2,724 research outputs found
Effects of the galactic winds on the stellar metallicity distribution of dwarf spheroidal galaxies
To study the effects of galactic winds on the stellar metallicity
distributions and on the evolution of Draco and Ursa Minor dwarf spheroidal
galaxies, we compared the predictions of several chemical evolution models,
adopting different prescriptions for the galactic winds, with the
photometrically-derived stellar metallicity distributions of both galaxies. The
chemical evolution models for Draco and Ursa Minor, which are able to reproduce
several observational features of these two galaxies, such as the several
abundance ratios, take up-to-date nucleosynthesis into account for
intermediate-mass stars and supernovae of both types, as well as the effect of
these objects on the energetics of the systems. For both galaxies, the model
that best fits the data contains an intense continuous galactic wind, occurring
at a rate proportional to the star formation rate. Models with a wind rate
assumed to be proportional only to the supernova rate also reproduce the
observed SMD, but do not match the gas mass, whereas the models with no
galactic winds fail to reproduce the observed SMDs. In the case of Ursa Minor,
the same model as in previous works reproduces the observed distribution very
well with no need to modify the main parameters of the model. The model for
Draco, on the other hand, is slightly modified. The observed SMD requires a
model with a lower supernova type Ia thermalization efficiency (
= 0.5 instead of = 1.0) in order to delay the galactic wind,
whereas all the other parameters are kept the same. The model results, compared
to observations, strongly suggest that intense and continuous galactic winds
play a very important role in the evolution of local dSphs.Comment: 11 pages, 7 figures, accepted for publication in Asttronomy &
Astrophysic
A possible theoretical explanation of metallicity gradients in elliptical galaxies
Models of chemical evolution of elliptical galaxies taking into account
different escape velocities at different galactocentric radii are presented. As
a consequence of this, the chemical evolution develops differently in different
galactic regions; in particular, we find that the galactic wind, powered by
supernovae (of type II and I) starts, under suitable conditions, in the outer
regions and successively develops in the central ones. The rate of star
formation (SFR) is assumed to stop after the onset of the galactic wind in each
region. The main result found in the present work is that this mechanism is
able to reproduce metallicity gradients, namely the gradients in the
index, in good agreement with observational data. We also find that in order to
honor the constant [Mg/Fe] ratio with galactocentric distance, as inferred from
metallicity indices, a variable initial mass function as a function of
galactocentric distance is required. This is only a suggestion since trends on
abundances inferred just from metallicity indices are still uncertain.Comment: 18 pages, LaTeX file with 4 figures using mn.sty, submitted to MNRA
The two regimes of the cosmic sSFR evolution are due to spheroids and discs
This 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
specific star formation rate 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 halos 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.Comment: 8 pages, 5 figures, MNRAS accepte
The Chemical Evolution of the Milky Way: the Three Infall Model
We present a new chemical evolution model for the Galaxy that assumes three
main infall episodes of primordial gas for the formation of halo, thick and
thin disk, respectively. We compare our results with selected data taking into
account NLTE effects. The most important parameters of the model are (i) the
timescale for gas accretion, (ii) the efficiency of star formation and (iii) a
threshold in the gas density for the star formation process, for each Galactic
component. We find that, in order to best fit the features of the solar
neighbourhood, the halo and thick disk must form on short timescales (~0.2 and
~1.25 Gyr, respectively), while a longer timescale is required for the
thin-disk formation. The efficiency of star formation must be maximum (10
Gyr-1) during the thick-disk phase and minimum (1 Gyr-1) during the thin-disk
formation. Also the threshold gas density for star formation is suggested to be
different in the three Galactic components. Our main conclusion is that in the
framework of our model an independent episode of accretion of extragalactic
gas, which gives rise to a burst of star formation, is fundamental to explain
the formation of the thick disk. We discuss our results in comparison to
previous studies and in the framework of modern galaxy formation theories.Comment: 12 pages, 7 figures, accepted for publication in MNRA
The galactic habitable zone of the Milky Way and M31 from chemical evolution models with gas radial flows
The galactic habitable zone is defined as the region with sufficient
abundance of heavy elements to form planetary systems in which Earth-like
planets could be born and might be capable of sustaining life, after surviving
to close supernova explosion events. Galactic chemical evolution models can be
useful for studying the galactic habitable zones in different systems. We apply
detailed chemical evolution models including radial gas flows to study the
galactic habitable zones in our Galaxy and M31. We compare the results to the
relative galactic habitable zones found with "classical" (independent ring)
models, where no gas inflows were included. For both the Milky Way and
Andromeda, the main effect of the gas radial inflows is to enhance the number
of stars hosting a habitable planet with respect to the "classical" model
results, in the region of maximum probability for this occurrence, relative to
the classical model results. These results are obtained by taking into account
the supernova destruction processes. In particular, we find that in the Milky
Way the maximum number of stars hosting habitable planets is at 8 kpc from the
Galactic center, and the model with radial flows predicts a number which is 38%
larger than what predicted by the classical model. For Andromeda we find that
the maximum number of stars with habitable planets is at 16 kpc from the center
and that in the case of radial flows this number is larger by 10 % relative to
the stars predicted by the classical model.Comment: Accepted by MNRA
Abundance gradients in spiral disks: is the gradient inversion at high redshift real?
We compute the abundance gradients along the disk of the Milky Way by means
of the two-infall model: in particular, the gradients of oxygen and iron and
their temporal evolution. First, we explore the effects of several physical
processes which influence the formation and evolution of abundance gradients.
They are: i) the inside-out formation of the disk, ii) a threshold in the gas
density for star formation, iii) a variable star formation efficiency along the
disk, iv) radial flows and their speed, and v) different total surface mass
density (gas plus stars) distributions for the halo. We are able to reproduce
at best the present day gradients of oxygen and iron if we assume an inside-out
formation, no threshold gas density, a constant efficiency of star formation
along the disk and radial gas flows. It is particularly important the choice of
the velocity pattern for radial flows and the combination of this velocity
pattern with the surface mass density distribution in the halo. Having selected
the best model, we then explore the evolution of abundance gradients in time
and find that the gradients in general steepen in time and that at redshift z~3
there is a gradient inversion in the inner regions of the disk, in the sense
that at early epochs the oxygen abundance decreases toward the Galactic center.
This effect, which has been observed, is naturally produced by our models if an
inside-out formation of the disk and and a constant star formation efficiency
are assumed. The inversion is due to the fact that in the inside-out formation
a strong infall of primordial gas, contrasting chemical enrichment, is present
in the innermost disk regions at early times. The gradient inversion remains
also in the presence of radial flows, either with constant or variable speed in
time, and this is a new result.Comment: 15 pages, 19 figures, accepted for publication in MNRA
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