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Theoretical cosmic Type Ia supernova rates
No full textThe purpose of this work is the computation of the cosmic Type Ia supernova rates, namely the frequency of Type Ia supernovae per unit time in a unitary volume of the Universe. Our main goal in this work is to predict the Type Ia supernova rates at very high redshifts and to check whether it is possible to select the best delay time distribution model, on the basis of the available observations of Type Ia supernovae. We compute the cosmic Type Ia supernova rates in different scenarios for galaxy formation and predict the expected number of explosions at high redshift (z⩾2). Moreover, we adopt various progenitor models in order to compute the Type Ia supernova rate in typical elliptical galaxies of initial luminous masses of 1010Msun,1011Msun and 1012Msun, and compute the total amount of iron produced by Type Ia supernovae in each case. In this analysis we assume that Type Ia supernovae are caused by thermonuclear explosions of C-O white dwarfs in binary systems and we consider the most popular frameworks: the single degenerate and the double degenerate scenarios. The two competing schemes for the galaxy formation, namely the monolithic collapse and the hierarchical clustering, are also taken into account, by considering the histories of star formation increasing and decreasing with redshift, respectively. We calculate the Type Ia supernova rates through an analytical formulation which rests upon the definition of the SN Ia rate following an instantaneous burst of star formation as a function of the time elapsed from the birth of the progenitor system to its explosion as a Type Ia supernova (i.e. the delay time). What emerges from this work is that: (i) we confirm the result of previous papers that it is not easy to select the best delay time distribution scenario from the observational data and this is because the cosmic star formation rate dominates over the distribution function of the delay times; (ii) the monolithic collapse scenario for galaxy formation predicts an increasing trend of the SN Ia rate at high redshifts (mainly due to the contribution by massive spheroids), whereas the predicted rate in the framework of a decreasing cosmic star formation rate, more in agreement with the hierarchical scenario, drops dramatically at high redshift; (iii) for the elliptical galaxies we note that the predicted maximum of the Type Ia supernova rate depends on the initial galactic mass. The maximum occurs earlier (at about 0.3 Gyr) in the most massive ellipticals, as a consequence of the assumed downsizing in star formation. In addition, we find that the Type Ia supernova rate per unit mass at the present time is higher in bluer ellipticals (i.e. the less massive ones)
Effects of tidal interactions on the gas flows of elliptical galaxies
No full textDuring 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 two-dimensional, time-dependent hydrodynamical models to investigate the effects of such perturbations on the gas flow inside elliptical galaxies, focusing on the expected X-ray properties. In particular, we studied in detail the modifications occurring in the scenario proposed in 1989 by A. D'Ercole and coworkers, in which the galactic interstellar medium produced by the aging galactic stellar population is heated by Type Ia supernovae (SNe Ia) at a decreasing rate. We find that although the tidal interaction in our models lasts less than 1 Gyr, its effect extends over several Gyr. The tidally induced turbulent hows create dense filaments which cool quickly and accrete onto the galactic center, producing large spikes in the global X-ray luminosity, L(x). Once this mechanism starts, it is fed by gravity and amplified by SNe Ia. This evolution is found to be virtually independent of the dynamical state of the gas flow at the beginning of the interaction. To better understand the role of SN Ia heating, we also considered a "pure" cooling flow model without supernovae; in this case the amplitude of the L(x) fluctuations due to the tidal interaction is substantially reduced. We conclude that if SNe Ia significantly contribute to the energetics of the gas flows in elliptical galaxies, then the observed spread in the L(X)-L(B) diagram at any fixed optical galaxy luminosity L(B) greater than or similar to 3 x 10(10) L. 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 how scenario applies
Stellar Archeology: A Cosmological View of dSphs
No full textThe origin of dwarf spheroidal galaxies (dSphs) is investigated in a global cosmological context by simultaneously following the evolution of the Galaxy and its dwarf satellites. This approach enables to study the formation of dSphs in their proper birth environment and to reconstruct their own merging histories. The proposed picture simultaneously accounts for several dSph and Milky Way properties, including the Metallicity Distribution Functions of metal-poor stars. The observed features are interpreted in terms of physical processes acting at high redshifts
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Star formation feedback and metal enrichment by Types Ia and II supernovae in dwarf spheroidal galaxies: the case of Draco
No full textWe 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 supernovae (SNe II) and Type Ia supernovae (SNe Ia), whose
different contribution is explicitly 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 bursts separated by quiescent periods. Diverse histories differ by
the number of bursts, but all have the same total duration and give rise to the same amount of
stars. 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 stellar [Fe/H] distributions found in our models
match very well the observed ones. 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 SNe Ia and SNe II rate, and by the different mixing of the metals produced by the
two types of supernovae. 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 break observed in the diagram is not connected with
the onset of a galactic wind or with a characteristic time-scale for the sudden switchover of
the SNe Ia, as sometimes claimed. Instead, 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
Galactic fountains and their connection with high and intermediate velocity clouds
No full textContext. Sequential supernova explosions create supershells which can break out a stratified medium, producing bipolar outflows. The gas of the supershells can fragment into clouds which eventually fall toward the disk producing so-called galactic fountains.
Aims. 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 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.
Methods. We use the Kompaneets (1960, Soviet Phys. Dokl., 5, 46) 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 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.
Results. Given the self-similarity of the 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 by ~1 kpc at most.
Conclusions. The range of heights and [O/Fe] ratios spanned by our clouds suggest that the high velocity clouds cannot have a Galactic origin, whereas intermediate velocity clouds have kinematic properties similar to our modeled clouds but have overabundances observed for the [O/Fe] ratios that can be reproduced only with initial metallicities that are too low compared to those of the Galaxy disk
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