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

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

The two regimes of the cosmic sSFR evolution are due to spheroids and discs

This paper aims at explaining the two phases in the observed specific star formation rate (sSFR), namely the high (>3/Gyr) values at z>2 and the smooth decrease since z=2. In order to do this, we compare to observations the specific star formation rate evolution predicted by well calibrated models of chemical evolution for elliptical and spiral galaxies, using the additional constraints on the mean stellar ages of these galaxies (at a given mass). We can conclude that the two phases of the sSFR evolution across cosmic time are due to different populations of galaxies. At z>2 the contribution comes from spheroids: the progenitors of present-day massive ellipticals (which feature the highest sSFR) as well as halos and bulges in spirals (which contribute with average and lower-than-average sSFR). In each single galaxy the sSFR decreases rapidly and the star formation stops in <1 Gyr. However the combination of different generations of ellipticals in formation might result in an apparent lack of strong evolution of the sSFR (averaged over a population) at high redshift. The z<2 decrease is due to the slow evolution of the gas fraction in discs, modulated by the gas accretion history and regulated by the Schmidt law. The Milky Way makes no exception to this behaviour.Comment: 8 pages, 5 figures, MNRAS accepte

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 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

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