only models with metal-enhanced galactic winds can reproduce the properties of IC10. The star formation must have proceeded in bursts rather than continuously and the bursts must have been less numerous than ~10 over the whole galactic lifetime. Finally, IC10 must have formed by a slow process of gas accretion with a timescale of the order of 8 Gyr.

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arXiv2010

A decline and fall in the future of Italian Astronomy?

On May 27th 2010, the Italian astronomical community learned with concern that the National Institute for Astrophysics (INAF) was going to be suppressed, and that its employees were going to be transferred to the National Research Council (CNR). It was not clear if this applied to all employees (i.e. also to researchers hired on short-term contracts), and how this was going to happen in practice. In this letter, we give a brief historical overview of INAF and present a short chronicle of the few eventful days that followed. Starting from this example, we then comment on the current situation and prospects of astronomical research in Italy.

astro-ph.IM↗

arXiv2010

Chemical Evolution Models for Spiral Disks: the Milky Way, M31 and M33

The distribution of chemical abundances and their variation in time are important tools to understand the chemical evolution of galaxies: in particular, the study of chemical evolution models can improve our understanding of the basic assumptions made for modelling our Galaxy and other spirals. To test a standard chemical evolution model for spiral disks in the Local Universe and study the influence of a threshold gas density and different efficiencies in the star formation rate (SFR) law on radial gradients (abundance, gas and SFR). We adopt a one-infall chemical evolution model where the Galactic disk forms inside-out by means of infall of gas, and we test different thresholds and efficiencies in the SFR. The model is scaled to the disk properties of three Local Group galaxies (the Milky Way, M31 and M33) by varying its dependence on the star formation efficiency and the time scale for the infalling gas into the disk. Using this simple model we are able to reproduce most of the observed constraints available in the literature for the studied galaxies. The radial oxygen abundance gradients and their time evolution are studied in detail. The present day abundance gradients are more sensitive to the threshold than to other parameters, while their temporal evolutions are more dependent on the chosen SFR efficiency. The most massive disks seem to have evolved faster (i.e. with more efficient star formation) than the less massive ones, thus suggesting a downsizing in star formation for spirals. The threshold and the efficiency of star formation play a very important role in the chemical evolution of spiral disks and an efficiency varying with radius can be used to regulate the star formation. The oxygen abundance gradient can steepen or flatten in time depending on the choice of this parameter

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arXiv2009

Stellar mass-loss, rotation and the chemical enrichment of early type galaxies

We present a comparison between the [Ca,C,N/Fe]-mass relations observed in local spheroids and the results of a chemical evolution model which already successfully reproduces the [Mg/Fe]-mass and the [Fe/H]-mass relations in these systems. We find that the [Ca/Fe]-mass relation is naturally explained by such a model without any additional assumption. In particular, the observed under-abundance of Ca with respect to Mg can be attributed to the different contributions from supernovae Type Ia and supernovae Type II to the nucleosynthesis of these two elements. For C and N, we consider new stellar yields that take into account stellar mass loss and rotation. These yields have been shown to successfully reproduce the C and N abundances in Milky Way metal-poor stars. The use of these new stellar yields produces a good agreement between the chemical evolution model predictions and the integrated stellar population observations for C. In the case of N, the inclusion of fast rotators and stellar mass-loss nucleosynthesis prescriptions improves our predictions for the slope of the [N/Fe] vs. sigma relation, but a zero point discrepancy of 0.3 dex remains. This work demonstrates that current stellar yields are unable to simultaneously reproduce the large mean stellar [ ] ratios inferred from integrated spectra of elliptical galaxies and the low N abundance measured in the gas of high redshift spheroids from absorption lines. However, it seems reasonable to suggest that there may be uncertainties in either the inferred stellar or gas-phase N abundances at the level of 0.3 dex. (abriged)

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arXiv2008

The timescales of chemical enrichment in the Galaxy

The time-scales of chemical enrichment are fundamental to understand the evolution of abundances and abundance ratios in galaxies. In particular, the time-scales for the enrichment by SNe II and SNe Ia are crucial in interpreting the evolution of abundance ratios such as [alpha/Fe]. In fact, the alpha-elements are produced mainly by SNe II on time-scales of the order of 3 to 30 Myr, whereas the Fe is mainly produced by SNe Ia on a larger range of time-scales, going from 30 Myr to a Hubble time. This produces differences in the [alpha/Fe] ratios at high and low redshift and it is known as "time-delay" model. In this talk we review the most common progenitor models for SNe Ia and the derived rates together with the effect of the star formation history on the [alpha/Fe] versus [Fe/H] diagram in the Galaxy. From these diagrams we can derive the timescale for the formation of the inner halo (roughly 2 Gyr), the timescale for the formation of the local disk (roughly 7-8 Gyr) as well the time-scales for the formation of the whole disk. These are functions of the galactocentric distance and vary from 2-3 Gyr in the inner disk up to a Hubble time in the outer disk (inside-out formation). Finally, the timescale for the formation of the bulge is found to be no longer than 0.3 Gyr, similar to the timescale for the formation of larger spheroids such as elliptical galaxies. We show the time-delay model applied to galaxies of different morphological type, identified by different star formation histories, and how it constrains differing galaxy formation models.

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arXiv2008

Are dry mergers of Ellipticals the way to reconcile model predictions with the downsizing?

To show that the bulk of the star formation and the galaxy assembly should occur simultaneously in order to reproduce at the same time the downsizing and the chemical properties of present-day massive spheroids within one effective radius.By means of chemical evolution models we create galactic building blocks of several masses and different chemical properties. We then construct a sample of possible merger histories going from a multiple minor merger scenario to a single major merger event aimed at reproducing a single massive elliptical galaxy. We compare our results against the mass-[Mg/Fe] and the mass-metallicity relations. We found that a series of multiple dry-mergers (no star formation in connection with the merger) involving building-blocks which have been created ad hoc in order to satisfy the [Mg/Fe]-mass relation cannot fit the mass-metallicity relation and viceversa. A major dry merger, instead, does not worsen the agreement with observation if it happens between galaxies which already obey to both the mass- or sigma-[Mg/Fe] and the mass-(sigma-) metallicity relations. However, this process alone cannot explain the physical reasons for these trends. Dry mergers alone cannot be the way to reconcile the need of a more efficient star formation in the most massive galaxies with the late time assembly suggested in the hierarchical paradigm in order to recover the galaxy downsizing.

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arXiv2008

Chemical evolution of the Milky Way and its Satellites

This paper contains the lectures I delivered during the 37th Saas-Fee Advanced Course in March 2007. It reviews all the main ingredients necessary to build galactic chemical evolution models with particular attention to the Milky Way and the dwarf spheroidals of the Local Group. Both analytical and numerical models are discussed. Model results are compared to observations in order to infer constraints on stellar nucleosynthesis and on the formation and evolution of galaxies.Particular attention is devoted to interpret abundance ratios in galaxies with different star formation histories. Finally, the cosmic chemical evolution is discussed

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arXiv2008

The formation of the [alpha/Fe] radial gradients in the stars of elliptical galaxies

The scope of this paper is two-fold: i) to test and improve our previous models of an outside-in formation for the majority of ellipticals in the context of the SN-driven wind scenario, by means of a careful study of gas inflows/outflows; ii) to explain the observed slopes, either positive or negative, in the radial gradient of the mean stellar [alpha/Fe], and their apparent lack of any correlation with all the other observables. In order to pursue these goals we present a new class of hydrodynamical simulations for the formation of single elliptical galaxies in which we implement detailed prescriptions for the chemical evolution of H, He, O and Fe. We find that all the models which predict chemical properties (such as the central mass-weighted abundance ratios, the colours as well as the [ ] gradient) within the observed ranges for a typical elliptical, also exhibit a variety of gradients in the [ ] ratio, in agreement with the observations (namely positive, null or negative). All these models undergo an outside-in formation, in the sense that star formation stops earlier in the outermost than in the innermost regions, owing to the onset of a galactic wind. The typical [ ] gradients predicted by our models have a slope of -0.3 dex per decade variation in radius, consistent with the mean values of several observational samples. We can safely conclude that the history of star formation is fundamental for the creation of abundance gradients in ellipticals but that radial flows with different velocity in conjunction with the duration and efficiency of star formation in different galactic regions are responsible for the gradients in the [ ] ratios.

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arXiv2008

The role of massive stars in galactic chemical evolution

I will review the role of massive stars in galactic evolution both from the nucleosynthesis and energetics point of view. In particular, I will highlight some important observational facts explained by means of massive stars in galaxies of different morphological type: the Milky Way, ellipticals and dwarf spheroidals. I will describe first the time-delay model and its interpretation in terms of abundance ratios in galaxies, then I will discuss the importance of mass loss in massive stars to reproduce the data in the Galactic bulge and disk. I will discuss also how massive stars can be important producers of primary nitrogen if rotation in stellar models is taken into account. Concerning elliptical galaxies, I will show that to reproduce the observed [Mg/Fe] versus Mass relation in these galaxies it is necessary to assume a more important role of massive stars in more massive galaxies and that this can be achieved by means of downsizing in star formation. I will discuss how massive stars are responsible in triggering galactic winds both in ellipticals and dwarf spheroidals. These latter systems show a low overabundance of alpha-elements relative to Fe with respect to Galactic stars of the same [Fe/H]: this is interpreted as due to a slow star formation coupled with very efficient galactic winds. Finally, I will show a comparison between the predicted Type Ib/c rates in galaxies and the observed GRB rate and how we can impose constraints on the mechanism of galaxy formation by studying the GRB rate at high redshift.

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arXiv2008

A comparison of the s- and r-process element evolution in local dwarf spheroidal galaxies and in the Milky Way

We study the nucleosynthesis of several neutron capture elements (barium, europium, lanthanum, and yttrium) in local group dwarf spheroidal (dSph) galaxies and in the Milky Way by comparing the evolution of [Ba/Fe], [Eu/Fe], [La/Fe], [Y/Fe], [Ba/Y], [Ba/Eu], [Y/Eu], and [La/Eu] observed in dSph galaxies and in our Galaxy with predictions of detailed chemical evolution models. The models for all dSph galaxies and for the Milky Way are able to reproduce several observational features of these galaxies, such as a series of abundance ratios and the stellar metallicities distributions. The Milky Way model adopts the two-infall scenario, whereas the most important features of the models for the dSph galaxies are the low star-formation rate and the occurrence of intense galactic winds. We predict that the [s-r/Fe] ratios in dSphs are generally different than the corresponding ratios in the Milky Way, at the same [Fe/H] values. This is interpreted as a consequence of the time-delay model coupled with different star formation histories. In particular, the star-formation is less efficient in dSphs than in our Galaxy and it is influenced by strong galactic winds. Our predictions are in very good agreement with the available observational data. The time-delay model for the galactic chemical enrichment coupled with different histories of star formation in different galaxies allow us to succesfully interpret the observed differences in the abundance ratios of s- and r- process elements, as well as of $α$-elements in dSphs and in the Milky Way. These differences strongly suggest that the main stellar populations of these galaxies could not have had a common origin and, consequently, that the progenitors of local dSphs might not be the same objects as the building blocks of our Galaxy.

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arXiv2008

The chemical evolution of a Milky Way-like galaxy: the importance of a cosmologically motivated infall law

We aim at finding a cosmologically motivated infall law to understand if the LambdaCDM cosmology can reproduce the main chemical characteristics of a Milky Way-like spiral galaxy. In this work we test several different gas infall laws, starting from that suggested in the two-infall model for the chemical evolution of the Milky Way by Chiappini et al., but focusing on laws derived from cosmological simulations which follows a concordance LambdaCDM cosmology. By means of a detailed chemical evolution model for the solar vicinity, we study the effects of the different gas infall laws on the abundance patterns and the G-dwarf metallicity distribution. The cosmological gas infall law predicts two main gas accretion episodes. By means of this cosmologically motivated infall law, we study the star formation rate, the SNIa and SNII rate, the total amount of gas and stars in the solar neighbourhood and the behaviour of several chemical abundances. We find that the results of the two-infall model are fully compatible with the evolution of the Milky Way with cosmological accretion laws. A gas assembly history derived from a DM halo, compatible with the formation of a late-type galaxy from the morphological point of view, can produce chemical properties in agreement with the available observations.

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