Cosmologically motivated gas infall laws and galactic chemical evolution

2007/2008The main aim of this thesis has been to find a cosmologically motivated infall law to understand if the CDM cosmology can reproduce the main chemical characteristics of a Milky Way-like spiral galaxy. We have tested several gas infall laws, starting from that suggested in the two-infall model of Chiappini et al. (1997) for the chemical evolution of the Milky Way, but focusing on laws derived from cosmological simulations which follow a concordance CDM cosmology. By means of a detailed chemical evolution model for the solar vicinity, we have studied the effects of the different gas infall laws on the abundance patterns and the G-dwarf metallicity distribution. Our best cosmological infall law, derived from dark matter halos having properties compatible with the formation of a disk galaxy like the Milky Way, and assuming that the baryons assemble like dark matter, resembles the infall law suggested by the two-infall model. It predicts two main gas accretion episodes. Minor infall episodes are predicted to have followed the second main one but they are of little significance compared to the previous two. By means of this cosmologically motivated infall law, we have studied 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 (O, Mg, Si, C, N, Fe). We have found that the results of the two-infall model are fully compatible with the evolution of the Milky Way with cosmological accretion laws. We have derived that the timescale for the formation of the stellar halo and the thick disk must have not been longer than 2 Gyr, whereas the disk in the solar vicinity assembled on a much longer timescale (∼ 6 Gyr). Then we have studied the abundance gradients along the Galactic disk produced by our best cosmological model and their dependence upon several parameters: a threshold in the surface gas density regulating star formation, the star formation efficiency, the timescale for the formation of the thin disk and the total surface mass density of the stellar halo. We have found that to reproduce at the same time the abundance, star formation rate and surface gas density gradients along the Galactic disk it is necessary to assume an inside-out formation for the disk. The threshold in the gas density is not necessary and the same effect could be reached 2 by assuming a variable star formation efficiency. However the derived new cosmological infall law contains a mild inside-out formation and is still not enough to reproduce the disk properties at best. We have also studied the effect of a cosmologically motivated infall law for the formation of a massive elliptical galaxy in order to understand the impact on the formation of the spheroids. We have found that such a model predicts a too low mean stellar value for the [Mg/Fe] ratio. This is, according to us, the most important result of our cosmological model applied to an early-type galaxy indicating that perhaps the hierarchical paradigm of galaxy formation should be revised for ellipticals. Moreover we have found that models for ellipticals without a galactic wind predict a too large current SNIa rate. In particular, the cosmological model produces a current SNIa which is about ten times higher than the observed values and predicts a large current SNII rate which is not observed. The predicted SNII rate for models with galactic wind is also in contrast with the actual star formation mesured by GALEX. The conclusions of our work are that 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. Moreover, a cosmologically derived infall law with an inside-out process for the disk formation and a variable star formation efficiency can indeed well reproduce all the properties of the disk. Higher resolution cosmological simulations, however, are necessary to better trace the radial properties of disk galaxies. Finally, a cosmologically derived infall law for an elliptical galaxy cannot well reproduce all the chemical constraints given by observations and this is an important result.XXI Ciclo198

Cosmological formation and chemical evolution of an elliptical galaxy

We aim at studying the effect of a cosmologically motivated gas infall law for the formation of a massive elliptical galaxy in order to understand its impact on the formation of the spheroids. We replace the empirical infall law of the model by Pipino & Matteucci with a cosmologically derived infall law for the formation of an elliptical galaxy. We constrast our predictions with observations. We also compare the obtained results with those of Pipino & Matteucci. We computed models with and without galactic winds: we found that models without wind predict a too large current SNIa rate. In particular, the cosmological model produces a current SNIa which is about ten times higher than the observed values. Moreover models without wind predict a large current SNII rate, too large even if compared with the recent GALEX data. The predicted SNII rate for the model with wind, on the other hand, is too low if compared with the star formation histories given by GALEX. Last but not least, the mean value for the [Mg/Fe] ratio in the dominant stellar population of the simulated galaxy, as predicted by the cosmological model, is too low if compared to observations. This is, a very important result indicating that the cosmological infall law is in contrast with the chemical evolution. A cosmologically derived infall law for an elliptical galaxy cannot reproduce all the chemical constraints given by the observations. The problem resides in the fact that the cosmologically derived infall law implies a slow gas accretion with consequent star formation rate active for a long period. In this situation low [Mg/Fe] ratios are produced for the dominant stellar population in a typical elliptical, at variance with observations.Comment: 8 pages, 6 figures, accepted for publication by A&

Effects of radial flows on the chemical evolution of the Milky Way disk

The majority of chemical evolution models assume that the Galactic disk forms by means of infall of gas and divide the disk into several independent rings without exchange of matter between them. However, if gas infall is important, radial gas flows should be taken into account as a dynamical consequence of infall. The aim of this paper is to test the effect of radial gas flows on detailed chemical evolution models (one-infall and two-infall) for the Milky Way disk with different prescriptions for the infall law and star formation rate. We found, that with a gas radial inflow of constant speed the metallicity gradient tends to steepen. Taking into account a constant time scale for the infall rate along the Galaxy disk and radial flows with a constant speed, we obtained a too flat gradient, at variance with data, implying that an inside-out formation and/or a variable gas flow speed are required. To reproduce the observed gradients the gas flow should increase in modulus with the galactocentric distance, both in the one-infall and two-infall models. However, the inside-out disk formation coupled with a threshold in the gas density (only in the two-infall model) for star formation and/or a variable efficiency of star formation with galactocentric distance can also reproduce the observed gradients without radial flows. We showed that the radial flows can be the most important process in reproducing abundance gradients but only with a variable gas speed. Finally, one should consider that uncertainties in the data concerning gradients prevent us to draw firm conclusions. Future more detailed data will help to ascertain whether the radial flows are a necessary ingredient in the formation and evolution of the Galactic disk and disks in general.Comment: Accepted by A&A; 11 pages, 16 figure

The origin of abundance gradients in the Milky Way: the predictions of different models

We aim at studying the abundance gradients along the Galactic disk and their dependence upon several parameters: a threshold in the surface gas density regulating star formation, the star formation efficiency, the timescale for the formation of the thin disk and the total surface mass density of the stellar halo. We test a model which considers a cosmological infall law. This law does not predict an inside-out disk formation, but it allows to well fit the properties of the solar vicinity. We study several cases. We find that to reproduce at the same time the abundance, star formation rate and surface gas density gradients along the Galactic disk it is necessary to assume an inside-out formation for the disk. The threshold in the gas density is not necessary and the same effect could be reached by assuming a variable star formation efficiency. A cosmologically derived infall law with an inside-out process for the disk formation and a variable star formation efficiency can indeed well reproduce all the properties of the disk. However, the cosmological model presented here does not have sufficient resolution to capture the requested inside-out formation for the disk.Comment: 13 pages, 17 figures and 2 tables. Accepted for publication in Astronomy & Astrophysic

Converted wave tomography: Developing a new inversion method for 3-D crustal shear wâve velocities, with application to the Central Alps

Passive seismological investigations typically image the Earth’s crust with direct P-waves or ambient noise correlation yielding S-wave information. While the first method requires local earthquakes to achieve high resolution, in the second method the depth penetration strongly depends on the recording network’s aperture. In this thesis I develop a new inversion method and implement the related software in which teleseismic P-to-S converted waves (receiver functions) are exploited to construct a fully 3-D structural and shear-wave velocity model of the crust. This method does not require local earthquakes, nor a large aperture seismic network, but a dense array of 3-component sensors with a station spacing similar to the expected crustal thickness. This new technique is first applied to the Central Alps, a tectonically complex area where imaging in 3-D is of pivotal interest. The new method is composed of the following main elements. (1) An accurate ray prop- agator, which respects Snell’s law in 3-D at any interface geometry, and allows P-to-S con- verted ray-paths to reach the recording station at <0.1 km accuracy. (2) A new model parameterization, with horizontally fixed but vertically flexible-position nodes, and layer- wise two velocity points defined to accommodate mapping both sharp discontinuities and gradients across layers. (3) A stochastic inversion procedure, combining simulated annealing and a pattern search algorithm, to find discontinuity depths and velocities across the crust by fitting grouped converted waves with synthetics. This inversion is performed locally for each point and its neighbours; it covers the entire study area step-wise with an overlap and at least two iterations. The application to the Central Alps uses 20 years of high-quality data from permanent broad-band stations and from the temporary AlpArray Seismic Network. The initial model includes a Moho depth map and a 3-D P wave velocity model derived from past investigations. The 3-D inversion results at 25 km horizontal resolution provide a series of maps and cross- sections. The crustal thickness generally reflects well the roots of the Alpine orogen, and its jump between the European and Adriatic plates, including the Ivrea Geophysical Body. The lower crustal thickness is less well resolved, yet appears fairly constant. Average crustal Vp/Vs ratios are relatively higher beneath the orogen. A low-Vp/Vs area in the European foreland correlates with lower crustal earthquakes, which we interpret as mechanical differences in rock properties, most likely inherited. Our results are generally similar to those found by 3-D ambient noise tomography in the area. The new method inherently performs better at localizing discontinuities, and less well at imaging bulk anomalies. Thanks to sub-vertically propagating rays, our method maps the full crustal structure across the entire area of a seismic network. Future developments can incorporate joint inversions with gravity or other seismological tomography methods. -- La sismologie passive image la croûte terrestre typiquement par des ondes P directes ou des ondes S basées sur la corrélation du bruit ambiant. Si la première méthode requiert des séismes locaux pour une imagerie haute résolution, la deuxième méthode est limitée dans sa pénétration en profondeur par l’ouverture du réseau enregistrant des signaux. Dans cette thèse je développe une nouvelle méthode d’inversion et j’implémente le logiciel correspondant pour exploiter des ondes converties P-en-S (fonctions récepteurs) pour con- struire un modèle 3-D structural et de vitesse d’onde S de la croûte. Cette méthode requiert ni séisme local, ni une grande ouverture du réseau, mais un déploiement dense de capteurs 3-composantes à un espacement comparable à l’épaisseur attendue de la croûte. La pre- mière application de cette nouvelle technique se focalise sur les Alpes Centrales, une région tectonique complexe où l’imagerie 3-D est un but important. La nouvelle méthode se compose des éléments principaux suivants. (1) Un propagateur de rai exact, qui respecte la loi de Snell en 3-D à une géométrie d’interface quelconque, et permet aux rais convertis P-en-S d’arriver à <0.1 km de la station. (2) Un nouveau paramétrage de modèle, avec des nœuds horizontalement fixes mais verticalement flexibles, et deux points de définition des vitesses par couche pour permettre à la fois l’imagerie des discontinuités et celle des gradients dans les couches. (3) Une procédure d’inversion stochastique, combinant recuit simulé et un recherche de motifs, pour trouver la profondeur des discontinuités et des vitesses à travers la croûte en ajustant des synthétiques à des groupes d’ondes converties. Cette inversion est appliquée localement à chaque point et ses voisins, la procédure couvre toute la zone d’étude pas-à-pas avec un recouvrement et au moins deux itérations. L’application aux Alpes Centrales utilise des données de haute qualité enregistrées sur plus de 20 ans par des stations large-bandes permanentes et par le du réseau temporaire du projet AlpArray. Le modèle initial inclut une carte de profondeur du Moho et un modèle 3-D en vitesse d’onde P d’études précédentes. Le résultat de l’inversion 3-D, à une résolution horizontale de 25 km, inclut une série de cartes et de profils. L’épaisseur de la croûte reflète bien la racine de l’orogène alpin, et le saut entre les plaques européenne et adriatique, y compris le corps d’Ivrée. L’épaisseur de la croûte inférieure est moins bien résolue mais paraît relativement constante. Le rapport Vp/Vs moyenne de la croûte est relativement plus élevé sous la chaîne. Une zone de Vp/Vs faible dans l’avant-pays européen coïncide avec des séismes dans la croûte inférieure, ce que nous interprétons comme une différence dans les propriétés mécaniques des roches, probablement héritée. Nos résultats sont généralement similaires à ceux trouvés par tomographie 3-D du bruit ambiant dans la région. La nouvelle méthode est plus performante à localiser des discontinu- ités, et moins bien pour l’imagerie des anomalies volumétriques. Grâce aux rais sub-verticaux, notre méthode image la structure de toute la croûte sous l’ensemble du réseau sismologique. Des développements futurs peuvent inclure des inversions conjointes avec la gravimétrie ou d’autres types de tomographies sismologiques

A Mechanism for the Oxygen and Iron Bimodal Radial Distribution Formation in the Disc of our Galaxy

Recently it has been proposed that there are two types of SN Ia progenitors -- short-lived and long-lived. On the basis of this idea, we develope a theory of a unified mechanism for the formation of the bimodal radial distribution of iron and oxygen in the Galactic disc. The underlying cause for the formation of the fine structure of the radial abundance pattern is the influence of spiral arms, specifically, the combined effect of the corotation resonance and turbulent diffusion. From our modelling we conclude that to explain the bimodal radial distributions simultaneously for oxygen and iron and to obtain approximately equal total iron output from different types of supernovae, the mean ejected iron mass per supernova event should be the same as quoted in literature if maximum mass of stars, that eject heavy elements, is $50 M_{\odot}$. For the upper mass limit of $70 M_{\odot}$ the production of iron by a supernova II explosion should be increased by about 1.5 times.Comment: 7 pages, 6 figures, MNRAS submitte

Modelling the chemical evolution

Advanced observational facilities allow to trace back the chemical evolution of the Universe, on the one hand, from local objects of different ages and, secondly, by direct observations of redshifted objects. The chemical enrichment serves as one of the cornerstones of cosmological evolution. In order to understand this chemical evolution in morphologically different astrophysical objects models are constructed based on analytical descriptions or numerical methods. For the comparison of their chemical issues, as there are element abundances, gradients, and ratios, with observations not only the present-day values are used but also their temporal evolution from the first era of metal enrichment. Here we will provide some insight into basics of chemical evolution models, highlight advancements, and discuss a few applications.Comment: 12 pages, 3 figures, to appear in the proceedings of "Chemical Abundances in the Universe: Connecting First Stars to Planets". IAU Symposium No. 265, 2009. K. Cunha, M. Spite & B. Barbuy, ed

Effects of the integrated galactic IMF on the chemical evolution of the solar neighbourhood

The initial mass function determines the fraction of stars of different intial mass born per stellar generation. In this paper, we test the effects of the integrated galactic initial mass function (IGIMF) on the chemical evolution of the solar neighbourhood. The IGIMF (Weidner & Kroupa 2005) is computed from the combination of the stellar intial mass function (IMF), i.e. the mass function of single star clusters, and the embedded cluster mass function, i.e. a power law with index beta. By taking into account also the fact that the maximum achievable stellar mass is a function of the total mass of the cluster, the IGIMF becomes a time-varying IMF which depends on the star formation rate. We applied this formalism to a chemical evolution model for the solar neighbourhood and compared the results obtained by assuming three possible values for beta with the results 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 and larger metal ejection rates. This translates into higher type Ia and II supernova rates, higher mass ejection rates from massive stars and a larger amount of gas available for star formation, coupled with lower present-day stellar mass densities. (abridged) We also discuss the importance of the present day stellar mass function (PDMF) in providing a way to disentangle among various assumptions for beta. Our results indicate that the model adopting the IGIMF computed with beta ~2 should be considered the best since it allows us to reproduce the observed PDMF and to account for most of the chemical evolution constraints considered in this work.Comment: 22 pages, 19 figure

Building the Urban Bioregion. Governance scenarios for urban and territorial planning

The book focuses on bioregionalist theories and experiences as an alternative way of reading and designing local contexts, based on the recovery of the co-evolutionary relationship between human settlements and territories to achieve a self-sustainable and non-hierarchical system of urban and rural centers, according to Alberto Magnaghi's vision. The work has developed a broader discussion among researchers from different European backgrounds about the ways in which processes related to bioregionalism, looked at in a transdisciplinary way, can lead to interesting applications and analytical insights, that are useful for reviewing and strengthening community self-organization and reflecting on the constitutive foundations of the relationship between communities and their territories. The collaboration between the French school of Bordeaux, the Tuscany school and the Cagliari school gives back a diversified overview of materials and references for the possible application of the bioregionalist model.The contributions discuss many issues related to the governance of metropolitan areas and the management of the urban-rural relationship with suggestions for interpretation and design in a bioregionalist perspective, the themes of urban green, land vulnerability, and agricultural supply chains in rural and peri-urban spaces and new food economies in metropolitan areas