Gustavo A. Lanfranchi

The predicted metallicity distribution of stars in dwarf spheroidal galaxies

We predict the metallicity distribution of stars and the age-metallicity relation for six dwarf spheroidal (dSph) galaxies of the Local Group by means of a chemical evolution model that is able to reproduce several observed abundance ratios, and the present-day total mass and gas content of these galaxies. The model adopts up-to-date nucleosynthesis and takes into account the role played by supernovae of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca and Fe). Each galaxy model is specified by the prescriptions of the star formation rate and by the galactic wind efficiency chosen to reproduce the main features of these galaxies. These quantities are constrained by the star formation histories of the galaxies as inferred by the observed colour-magnitude diagrams (CMD). The main conclusions are: (i) five of the six dSph galaxies are characterized by very low star formation efficiencies (v = 0.005-0.5 Gyr -1 ) with only Sagittarius having a higher one (ν = 1.0-5.0 Gyr -1 ); (ii) the wind rate is proportional to the star formation rate and the wind efficiency is high for all galaxies, in the range w i = 6-15; (iii) a high wind efficiency is required in order to reproduce the abundance ratios and the present-day gas mass of the galaxies; (iv) the predicted age-metallicity relation implies that the stars of the dSphs reach solar metallicities in a time-scale of the order of 2-6 Gyr, depending on the particular galaxy; (v) the metallicity distributions of stars in dSphs exhibit a peak around [Fe/H] ∼ -1.8 to -1.5 dex, with the exception of Sagittarius, which shows a peak around [Fe/H] ∼ -0.8 dex; (iv) the predicted metallicity distributions of stars suggest that the majority of stars in dSphs are formed in a range of metallicity in agreement with the one of the observed stars.

Chemical evolution of dwarf spheroidal and blue compact galaxies

We studied the star formation and chemical evolution in a sample of eight dwarf spheroidal (dSph) galaxies of the Local Group and in blue compact galaxies (BCGs) by means of comparison between the predictions of chemical evolution models and several observed abundance ratios. Detailed models with up-to-date nucleosynthesis taking into account the role played by supernovae of different types (II, Ia) were developed for both types of galaxies, allowing us to follow the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca and Fe). The models are specified by the prescriptions of the star formation and galactic wind efficiencies chosen to reproduce the main features of these galaxies. The BCGs are characterized by a star formation proceeding in several short bursts separated by long quiescent periods and by a low wind efficiency, whereas one or two long bursts and a very efficient wind well describe the dSph galaxies. We also investigated a possible connection in the evolution of dSph and BCGs and compared the predictions of the models to the abundance ratios observed in damped Lyman a systems (DLAs). The main conclusions are: (i) the observed distribution of [a/Fe] versus [Fe/H] in dSph galaxies is mainly a result of the star formation rate coupled with the wind efficiency; (ii) a low star formation efficiency (v = 0.01-1 Gyr - 1 ) and a high wind efficiency (w i ∼ 5-15) are required to reproduce the observational data for dSph galaxies; (iii) the low gas content of these galaxies is the result of the combined action of gas consumption by star formation and gas removal by galactic winds; (iv) the abundance ratios of the BCGs are reproduced by models with two to seven bursts of star formation and an efficiency in the range v = 0.1-0.9 Gyr - 1 ; (v) the low values of N/O observed in BCGs are the natural result of a bursting star formation; (vi) a connection between dSph and BCGs in a unified evolutionary scenario is unlikely; (vii) the models for both the dSph galaxies and BCGs reproduce the abundance ratios observed in DLAs, but imply different formation scenarios for these objects; (viii) a suitable amount of primary N produced in massive stars can be perhaps an explanation for the low plateau in the [N/a] distribution observed in DLAs, if real.

The effect of differential galactic winds on the chemical evolution of galaxies

Aims. The aim of this paper is to study the basic equations of the chemical evolution of galaxies with gas flows. In particular, we focus on models in which the outflow is differential, namely in which the heavy elements (or some of the heavy elements) can leave the parent galaxy more easily than other chemical species such as H and He. Methods. We study the chemical evolution of galaxies in the framework of simple models, namely we make simplifying assumptions about the lifetimes of stars and the mixing of freshly produced metals. This allows us to solve analytically the equations for the evolution of gas masses and metallicities. In particular, we find new analytical solutions for various cases in which the effects of winds and infall are taken into account. Results. Differential galactic winds, namely winds carrying out preferentially metals, have the effect of reducing the global metallicity of a galaxy, with the amount of reduction increasing with the ejection efficiency of the metals. Abundance ratios are predicted to remain constant throughout the whole evolution of the galaxy, even in the presence of differential winds. One way to change them is by assuming differential winds with different ejection efficiencies for different elements. However, simple models apply only to elements produced on short timescales, namely all by type II SNe, and therefore large differences in the ejection efficiencies of different metals are unlikely. Conclusions. Variations in abundance ratios such as [O/Fe] in galaxies, without including the Fe production by type Ia supernovae, can in principle be obtained by assuming an unlikely different efficiency in the loss of O relative to Fe from type II supernovae. Therefore, we conclude that it is not realistic to ignore type Ia supernovae and that the delayed production of some chemical elements relative to others (time-delay model) remains the most plausible explanation for the evolution of α-elements relative to Fe.

The evolution of barium and europium in local dwarf spheroidal galaxies

By means of a detailed chemical evolution model, we follow the evolution of barium and europium in four Local Group Dwarf Spheroidal Galaxies, in order to set constraints on the nucleosynthesis of these elements and on the evolution of this type of galaxies compared with the Milky Way. The model, which is able to reproduce several observed abundance ratios and the present day total mass and gas mass content of these galaxies, adopts up to date nucleosynthesis and takes into account the role played by supernovae of different types (II, Ia) allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba and Eu). By assuming that barium is a neutron capture element produced in low mass AGB stars by s-process but also in massive stars (in the mass range 10 - 30 M⊙) by r-process, during the explosive event of supernovae of type II, and that europium is a pure r-process element synthesized in massive stars also in the range of masses 10 - 30 M⊙, we are able to reproduce the observed [Ba/Fe] and [Eu/Fe] as functions of [Fe/H] in all four galaxies studied. We confirm also the important role played by the very low star formation efficiencies ( = 0.005 - 0.5 Gyr −1 ) and by the intense galactic winds (6-13 times the star formation rate) in the evolution of these galaxies. These low star formation efficiencies (compared to the one for the Milky Way disc) adopted for the Dwarf Spheroidal Galaxies are the main reason for the differences between the trends of [Ba/Fe] and [Eu/Fe] predicted and observed in these galaxies and in the metal-poor stars of our Galaxy. Finally, we provide predictions for Sagittarius galaxy for which data of only two stars are available.

Clues to the nature of damped Lyman α systems from chemical evolution models

The evolution of the metallicity of damped Lyman a systems (DLAs) is investigated in order to explore several scenarios for the nature of these systems. The observational data on chemical abundances of DLAs are analysed with robust statistical methods, and the abundances are corrected for dust depletion. The results of this analysis are compared with predictions of several classes of chemical evolution models describing a variety of scenarios for DLAs: one-zone dwarf galaxy models, multizone disc models and chemodynamical models representing dwarf galaxies. In order to settle constraints for star formation time-scales and metal production in DLAs, we compare the observational data on the [α/Fe] and [N/α] ratios to the predictions from the models. In DLAs, these ratios are only partially reproduced by the dwarf galaxy one-zone model and by the disc model. On the other hand, the chemodynamical model for dwarf galaxies reproduces the properties of nearly all DLAs. The connection between the gas flow evolution and the star formation rate is the reason for the ability of this model in reproducing the range of abundance ratios seen in DLAs. The comparison of the observed [α/Fe] and [N/a] trends with the predictions of the chemodynamical model is used to derive the formation epoch of dwarf galaxies. The chemodynamical model predicts that dwarf galaxies make a significant contribution to the observed total neutral gas density in DLAs, and that this contribution is more important at high redshifts (z ≥ 2-3). This is consistent with a scenario in which the DLA population is dominated by dwarf galaxies at high redshifts and by discs at lower redshifts. The relation between DLAs and Lyman break galaxies (LBGs) is investigated with chemodynamical models describing LBGs. Our results calls for a smoother progression in the evolutionary history of DLAS and LBGs rather than a sharp dichotomy between the two populations. LBGs and DLAs may constitute a sequence of increasing star formation rate, with the LBGs being systems with typically short star formation time-scales (∼10 8 yr), and the DLAs having slower star formation. We also raise the possibility that we could be missing a whole population of high H t density column objects, with metallicities intermediate between those of DLAs and LBGs. Finally, we discuss the possibility that relying only on the observations of DLAs could lead to an underestimate of the metal content of the high-redshift Universe.

The chemical evolution of IC 10

Context. Dwarf irregular galaxies are relatively simple unevolved objects where it is easy to test models of galactic chemical evolution. Aims. We attempt to determine the star formation and gas accretion history of IC 10, a local dwarf irregular for which abundance, gas, and mass determinations are available. Methods. 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 IC 10. 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. Results. We find that only models with metal-enhanced galactic winds can reproduce the properties of IC 10. 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, IC 10 must have formed by a slow process of gas accretion with a timescale of the order of 8 Gyr.

The mass-loss process in dwarf galaxies from 3D hydrodynamical simulations: the role of dark matter and starbursts

Theoretical

The evolution of the photometric properties of Local Group dwarf spheroidal galaxies

\Lambda

The IGIMF and other IMFs in dSphs: the case of Sagittarius

CDM cosmological models predict a much larger number of low mass dark matter haloes than has been observed in the Local Group of galaxies. One possible explanation is the increased difficulty of detecting these haloes if most of the visible matter is lost at early evolutionary phases through galactic winds. In this work we study the current models of triggering galactic winds in dwarf spheroidal galaxies (dSph) from supernovae, and study, based on 3D hydrodynamic numerical simulations, the correlation of the mass loss rates and important physical parameters as the dark matter halo mass and its radial profile, and the star formation rate. We find that the existence of winds is ubiquitous, independent on the gravitational potential. Our simulations revealed that the Rayleigh-Taylor Instability (RTI) may play a major role on pushing matter out of these systems, even for very massive haloes. The instability is responsible for 5 - 40% of the mass loss during the early evolution of the galaxy, being less relevant at

Gas Removal in the Ursa Minor Galaxy: Linking Hydrodynamics and Chemical Evolution Models

t > 200