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

Chemo-dynamical Evolution of the ISM in Galaxies

Chemo-dynamical models have been introduced in the late eighties and are a generally accepted tool for understanding galaxy evolution. They have been successfully applied to one-dimensional problems, e.g. the evolution of non-rotating galaxies, and two-dimensional problems, e.g. the evolution of disk galaxies. Recently, also three-dimensional chemo-dynamical models have become available. In these models the dynamics of different components, i.e. dark matter, stars and a multi-phase interstellar medium, are treated in a self-consistent way and several processes allow for an exchange of matter, energy and momentum between the components or different gas phases. Some results of chemo-dynamical models and their comparison with observations of chemical abundances or star formation histories will be reviewed.Comment: 10 Pages, 5 Figures, to appear in "From Observations to Self-Consistent Modelling of the ISM in Galaxies", 2003, eds M. Avillez et a

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.Comment: 2 pages, 1 figure, to appear in the Proceedings of the CRAL conference "Chemodynamics: from first stars to local galaxies", Lyon, France, 10-14 July 200

The Exceptionally Soft X-ray Spectrum of the Low-mass Starburst Galaxy NGC 1705

NGC 1705 is one of the optically brightest and best studied dwarf galaxies. It appears to be in the late stage of a major starburst and contains a young super star cluster. Type II supernovae are therefore likely to have been a major effect in the recent evolution of this galaxy and are likely to have produced a superbubble whose affects on the low-density ambient interstellar medium can be ideally studied. ROSAT PSPC observations of this galaxy reveal two striking blobs of X-ray emission embedded in \Ha loops which can be interpreted as both sides of the upper plumes of the same superbubble. These sources are a surprise. They are much softer than those observed from other starburst dwarf galaxies, and are so soft that they should have been blocked if the observed Galactic HI column density were uniformly distributed across NGC 1705 or if the sources were embedded in the HI disk of NGC 1705. In addition, the total X-ray luminosity in the ROSAT energy band of 1.2x10^{38} erg s^{-1} is low in comparison to similar objects. We discuss possible models for the two X-ray peaks in NGC 1705 and find that the sources most likely originate from relatively cool gas of one single superbubble in NGC 1705. The implications of the exceptional softness of these sources are addressed in terms of intrinsic properties of NGC 1705 and the nature of the foreground Galactic absorption.Comment: 7 pages, 2 ps-figures, LATEX-file; accepted for publication in ApJ.Letter

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.Comment: 6 pages, 2 figures, to appear in the Proceedings of the CRAL conference "Chemodynamics: from first stars to local galaxies", Lyon, France, 10-14 July 200

Can filamentary accretion explain the orbital poles of the Milky Way satellites?

Several scenarios have been suggested to explain the phase-space distribution of the Milky Way (MW) satellite galaxies in a disc of satellites (DoS). To quantitatively compare these different possibilities, a new method analysing angular momentum directions in modelled data is presented. It determines how likely it is to find sets of angular momenta as concentrated and as close to a polar orientation as is observed for the MW satellite orbital poles. The method can be easily applied to orbital pole data from different models. The observed distribution of satellite orbital poles is compared to published angular momentum directions of subhalos derived from six cosmological state-of-the-art simulations in the Aquarius project. This tests the possibility that filamentary accretion might be able to naturally explain the satellite orbits within the DoS. For the most likely alignment of main halo and MW disc spin, the probability to reproduce the MW satellite orbital pole properties turns out to be less than 0.5 per cent in Aquarius models. Even an isotropic distribution of angular momenta has a higher likelihood to produce the observed distribution. The two Via Lactea cosmological simulations give results similar to the Aquarius simulations. Comparing instead with numerical models of galaxy-interactions gives a probability of up to 90 per cent for some models to draw the observed distribution of orbital poles from the angular momenta of tidal debris. This indicates that the formation as tidal dwarf galaxies in a single encounter is a viable, if not the only, process to explain the phase-space distribution of the MW satellite galaxies.Comment: 14 pages, 4 figures, 3 tables. Accepted for publication in MNRA