900 research outputs found
The effect of clouds on the dynamical and chemical evolution of gas-rich dwarf galaxies
We study the effects of clouds on the dynamical and chemical evolution of
gas-rich dwarf galaxies, in particular focusing on two model galaxies similar
to IZw18 and NGC1569. We consider both scenarios, clouds put at the beginning
of the simulation and continuously created infalling ones. Due to dynamical
processes and thermal evaporation, the clouds survive only a few tens of Myr,
but during this time they act as an additional cooling agent and the internal
energy of cloudy models is typically reduced by 20 - 40% in comparison with
models without clouds. The clouds delay the development of large-scale
outflows, therefore helping to retain a larger amount of gas inside the galaxy.
However, especially in models with continuous creation of infalling clouds,
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 (by up to ~ 0.4 dex)
with respect to the one attained by diffuse models.Comment: 3 pages, 3 figures, to be published in Astronomische Nachrichten
(proceedings of Symposium 6 of the JENAM 2008, Vienna
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.Comment: 3 pages, 1 figure, to appear in the Proceedings of the Conference
"Cosmic Evolution", Paris, November 200
The fate of heavy elements in dwarf galaxies - the role of mass and geometry
Energetic feedback from Supernovae and stellar winds can drive galactic
winds. Dwarf galaxies, due to their shallower potential wells, are assumed to
be more vulnerable to this phenomenon. Metal loss through galactic winds is
also commonly invoked to explain the low metal content of dwarf galaxies. Our
main aim in this paper is to show that galactic mass cannot be the only
parameter determining the fraction of metals lost by a galaxy. In particular,
the distribution of gas must play an equally important role. We perform 2-D
chemo-dynamical simulations of galaxies characterized by different gas
distributions, masses and gas fractions. The gas distribution can change the
fraction of lost metals through galactic winds by up to one order of magnitude.
In particular, disk-like galaxies tend to loose metals more easily than
roundish ones. Consequently, also the final metallicities attained by models
with the same mass but with different gas distributions can vary by up to one
dex. Confirming previous studies, we also show that the fate of gas and freshly
produced metals strongly depends on the mass of the galaxy. Smaller galaxies
(with shallower potential wells) more easily develop large-scale outflows,
therefore the fraction of lost metals tends to be higher.Comment: 13 pages, 11 figures, accepted for publication on Astronomy and
Astrophysic
Modelling the chemical evolution
Advanced observational facilities allow to trace back the chemical evolution
of the Universe, on the one hand, from local objects of different ages and,
secondly, by direct observations of redshifted objects. The chemical enrichment
serves as one of the cornerstones of cosmological evolution. In order to
understand this chemical evolution in morphologically different astrophysical
objects models are constructed based on analytical descriptions or numerical
methods. For the comparison of their chemical issues, as there are element
abundances, gradients, and ratios, with observations not only the present-day
values are used but also their temporal evolution from the first era of metal
enrichment. Here we will provide some insight into basics of chemical evolution
models, highlight advancements, and discuss a few applications.Comment: 12 pages, 3 figures, to appear in the proceedings of "Chemical
Abundances in the Universe: Connecting First Stars to Planets". IAU Symposium
No. 265, 2009. K. Cunha, M. Spite & B. Barbuy, ed
Non-isothermal filaments in equilibrium
The physical properties of the so-called Ostriker isothermal filament
(Ostriker 1964) have been classically used as benchmark to interpret the
stability of the filaments observed in nearby clouds. However, recent continuum
studies have shown that the internal structure of the filaments depart from the
isothermality, typically exhibiting radially increasing temperature gradients.
The presence of internal temperature gradients within filaments suggests that
the equilibrium configuration of these objects should be therefore revisited.
The main goal of this work is to theoretically explore how the equilibrium
structure of a filament changes in a non-isothermal configuration. We solve the
hydrostatic equilibrium equation assuming temperature gradients similar to
those derived from observations. We obtain a new set of equilibrium solutions
for non-isothermal filaments with both linear and asymptotically constant
temperature gradients. Our results show that, for sufficiently large internal
temperature gradients, a non-isothermal filament could present significantly
larger masses per unit length and shallower density profiles than the
isothermal filament without collapsing by its own gravity. We conclude that
filaments can reach an equilibrium configuration under non-isothermal
conditions. Detailed studies of both the internal mass distribution and
temperature gradients within filaments are then needed in order to judge the
physical state of filaments.Comment: 5 pages, 2 figures, accepted for publication in A&
The mass-metallicity relation of tidal dwarf galaxies
Dwarf galaxies generally follow a mass-metallicity (MZ) relation, where more
massive objects retain a larger fraction of heavy elements. Young tidal dwarf
galaxies (TDGs), born in the tidal tails produced by interacting gas-rich
galaxies, have been thought to not follow the MZ relation, because they inherit
the metallicity of the more massive parent galaxies. We present chemical
evolution models to investigate if TDGs that formed at very high redshifts,
where the metallicity of their parent galaxy was very low, can produce the
observed MZ relation. Assuming that galaxy interactions were more frequent in
the denser high-redshift universe, TDGs could constitute an important
contribution to the dwarf galaxy population. The survey of chemical evolution
models of TDGs presented here captures for the first time an initial mass
function (IMF) of stars that is dependent on both the star formation rate and
the gas metallicity via the integrated galactic IMF (IGIMF) theory. As TDGs
form in the tidal debris of interacting galaxies, the pre-enrichment of the
gas, an underlying pre-existing stellar population, infall, and mass dependent
outflows are considered. The models of young TDGs that are created in strongly
pre-enriched tidal arms with a pre-existing stellar population can explain the
measured abundance ratios of observed TDGs. The same chemical evolution models
for TDGs, that form out of gas with initially very low metallicity, naturally
build up the observed MZ relation. The modelled chemical composition of ancient
TDGs is therefore consistent with the observed MZ relation of satellite
galaxies.Comment: 7 pages, 3 figures, MNRAS accepte
Galactic consequences of clustered star formation
If all stars form in clusters and both the stars and the clusters follow a
power law distribution which favours the creation of low mass objects, then the
numerous low mass clusters will be deficient in high mass stars. Therefore, the
mass function of stars, integrated over the whole galaxy (the Integrated
Galactic Initial Mass Function, IGIMF) will be steeper at the high mass end
than the underlying IMF of the stars. We show how the steepness of the IGIMF
depends on the sampling method and on the assumptions made for the star cluster
mass function. We also investigate the O-star content, integrated photometry
and chemical enrichment of galaxies that result from several IGIMFs, as
compared to more standard IMFs.Comment: 4 pages, 2 figures, to appear in online version of Proceedings of IAU
S266, a two page version will appear in the Proceedings of IAU S26
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