We have explored the Eu production in the Milky Way by means of a very detailed chemical evolution model. In particular, we have assumed that Eu is formed in merging neutron star (or neutron star black hole) binaries as well as in type II supernovae.
We have tested the effects of several important parameters influencing the production of Eu during the merging of two neutron stars, such as: i) the time scale of coalescence, ii) the Eu yields and iii) the range of initial masses for the progenitors of the neutron stars. The yields of Eu from type II supernovae are very uncertain, more than those from coalescing neutron stars, so we have explored several possibilities. We have compared our model results with the observed rate of coalescence of neutron stars, the solar Eu abundance, the [Eu/Fe] versus [Fe/H] relation in the solar vicinity and the [Eu/H] gradient along the Galactic disc. Our main results can be summarized as follows: i) neutron star mergers can be entirely responsible for the production of Eu in the Galaxy if the coalescence time scale is no longer than 1 Myr for the bulk of binary systems, the Eu yield is around $3 times 10^{-7}$ M$_odot$, and the mass range of progenitors of neutron stars is 9-50 M$_odot$; ii) both type II supernovae and merging neutron stars can produce the right amount of Eu if the neutron star mergers produce $2 times 10^{-7}$ M$_odot$ per system and type II supernovae, with progenitors in the range 20-50 M$_odot$, produce yields of Eu of the order of $10^{-8}-10^{-9}$ M$_odot$; iii) either models with only neutron stars producing Eu or mixed ones can reproduce the observed Eu abundance gradient along the Galactic disc.
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.
The cosmic star formation rate (CSFR), is an important clue to investigate the history of the assembly and evolution of galaxies. Here, we develop a method to study the CSFR from a purely theoretical point of view. Starting from detailed models of ch
emical evolution, we obtain the histories of star formation of galaxies of different morphological types. These histories are then used to determine the luminosity functions of the same galaxies by means of a spectro-photometric code. We obtain the CSFR under different hypothesis. First, we study the hypothesis of a pure luminosity evolution scenario, in which all galaxies are supposed to form at the same redshift and then evolve only in luminosity. Then we consider scenarios in which the number density or the slope of the LFs are assumed to vary with redshift. After comparison with available data we conclude that a pure luminosity evolution does not provide a good fit to the data, especially at very high redshift, although many uncertainties are still present in the data. On the other hand, a variation in the number density of ellipticals and spirals as a function of redshift can provide a better fit to the observed CSFR. We also explore cases of variable slope of the LFs with redshift and variations of number density and slope at the same time. We cannot find any of those cases which can improve the fit to the data respect to the solely number density variation. Finally, we compute the evolution of the average cosmic metallicity in galaxies with redshift.
In order to trace the instantaneous star formation rate at high redshift, and hence help understanding the relation between the different emission mechanisms related to star formation, we combine the recent 4 Ms Chandra X-ray data and the deep VLA ra
dio data in the Extended Chandra Deep Field South region. We find 268 sources detected both in the X-ray and radio band. The availability of redshifts for $sim 95$ of the sources in our sample allows us to derive reliable luminosity estimates and the intrinsic properties from X-ray analysis for the majority of the objects. With the aim of selecting sources powered by star formation in both bands, we adopt classification criteria based on X-ray and radio data, exploiting the X-ray spectral features and time variability, taking advantage of observations scattered across more than ten years. We identify 43 objects consistent with being powered by star formation. We also add another 111 and 70 star forming candidates detected only in the radio or X-ray band, respectively. We find a clear linear correlation between radio and X-ray luminosity in star forming galaxies over three orders of magnitude and up to $z sim 1.5$. We also measure a significant scatter of the order of 0.4 dex, higher than that observed at low redshift, implying an intrinsic scatter component. The correlation is consistent with that measured locally, and no evolution with redshift is observed. Using a locally calibrated relation between the SFR and the radio luminosity, we investigate the L_X(2-10keV)-SFR relation at high redshift. The comparison of the star formation rate measured in our sample with some theoretical models for the Milky Way and M31, two typical spiral galaxies, indicates that, with current data, we can trace typical spirals only at z<0.2, and strong starburst galaxies with star-formation rates as high as $sim 100 M_odot yr^{-1}$, up to $zsim 1.5$.
The existence of a mass-metallicity (MZ) relation in star forming galaxies at all redshift has been recently established. We aim at studying some possible physical mechanisms contributing to the MZ relation by adopting analytical solutions of chemica
l evolution models including infall and outflow. We explore the hypotheses of a variable galactic wind rate, infall rate and yield per stellar generation (i.e. a variation in the IMF), as possible causes for the MZ relation. By means of analytical models we compute the expected O abundance for galaxies of a given total baryonic mass and gas mass.The stellar mass is derived observationally and the gas mass is derived by inverting the Kennicutt law of star formation, once the star formation rate is known. Then we test how the parameters describing the outflow, infall and IMF should vary to reproduce the MZ relation, and we exclude the cases where such a variation leads to unrealistic situations. We find that a galactic wind rate increasing with decreasing galactic mass or a variable IMF are both viable solutions for the MZ relation. A variable infall rate instead is not acceptable. It is difficult to disentangle among the outflow and IMF solutions only by considering the MZ relation, and other observational constraints should be taken into account to select a specific solution. For example, a variable efficiency of star formation increasing with galactic mass can also reproduce the MZ relation and explain the downsizing in star formation suggested for ellipticals. The best solution could be a variable efficiency of star formation coupled with galactic winds, which are indeed observed in low mass galaxies.
Our aim is to show how different hypotheses about Type Ia supernova progenitors can affect Galactic chemical evolution. We include different Type Ia SN progenitor models, identified by their distribution of time delays, in a very detailed chemical ev
olution model for the Milky Way which follows the evolution of several chemical species. We test the single degenerate and the double degenerate models for supernova Ia progenitors, as well as other more empirical models based on differences in the time delay distributions. We find that assuming the single degenerate or the double degenerate scenario produces negligible differences in the predicted [O/Fe] vs. [Fe/H] relation. On the other hand, assuming a percentage of prompt (exploding in the first 100 Myr) Type Ia supernovae of 50%, or that the maximum Type Ia rate is reached after 3-4 Gyr from the beginning of star formation, as suggested by several authors, produces more noticeable effects on the [O/Fe] trend. However, given the spread still existing in the observational data no model can be firmly excluded on the basis of only the [O/Fe] ratios. On the other hand, when the predictions of the different models are compared with the G-dwarf metallicity distribution, the scenarios with very few prompt Type Ia supernovae can be excluded. Models including the single degenerate or double degenerate scenario with a percentage of 10-13% of prompt Type Ia supernovae produce results in very good agreement with the observations. A fraction of prompt Type Ia supernovae larger than 30% worsens the agreement with observations and the same occurs if no prompt Type Ia supernovae are allowed. In particular, two empirical models for the Type Ia SN progenitors can be excluded: the one without prompt Type Ia supernovae and the one assuming delay time distribution going like t^{-0.5}.
Aims. To model the chemical evolution of manganese relative to iron in three different stellar systems: the solar neighbourhood, the Galactic bulge and the Sagittarius dwarf spheroidal galaxy, and compare our results with the recent and homogeneous o
bservational data. Methods. We adopt three chemical evolution models well able to reproduce the main properties of the solar vicinity, the galactic Bulge and the Sagittarius dwarf spheroidal. Then, we compare different stellar yields in order to identify the best set to match the observational data in these systems. Results. We compute the evolution of manganese in the three systems and we find that in order to reproduce simultaneously the [Mn/Fe] versus [Fe/H] in the Galactic bulge, the solar neighbourhood and Sagittarius, the type Ia SN Mn yield must be metallicity-dependent. Conclusions. We conclude that the different histories of star formation in the three systems are not enough to reproduce the different behaviour of the [Mn/Fe] ratio, unlike the situation for [alpha/Fe]; rather, it is necessary to invoke metallicity-dependent type Ia SN Mn yields, as originally suggested by McWilliam, Rich & Smecker-Hane in 2003.
We investigate the present-day photometric properties of the dwarf spheroidal galaxies in the Local Group. From the analysis of their integrated colours, we consider a possible link between dwarf spheroidals and giant ellipticals. From the analysis o
f the V vs (B-V) plot, we search for a possible evolutionary link between dwarf spheroidal galaxies (dSphs) and dwarf irregular galaxies (dIrrs). By means of chemical evolution models combined with a spectro-photometric model, we study the evolution of six Local Group dwarf spheroidal galaxies (Carina, Draco, Sagittarius, Sculptor, Sextans and Ursa Minor). The chemical evolution models, which adopt up-to-date nucleosynthesis from low and intermediate mass stars as well as nucleosynthesis and energetic feedback from supernovae type Ia and II, reproduce several observational constraints of these galaxies, such as abundance ratios versus metallicity and the metallicity distributions. The proposed scenario for the evolution of these galaxies is characterised by low star formation rates and high galactic wind efficiencies. Such a scenario allows us to predict integrated colours and magnitudes which agree with observations. Our results strongly suggest that the first few Gyrs of evolution, when the star formation is most active, are crucial to define the luminosities, colours, and other photometric properties as observed today. After the star formation epoch, the galactic wind sweeps away a large fraction of the gas of each galaxy, which then evolves passively. Our results indicate that it is likely that at a certain stage of their evolution, dSphs and dIrrs presented similar photometric properties. However, after that phase, they evolved along different paths, leading them to their currently disparate properties.
