1,721,033 research outputs found
Photochemical evolution of elliptical galaxies - II. The impact of merging-induced starbursts
No full textThe effects of Population III stars on the cemical and photometrical evolution of ellipticals
No full textPhotochemical evolution of elliptical galaxies - I. The high-redshift formation scenario
No full textIn this paper we compute new multizone photochemical evolution models for elliptical galaxies, taking into account detailed nucleosynthetic yields, feedback from supernovae and an initial infall episode. By comparing model predictions with observations, we derive a picture of galaxy formation in which the higher the mass of the galaxy, the shorter the infall and the star formation time-scales. Therefore, in this scenario, the most massive objects are older than the less massive ones, in the sense that larger galaxies stop forming stars at earlier times. Each galaxy is created outside-in, i.e. the outermost regions accrete gas, form stars and develop a galactic wind very quickly, compared with the central core in which the star formation can last up to ~1.3 Gyr. In particular, we suggest that both the duration of the star formation and the infall time-scale decrease with galactic radius. In order to convert theoretical predictions into line-strength indices, different calibrations are adopted and discussed, focusing in particular on their dependence on the α enhancement.
By means of our model, we are able to match the observed mass-metallicity and colour-magnitude relations for the centre of the galaxies as well as to reproduce the overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass. Furthermore, we find that the observed Ca underabundance relative to Mg can be real, owing to the non-negligible contribution of type Ia SNe to the production of this element. We predict metallicity and colour gradients inside the galaxies that are in good agreement with the mean value of the observed ones.
Finally, we conclude that models with a Salpeter initial mass function (IMF) are the best ones in reproducing the majority of the properties of ellipticals, although a slightly flatter IMF seems to be required in order to explain the colours of the most massive galaxies
Cosmological formation and chemical evolution of an elliptical galaxy
No full textAims: We aim to study 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 spheroids.
Methods: 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 compare our predictions with observations. We also compare the obtained results with those of Pipino & Matteucci.
Results: 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 value which is about ten times higher than the observed values. Moreover models without wind predict a high 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. 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 contradiction to the chemical evolution.
Conclusions: A cosmologically derived infall law for an elliptical galaxy cannot reproduce all the chemical constraints given by observations. The problem resides in the fact that the cosmologically derived infall law implies a slow gas accretion with a 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
The two regimes of the cosmic sSFR evolution are due to spheroids and discs
No full textThis 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 sSFR 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 haloes 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
Are dry mergers of ellipticals the way to reconcile model predictions with downsizing?
Full text linkAims. We show that the bulk of the star formation and the galaxy assembly should occur simultaneously
in order to reproduce at the same time the downsizing and the chemical properties of present-day massive spheroids within one effective radius.
Methods. By means of chemical evolution models, we create galactic building blocks of several masses
and different chemical properties. We then construct a sample of possible merger histories
going from a multiple minor merger scenario to a single major merger event aimed at reproducing
a single massive elliptical galaxy. We compare our results against the mass-[Mg/Fe] and the mass-metallicity relations.
Results. We found that a series of multiple dry mergers (no star formation in connection with the merger) involving building-blocks that have been created ad hoc to satisfy
the [Mg/Fe]-mass relation cannot fit the mass-metallicity relation and viceversa.
A major dry merger, instead, does not make the agreement with observations worse if it happens
between galaxies that already obey both the mass(σ)-[Mg/Fe] and the mass(σ)-metallicity relations. However, this process alone cannot explain the physical reasons for these trends.
Conclusions. Dry mergers alone are not be the way to reconcile the need of a more efficient
star formation in the most massive galaxies with the late-time assembly
suggested in the hierarchical paradigm to recover the galaxy downsizing
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