Chemical Evolution of M31

We review chemical evolution models developed for M31 as well as the abundance determinations available for this galaxy. Then we present a recent chemical evolution model for M31 including radial gas flows and galactic fountains along the disk, as well as a model for the bulge. Our models are predicting the evolution of the abundances of several chemical species such as H, He, C, N, O, Ne, Mg, Si, S, Ca and Fe. From comparison between model predictions and observations we can derive some constraints on the evolution of the disk and the bulge of M31. We reach the conclusions that Andromeda must have evolved faster than the Milky Way and inside-out, and that its bulge formed much faster than the disk on a timescale $\leq$ 0.5 Gyr. Finally, we present a study where we apply the model developed for the disk of M31 in order to study the probability of finding galactic habitable zones in this galaxy.Comment: To be published in:"Lessons from the Local Group: A Conference in Honour of David Block and Bruce Elmegreen" Editors: Prof. Dr. Kenneth Freeman, Dr. Bruce Elmegreen, Prof. Dr. David Block, Matthew Woolway, Springe

Modelling the chemical evolution of the Galaxy halo

We study the chemical evolution and formation of the Galactic halo through the analysis of its stellar metallicity distribution function and some key elemental abundance patterns. Starting from the two-infall model for the Galaxy, which predicts too few low-metallicity stars, we add a gas outflow during the halo phase with a rate proportional to the star formation rate through a free parameter, lambda. In addition, we consider a first generation of massive zero-metal stars in this two-infall + outflow model adopting two different top-heavy initial mass functions and specific population III yields. The metallicity distribution function of halo stars, as predicted by the two-infall + outflow model shows a good agreement with observations, when the parameter lambda=14 and the time scale for the first infall, out of which the halo formed, is not longer than 0.2 Gyr, a lower value than suggested previously. Moreover, the abundance patterns [X/Fe] vs. [Fe/H] for C, N and alpha-elements O, Mg, Si, S, Ca show a good agreement with the observational data. If population III stars are included, under the assumption of different initial mass functions, the overall agreement of the predicted stellar metallicity distribution function with observational data is poorer than in the case without population III. We conclude that it is fundamental to include both a gas infall and outflow during the halo formation to explain the observed halo metallicity distribution function, in the framework of a model assuming that the stars in the inner halo formed mostly in situ. Moreover, we find that it does not exist a satisfactory initial mass function for population III stars which reproduces the observed halo metallicity distribution function. As a consequence, there is no need for a first generation of only massive stars to explain the evolution of the Galactic halo.Comment: Accepted for publication in A&A. 11 pages, 5 figure

The Chemical Evolution of the Galaxy: the two-infall model

In this paper we present a new chemical evolution model for the Galaxy which assumes two main infall episodes for the formation of halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin-disk is much longer than that of the thick disk, implying that the infalling gas forming the thin-disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin-disk is assumed to be a function of the galactocentric distance, leading to an inside-out picture for the Galaxy building. The model takes into account the most up to date nucleosynthesis prescriptions and adopts a threshold in the star formation process which naturally produces a hiatus in the star formation rate at the end of the thick disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints. Among these constraints, the tightest one is the metallicity distribution of the G-dwarf stars for which new data are now available. Our model fits very well these new data. We show that in order to reproduce most of these constraints a timescale $\le 1$ Gyr for the (halo)-thick-disk and of 8 Gyr for the thin-disk formation in the solar vicinity are required. We predict that the radial abundance gradients in the inner regions of the disk ($R< R_{\odot}$) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner ones steepen in time during the Galactic lifetime.Comment: 48 pages, 20 Postscript figures, AASTex v.4.0, to be published in Astrophysical Journa

The Galactic habitable zone around M and FGK stars with chemical evolution models with dust

The Galactic habitable zone is defined as the region with highly enough metallicity to form planetary systems in which Earth-like planets could be born and might be capable of sustaining life surviving to the destructive effects of nearby supernova explosion events. Galactic chemical evolution models can be useful tools for studying the galactic habitable zones in different systems. Our aim here is to find the Galactic habitable zone using chemical evolution models for the Milky Way disc, adopting the most recent prescriptions for the evolution of dust and for the probability of finding planetary systems around M and FGK stars. Moreover, for the first time, we will express those probabilities in terms of the dust-to-gas ratio of the ISM in the solar neighborhood as computed by detailed chemical evolution models. At a fixed Galactic time and Galactocentric distance we determine the number of M and FGK stars having Earths (but no gas giant planets) which survived supernova explosions, using the formalism of our Paper I. The probabilities of finding terrestrial planets but not gas giant planets around M stars deviate substantially from the ones around FGK stars for supersolar values of [Fe/H]. For both FGK and M stars the maximum number of stars hosting habitable planets is at 8 kpc from the Galactic Centre, if destructive effects by supernova explosions are taken into account. At the present time the total number of M stars with habitable planets are $\simeq$ 10 times the number of FGK stars. Moreover, we provide a sixth order polynomial fit (and a linear one but more approximated) for the relation found with chemical evolution models in the solar neighborhood between the [Fe/H] abundances and the dust-to-gas ratio.Comment: Accepted for publication in A&A, 10 pages 6 figure

A possible theoretical explanation of metallicity gradients in elliptical galaxies

Models of chemical evolution of elliptical galaxies taking into account different escape velocities at different galactocentric radii are presented. As a consequence of this, the chemical evolution develops differently in different galactic regions; in particular, we find that the galactic wind, powered by supernovae (of type II and I) starts, under suitable conditions, in the outer regions and successively develops in the central ones. The rate of star formation (SFR) is assumed to stop after the onset of the galactic wind in each region. The main result found in the present work is that this mechanism is able to reproduce metallicity gradients, namely the gradients in the $Mg_2$ index, in good agreement with observational data. We also find that in order to honor the constant [Mg/Fe] ratio with galactocentric distance, as inferred from metallicity indices, a variable initial mass function as a function of galactocentric distance is required. This is only a suggestion since trends on abundances inferred just from metallicity indices are still uncertain.Comment: 18 pages, LaTeX file with 4 figures using mn.sty, submitted to MNRA

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.Comment: To appear in Proceeding of Science: Frontier Research in Astrophysics - II 23-28 May 2016 Mondello (Palermo), Ital

Galactic Winds in Irregular Starburst Galaxies

In this paper we present some results concerning the study of the development of galactic winds in blue compact galaxies. In particular, we model a situation very similar to that of the galaxy IZw18, the most metal poor and unevolved galaxy known locally. To do that we compute the chemo-dynamical evolution of a galaxy in the case of one istantaneous isolated starburst as well as in the case of two successive instantaneous starbursts. We show that in both cases a metal enriched wind develops and that the metals produced by the type Ia SNe are lost more efficiently than those produced by type II SNe. We also find that one single burst is able to enrich chemically the surrounding region in few Myr. Both these results are the effect of the assumed efficiency of energy transfer from SNe to ISM and to the consideration of type Ia SNe in this kind of problem. The comparison with observed abundances of IZw18 suggests that this galaxy is likely to have suffered two bursts in its life, with the previous being less intense than the last one.Comment: 3 pages, 1 figure, to appear in the Proceedings of the Conference "Cosmic Evolution", Paris, November 200