Francesca Matteucci

Abundances Of Light Elements In Metal-Poor Stars IV. Fe/O And Fe/Mg Ratios And The History Of Star Formation In The Solar Neighborhood

The accurate O, Mg and Fe abundances derived in previous papers of this series from a homogeneous reanalysis of high quality data for a large sample of stars are combined with stellar kinematics in order to discuss the history of star for- mation in the solar neighborhood. We found that the Fe/O and Fe/Mg abundance ratios are roughly constant in the (inner) halo and the thick disk; this means that the timescale of halo col- lapse was shorter than or of the same order of typical lifetime of progenitors of type Ia SNe (� 1 Gyr), this conclusion being somewhat relaxed (referring to star formation in the individual fragments) in an accretion model for the Galaxy formation. Both Fe/O and Fe/Mg ratios raised by � 0.2 dex while the O/H and Mg/H ratios hold constant during the transition from the thick to thin disk phases, indicating a sudden decrease in star forma- tion in the solar neighbourhood at that epoch. These results are discussed in the framework of current views of Galaxy forma- tion; they fit in a scenario where both dissipational collapse and accretions were active on a quite similar timescale.

The effects of the initial mass function on the chemical evolution of elliptical galaxies

We describe the use of our chemical evolution model to reproduce the abundance patterns observed in a catalog of elliptical galaxies from the SDSS DR4. The model assumes ellipticals form by fast gas accretion, and suffer a strong burst of star formation followed by a galactic wind which quenches star formation. Models with fixed IMF failed in simultaneously reproducing the observed trends with the galactic mass. So, we tested a varying IMF; contrary to the diffused claim that the IMF should become bottom heavier in more massive galaxies, we find a better agreement with data by assuming an inverse trend, where the IMF goes from being bottom heavy in less massive galaxies to top heavy in more massive ones. This naturally produces a downsizing in star formation, favoring massive stars in largest galaxies. Finally, we tested the use of the Integrated Galactic IMF, obtained by averaging the canonical IMF over the mass distribution function of the clusters where star formation is assumed to take place. We combined two prescriptions, valid for different SFR regimes, to obtain the IGIMF values along the whole evolution of the galaxies in our models. Predicted abundance trends reproduce the observed slopes, but they have an offset relative to the data. We conclude that bottom-heavier IMFs do not reproduce the properties of the most massive ellipticals, at variance with previous suggestions. On the other hand, an IMF varying with galactic mass from bottom-heavier to top-heavier should be preferred

Fluorine in the solar neighborhood: Chemical evolution models

Context. In light of new observational data related to fluorine abundances in solar neighborhood stars, we present chemical evolution models testing various fluorine nucleosynthesis prescriptions with the aim to best fit those new data. Aim. We consider chemical evolution models in the solar neighborhood testing various nucleosynthesis prescriptions for fluorine production with the aim of reproducing the observed abundance ratios [F/O] versus [O/H] and [F/Fe] versus [Fe/H]. We study in detail the effects of various stellar yields on fluorine production. Methods. We adopted two chemical evolution models: the classical two-infall model, which follows the chemical evolution of halo-thick disk and thin disk phases; and the one-infall model, which is designed only for thin disk evolution. We tested the effects on the predicted fluorine abundance ratios of various nucleosynthesis yield sources, that is, asymptotic giant branch (AGB) stars, Wolf-Rayet (W-R) stars, Type II and Type Ia supernovae, and novae. Results. The fluorine production is dominated by AGB stars but the W-R stars are required to reproduce the trend of the observed data in the solar neighborhood with our chemical evolution models. In particular, the best model both for the two-infall and one-infall cases requires an increase by a factor of 2 of the W-R yields. We also show that the novae, even if their yields are still uncertain, could help to better reproduce the secondary behavior of F in the [F/O] versus [O/H] relation. Conclusions. The inclusion of the fluorine production by W-R stars seems to be essential to reproduce the new observed ratio [F/O] versus [O/H] in the solar neighborhood. Moreover, the inclusion of novae helps to reproduce the observed fluorine secondary behavior substantially. (Less)

ALMA Constrains the Stellar Initial Mass Function of Dusty Starburst Galaxies

The Messenger 172 – June 2018 (for example, in excess of 1000 M⊙ yr –1, see Ivison et al., 1998) — have their ultraviolet and optical stellar light heavily obscured by dust (see Figure 1). However, according to theories and cosmological simulations, it is in exactly these systems where the most extreme IMF variations would arise. Are there any other sensible, indirect methods to probe the IMF in these important, dust-shrouded systems?

The connection between the Galactic halo and ancient Dwarf Satellites

We explore the hypothesis that the classical and ultra-faint dwarf spheroidal satellites of the Milky Way have been the building blocks of the Galactic halo by comparing their [O/Fe] and [Ba/Fe] versus [Fe/H] patterns with the ones observed in Galactic halo stars. Oxygen abundances deviate substantially from the observed abundances in the Galactic halo stars for [Fe/H] values larger than -2 dex, while they overlap for lower metallicities. On the other hand, for the [Ba/Fe] ratio the discrepancy is extended at all [Fe/H] values, suggesting that the majority of stars in the halo are likely to have been formed in situ. Therefore, we suggest that [Ba/Fe] ratios are a better diagnostic than [O/Fe] ratios. Moreover, we show the effects of an enriched infall of gas with the same chemical abundances as the matter ejected and/or stripped from dwarf satellites of the Milky Way on the chemical evolution of the Galactic halo. We find that the resulting chemical abundances of the halo stars depend on the assumed infall time scale, and the presence of a threshold in the gas for star formation.

On the origin of the helium-rich population in the peculiar globular cluster Omega Centauri

In this contribution we discuss the origin of the extreme helium-rich stars which inhabit the blue main sequence (bMS) of the Galactic globular cluster Omega Centauri. In a scenario where the cluster is the surviving remnant of a dwarf galaxy ingested by the Milky Way many Gyr ago, the peculiar chemical composition of the bMS stars can be naturally explained by considering the effects of strong differential galactic winds, which develop owing to multiple supernova explosions in a shallow potential well.

Formation and evolution of massive elliptical galaxies in clusters: a consistent picture from optical and X-ray properties

We present a new chemical evolution model meant to be a first step in the self-consistent study of both optical and X-ray properties of elliptical galaxies in cluster of galaxies. Detailed cooling and heating processes in the interstellar medium are taken into account using a mono-phase onezone treatment which allows a more reliable modelling of the galactic wind regime with respect to previous work. The model successfully reproduces simultaneously the mass-metallicity, colour-magnitude and the LX LB relations, as well as the observed trend of the [Mg/Fe] ratio as a function of sigma, by assuming that the gas infall and star formation timescales are shorter for brighter objects. We found that a late secondary accretion of gas from the environment plays a fundamental role in driving the LX LB and LX T relations and can explain their large observational scatter. The iron discrepancy, namely the too high predicted iron abundance in Xray haloes of ellipticals compared to observations, still persists. On the other hand, we predict [O/Fe] in the ISM which is in good agreement with the most recent observations. New predictions for the amounts of iron, oxygen and energy ejected into the intracluster medium are presented and we conclude that Type Ia supernovae play a fundamental role in the ICM enrichment. SNe Ia activity, in fact, may power a galactic wind lasting for a considerable amount of the galactic lifetime, even in the case for which the efficiency of energy transfer into the ISM per SN Ia event is much less than unity.

The Role of Novae in Galactic Chemical Evolution

We discuss the role of novae as producers of 7Li and CNO isotopes in the Milky Way. A detailed model for the chemical evolution of the Milky Way including novae, Type Ia supernovae, Type II supernovae as well as single low‐ and intermediate‐mass stars is adopted and the results are compared with the available observational constraints. It is shown that novae are among the most promising candidates in order to explain the steep rise off the Li plateau in the log e(7Li)‐[Fe/H] diagram observed for stars in the solar vicinity. We also find that novae are likely to be the main producers of 15N, whereas they should only partly contribute to 13C and 17O.

The chemical evolution of dwarf starburst galaxies

We will review the most popular models for the chemical evolution of some starburst galaxies, in particular dwarf irregular galaxies. These galaxies are relatively simple and unevolved objects with low metallicities and large gas contents, suggesting that they are either young or have undergone discontinuous star formation activity. Some dwarf irregulars are starburst galaxies currently experiencing an intense star formation event and they are known as blue compact galaxies or extragalactic H II regions. We will discuss the effects of the presence of dark matter halos together with stellar energetics (stellar winds and supernovae) on the development of a galactic wind in these systems. Particular emphasis will be given to the role of massive stars in driving the thermal and chemical evolution of the gas, in particular to type II supernovae. A comparison between different model predictions for abundances and abundance ratios will be used to impose constraints on the star formation history and on the amount of dark matter, which we found to be extremely important in these systems.

On the origin of Damped Lyman-alpha systems: a case for LSB galaxies?

We use a model of galaxy disk formation to explore the metallicities, dust content, and neutral-gas mass density of damped Lyman-