arXiv subjects

S. Recchi

Publications and source records attributed to S. Recchi.

36 records · Page 2Linked to original sources

arXiv2008

Galactic fountains and their connection with high and intermediate velocity clouds

The aim of this paper is to calculate the expansion law and chemical enrichment of a supershell powered by the energetic feedback of a typical Galactic OB association at various galactocentric radii. We study then the orbits of the fragments created when the supershell breaks out and we compare their kinetic and chemical properties with the available observations of high - and intermediate - velocity clouds. We use the Kompaneets (1960) approximation for the evolution of the superbubble driven by sequential supernova explosions and we compute the abundances of oxygen and iron residing in the thin cold supershell. We assume that supershells are fragmented by means of Rayleigh-Taylor instabilities and we follow the orbit of the clouds either ballistically or by means of a hybrid model considering viscous interaction between the clouds and the extra-planar gas.Given the self-similarity of the Kompaneets solutions, clouds are always formed ~ 448 pc above the plane. If the initial metallicity is solar, the pollution from dying stars of the OB association has a negligible effect on the chemical composition of the clouds. The maximum height reached by the clouds above the plane seldom exceeds 2 kpc and when averaging over different throwing angles, the landing coordinate differs from the throwing coordinate ~ 1 kpc at most. The range of heights and [O/Fe] ratios spun by our clouds suggest us that the high velocity clouds cannot have a Galactic origin, whereas intermediate velocity clouds have kinematic properties similar to our modeled clouds but overabundance observed for the [O/Fe] ratios which can be reproduced only with initial metallicities which are too low compared for those of the Galaxy disk.

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arXiv2007

The effect of clouds in a galactic wind on the evolution of gas-rich dwarf galaxies

(Abridged) We study the effects of interstellar clouds on the dynamical and chemical evolution of gas-rich dwarf galaxies. In particular, we focus on two model galaxies similar to IZw18 and NGC1569 in comparison to models in which a smooth initial distribution of gas is assumed. We use a 2-D hydrodynamical code coupled with a series of routines able to trace the chemical products of SNeII, SNeIa and intermediate-mass stars. Clouds are simulated by adding overdense regions in the computational grid, whose locations are chosen randomly and whose density profiles match observed ones. The clouds are inherently dynamically coupled to the diffuse gas, and they experience heat conduction from a hot surrounding gas. Due to dynamical processes and thermal evaporation, the clouds survive only a few tens of Myr. Due to the additional cooling agent, the internal energy of cloudy models is typically reduced by 20 - 40% compared with models of diffuse gas alone. The clouds delay the development of large-scale outflows by mass loading, therefore helping to retain a larger amount of gas inside the galaxy. However, their bullet effect can pierce the expanding supershell and create holes through which the superbubble can vent freshly produced metals. Moreover, assuming a pristine chemical composition for the clouds, their interaction with the superbubble dilutes the gas, reducing the metallicity. The resulting final metallicity is therefore generally lower (by ~ 0.2 - 0.4 dex) than the one attained by diffuse models.

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arXiv2007

The early evolution of tidal dwarf galaxies

Context: Dwarf galaxies can arise from self-gravitating structures emerging from tidal tails. What fraction of the known dwarf galaxies in the Local Universe can have this origin is still a matter of debate. Aims: In our effort to understand the origin and evolution of tidal dwarf galaxies and their correspondence with local objects, the first step is to understand how these galaxies (which are supposed to have a limited amount of dark matter) react to the feedback of the ongoing star formation. Methods: We make use of 2-D chemodynamical calculations in order to study the early evolution of isolated, dark matter-free dwarf galaxies. We present models in which feedback parameters are varied. We also compare the results with dark matter-dominated dwarf galaxy models. Results: All the considered models show that the star formation proceeds for more than 300 Myr, therefore dwarf galaxies without large dark matter halos are not necessarily quickly destroyed. The chemical evolution of these objects is consistent with the main chemical properties of the dSphs of the Local Group. Models with large dark matter halos show results consistent with models free of dark matter, indicating that the distribution of gas is more important than the depth of the potential well in determining the global behaviour of dSph-sized dwarf galaxies.

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arXiv2006

About the evolution of Dwarf Spheroidal Galaxies

We present 3D hydrodynamic simulations aimed at studying the dynamical and chemical evolution of the interstellar medium in dwarf spheroidal galaxies. This evolution is driven by the explosions of Type II and Type Ia supernovae, whose different contribution is explicitly taken into account in our models. We compare our results with avaiable properties of the Draco galaxy. Despite the huge amount of energy released by SNe explosions, in our model the galaxy is able to retain most of the gas allowing a long period ($> 3$ Gyr) of star formation, consistent with the star formation history derived by observations. The stellar [Fe/H] distribution found in our model matches very well the observed one. The chemical properties of the stars derive from the different temporal evolution between Type Ia and Type II supernova rate, and from the different mixing of the metals produced by the two types of supernovae. We reproduce successfully the observed [O/Fe]-[Fe/H] diagram.

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arXiv2006

Star Formation Regulation, Gas cycles and the Chemical Evolution of Dwarf Irregular Galaxies

Due to their low gravitational energies, dwarf galaxies are greatly exposed to energetical influences from internal and external sources. By means of chemodynamical models we show that their star formation is inherently self-regulated, that peculiar abundance ratios can only be achieved assuming different star-formation episodes and that evaporation of interstellar clouds embedded in a hot phase can lead to a fast mixing of the interstellar gas. Metal-enriched hot outflows can accrete onto infalling clouds by means of condensation leading to a large range of timescales for the self-enrichment of the ISM from local scales within a few tens of Myr up to a few Gyr for the large-range circulation. Infall of clouds is also required to explain abundance ratios of metal-poor galaxies at evolved stages because it reduces the metallicity altering only marginally the abundance ratios.

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arXiv2006

Early evolution of Tidal Dwarf Galaxies

Our aim is to study the evolution of tidal dwarf galaxies. The first step is to understand whether a model galaxy without Dark Matter can sustain the feedback of the ongoing star formation. We present tests of the evolution of models in which star formation efficiency, temperature threshold, initial distribution of gas and infall are varied. We conclude that it is feasible to keep a fraction of gas bound for several hundreds of Myr and that the development of galactic winds does not necessarily stop continuous star formation.

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arXiv2006

A new formulation of the Type Ia SN rate and its consequences on galactic chemical evolution

In recent papers Mannucci et al. (2005, 2006) suggested, on the basis of observational arguments, that there is a bimodal distribution of delay times for the explosion of Type Ia SNe. In this paper, we test this hypothesis in models of chemical evolution of galaxies of different morphological type: ellipticals, spirals and irregulars. We show that this proposed scenario is compatible also with the main chemical properties of galaxies. When the new rate is introduced in the two-infall model for the Milky Way, the derived Type Ia SN rate as a function of cosmic time shows a high and broad peak at very early epochs thus influencing the chemical evolution of the galactic halo more than in the previous widely adopted formulations for the SNIa rate. As a consequence of this, the [O/Fe] ratio decreases faster for [Fe/H] > -2.0 dex, relative to the old models. For a typical elliptical of 10^11 M_sun of luminous mass, the new rate produces average [alpha/ Fe] ratios in the dominant stellar population still in agreement with observations. The Type Ia SN rate also in this case shows an earlier peak and a subsequent faster decline relative to the previous results, but the differences are smaller than in the case of our Galaxy. We have also checked the effects of the new Type Ia SN rate on the evolution of the Fe content in the ICM, as a consequence of its production from cluster ellipticals and we found that less Fe in the ICM is produced with the new rate, due to the higher fraction of Fe synthesized at early times and remaining locked into the stars in ellipticals. For dwarf irregular galaxies suffering few bursts of star formation we obtain [O/Fe] ratios larger by 0.2 dex relative to the previous models.

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arXiv2006

Star formation feedback and metal enrichment by SN Ia and SN II in dwarf spheroidal galaxies: the case of Draco

We present 3D hydrodynamical simulations aimed to study the dynamical and chemical evolution of the interstellar medium in dwarf spheroidal galaxies. This evolution is driven by the explosions of Type II and Type Ia supernovae, whose different contribution is explicity taken into account in our models. We compare our results with detailed observations of the Draco galaxy. We assume star formation histories consisting of a number of instantaneous burst separated by quiescent periods. Because of the large effectiveness of the radiative losses and the extended dark matter halo, no galactic wind develops, despite the total energy released by the supernovae is much larger than the binding energy of the gas. This explains why the galaxy is able to form stars for a long period (> 3 Gyr), consistently with observations. In this picture, the end of the star formation and gas removal must result from external mechanisms, such as ram pressure and/or tidal interaction with the Galaxy. The metallicity distributions of the stars found in our models agree very well with the observed one. We find a mean value <[Fe/H]>=-1.65 with a spread of ~1.5 dex. The chemical properties of the stars derive by the different temporal evolution between Type Ia and Type II supernova rate, and by the different mixing of the metals produced by the two types of SNe. We reproduce successfully the observed [O/Fe]-[Fe/H] diagram. However, our interpretation of this diagram differs from that generally adopted by previous chemical models. In fact, we find that the chemical properties of the stars derive, besides the different temporal evolution of the SNe II and SNe Ia rates, from the spatial inhomogeneous chemical enrichment due to the different dynamical behaviour between the remnants of the two types of supernovae.

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arXiv2005

Dynamical and chemical evolution of NGC1569

Blue Compact Dwarf and Dwarf Irregular galaxies are generally believed to be unevolved objects, due to their blue colors, compact appearance and large gas fractions. Many of these objects show an ongoing intense burst of star formation or have experienced it in the recent past. By means of 2-D hydrodynamical simulations, coupled with detailed chemical yields originating from SNeII, SNeIa, and intermediate-mass stars, we study the dynamical and chemical evolution of model galaxies with structural parameters similar to NGC1569, a prototypical starburst galaxy. A burst of star formation with short duration is not able to account for the chemical and morphological properties of this galaxy. The best way to reproduce the chemical composition of this object is by assuming long-lasting episodes of star formation and a more recent burst, separated from the previous episodes by a short quiescent period. The last burst of star formation, in most of the explored cases, does not affect the chemical composition of the galaxy, since the enriched gas produced by young stars is in a too hot phase to be detectable with the optical spectroscopy. Models assuming the infall of a big cloud towards the center of the galaxy reproduce the chemical composition of the NGC1569, but the pressure exercised by the cloud hampers the expansion of the galactic wind, at variance with what observed in NGC1569.

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arXiv2005

Self-enrichment in globular clusters. I. An analytic approach

By means of analytical calculations, we explore the self-enrichment scenario for Globular Cluster formation. According to this scenario, an initial burst of star formation occurs inside the core radius of the initial gaseous distribution. The outward-propagating shock wave sweeps up a shell in which gravitational instabilities may arise, leading to the formation of a second, metal-enriched, population of stars. We find a minimum mass of the proto-globular cluster of the order of 10^6 solar masses. We also find that the observed spread in the Magnitude-Metallicity relation can be explained assuming cluster-to-cluster variations of some parameters like the thermalization efficiency, the mixing efficiency and the Initial Mass Function, as well as variations of the external pressure.

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arXiv2004

Continuous star formation in gas-rich dwarf galaxies

Blue Compact Dwarf and Dwarf Irregular galaxies are generally believed to be unevolved objects, due to their blue colors, compact appearance and large gas fractions. Many of these objects show an ongoing intense burst of star formation or have experienced it in the recent past. By means of 2-D hydrodynamical simulations, coupled with detailed chemical yields originating from SNeII, SNeIa and intermediate-mass stars, we study the dynamical and chemical evolution of model galaxies with structural parameters similar to IZw18 and NGC1569. Bursts of star formation with short duration are not able to account for the chemical and morphological properties of these galaxies. The best way to reproduce the chemical composition of these objects is by assuming long-lasting episodes of star formation and a more recent burst, separated from the previous episodes by a short quiescent period. The last burst of star formation, in most of the explored cases, does not affect the chemical composition of the galaxy observable in HII regions, since the enriched gas produced by young stars is in a too hot phase to be detectable with the optical spectroscopy.

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arXiv2004

Continuous star formation in IZw18

We study the dynamical and chemical evolution of a galaxy similar to IZw18 under the assumption of a continuous star formation during bursts. We adopt a 2-D hydrocode coupled with detailed chemical yields originating from SNeII, SNeIa and from single intermediate-mass stars. Different nucleosynthetic yields and different IMF slopes are tested. In most of the explored cases, a galactic wind develops, mostly carrying out of the galaxy the metal-enriched gas produced by the burst itself. The chemical species with the largest escape probabilities are Fe and N. Consequently, we predict that the [$α$/Fe] and [$α$/N] ratios outside the galaxy are lower than inside. In order to reproduce the chemical composition of IZw18, the best choice seems to be the adoption of the yields of Meynet & Maeder (2002) which take into account stellar rotation, although these authors do not follow the whole evolution of all the stars. Models with a flat IMF (x=0.5) seem to be able to better reproduce the chemical properties of IZw18, but they inject in the gas a much larger amount of energy and the resulting galactic wind is very strong, at variance with observations. We also predict the evolution of the abundances in the \hi medium and compare them with recent {\sl FUSE} observations.

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arXiv2004

Simulating the metal enrichment of the ICM

We present results from Tree+SPH simulations of a galaxy cluster, aimed at studying the metal enrichment of the intra--cluster medium (ICM). The simulation code includes a fairly advanced treatment of star formation, as well as the release of energy feedback and detailed yields from both type-II and type-Ia supernovae, also accurately accounting for the lifetimes of different stellar populations. We perform simulations of a cluster with virial mass ~ 3.9x 10^14 Msun, to investigate the effect of varying the feedback strength and the stellar initial mass function (IMF). Although most of the models are able to produce acceptable amounts of Fe mass, we find that the profiles of the iron abundance are always steeper than observed. The [O/Fe] ratio is found to be sub--solar for a Salpeter IMF, with [O/Fe] -0.2 at R >~ 0.1R200, whereas increasing to super-solar values in central regions, as a result of recent star formation. Using a top--heavier IMF gives a larger [O/Fe] over the whole cluster, at variance with observations. On the other hand, the adoption of a variable IMF, which becomes top-heavier at z>2, provides a roughly solar [O/Fe] ratio. Our results indicate that our simulations still lack a feedback mechanism which should quench star formation at low redshift and transport metals away from the star forming regions.

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arXiv2001

Multiple starbursts in Blue Compact Galaxies

In this paper we present some results concerning the effects of two instantaneous starbursts, separated by a quiescent period, on the dynamical and chemical evolution of blue compact dwarf galaxies. In particular, we compare the model results to the galaxy IZw18, which is a very metal-poor, gas-rich dwarf galaxy, possibly experiencing its first or second burst of star formation. We follow the evolution of a first weak burst of star formation followed by a second more intense one occurring after several hundreds million years. We find that a galactic wind develops only during the second burst and that metals produced in the burst are preferentially lost relative to the hydrogen gas. We predict the evolution of several chemical abundances (H, He, C, N, O, α-elements, Fe) in the gas inside and outside the galaxy, by taking into account in detail the chemical and energetical contributions from type II and Ia supernovae. We find that the abundances predicted for the star forming region are in good agreement with the HII region abundances derived for IZw18. We also predict the abundances of C, N and O expected for the HI gas to be compared with future FUSE abundance determinations. We conclude that IZw18 must have experienced two bursts of star formation, one occurred \sim 300 Myr ago and a present one with an age between 4-7 Myr. However, by taking into account also other independent estimates, such as the color-magnitude diagram and the spectral energy distribution of stars in IZw18, and the fact that real starbursts are not instantaneous, we suggest that it is more likely that the burst age is between 4 and 15 Myr.

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arXiv2001

Galactic Winds in Starburst Irregular Galaxies

In this paper we present some results of numerical simulations concerning the development of galactic winds in starburst galaxies. In particular, we focus on a galaxy similar to IZw18, the most metal-poor galaxy locally known. We compute the chemio-dynamical evolution of this galaxy, considering the energetic input and the chemical yields originating from Supernovae (SNe) of Type II and Ia and from intermediate-mass stars. We consider both single, instantaneous starburst and two starbursts separated by a quiescent period. In all considered cases a metal enriched winds develops and in particular the metals produced by Type Ia SNe are ejected more efficiently than the other metals. We suggest that two burst of star formation, the first being weaker and the last having an age of some tenth of Myr, can satisfactorily reproduce the abundances and abundance ratios found in literature for IZw18

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arXiv2001

On the typical timescale for the chemical enrichment from SNeIa in Galaxies

We calculate the type Ia supernova rate for different star formation histories in galaxies by adopting the most popular and recent progenitor models. We show that the timescale for the maximum in the type Ia supernova rate, which corresponds also to time of the maximum enrichment, is not unique but is a strong function of the adopted stellar lifetimes, initial mass function and star formation rate. This timescale varies from $\sim 40-50$ Myr for an instantaneous starburst to $\sim$ 0.3 Gyr for a typical elliptical galaxy to $\sim 4.0-5.0$ Gyr for a disk of a spiral Galaxy like the Milky Way. We also show that the typical timescale of 1 Gyr, often quoted as the typical timescale for the type Ia supernovae, is just the time at which, in the solar neighbourhood, the Fe production from supernovae Ia starts to become important and not the time at which SNe Ia start to explode. As a cosequence of this, a change in slope in the [O/Fe] ratio is expected in correspondance of this timescale. We conclude that the suggested lack of supernovae Ia at low metallicities produces results at variance with the observed [O/Fe] vs. [Fe/H] relation in the solar region. We also compute the supernova Ia rates for different galaxies as a function of redshift and predict an extended maximum between redshift $z \sim 3.6$ and $z \sim 1.6$ for elliptical galaxies, and two maxima, one at $z \sim 3$ and the other at $z \sim 1$, for spiral galaxies, under the assumption that galaxies start forming stars at $z_f \sim 5$ and $Ω_M = 0.3$, $Ω_Λ = 0.7$.

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arXiv2001

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.

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arXiv1999

Effects of tidal interactions on the gas flows of elliptical galaxies

During a Hubble time, cluster galaxies may undergo several mutual encounters close enough to gravitationally perturb their hot, X-ray emitting gas flows. We ran several 2D, time dependent hydrodynamical models to investigate the effects of such perturbations on the gas flow inside elliptical galaxies. In particular, we studied in detail the modifications occurring in the scenario proposed by D'Ercole et al. (1989), in which the galactic interstellar medium produced by the aging galactic stellar population, is heated by SNIa at a decreasing rate. We find that, although the tidal interaction in our models lasts less than 1 Gyr, its effect extends over several Gyrs. The tidally induced turbulent flows create dense filaments which cool quickly and accrete onto the galactic center, producing large spikes in the global Lx. Once this mechanism starts, it is fed by gravity and amplified by SNIa. In cooling flow models without supernovae the amplitude of the Lx fluctuations due to the tidal interaction is substantially reduced. We conclude that, if SNIa significantly contribute to the energetics of the gas flows in ellipticals, then the observed spread in the Lx-Lb diagram may be caused, at least in part, by this mechanism. On the contrary, tidal interactions cannot be responsible for the observed spread if the pure cooling flow scenario applies (abridged).

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