Galactic wind

The most metal-rich stars in dwarf spheroidals (dSph) have been shown to have significantly lower even-Z abundance ratios than stars of similar metallicity in the Milky Way (MW). In addition, the most metal-rich dSph stars are dominated by an s-process abundance pattern in comparison to stars of similar metallicity in the MW. This has been interpreted as excessive contamination by Type la super-novae (SN) and asymptotic giant branch (AGB) stars ( Bonifacio et al. 2000, Shetrone et al. 2001, Smecker-Hane McWilliam 2002). By comparing these results to MW chemical evolution, Lanfranchi Matteucci (2003) conclude that the dSph galaxies have had a slower star formation rate than the MW (Lanfranchi Matteucci 2003). This slow star formation, when combined with an efficient galactic wind, allows the contribution of Type la SN and AGB stars to be incorporated into the ISM before the Type II SN can bring the metallicity up to MW thick disk metallicities. 

We present chemical evolution models for NGC 6822 computed with five fixed parameters, all constrained by observations, and only a free parameter, related with galactic winds. The fixed parameters are i) the infall history that has produced NGC 6822 is derived from its rotation curve and a cosmological model ii) the star formation history of the whole galaxy based on star formation histories for 8 zones inferred from H-R diagrams iii) the IMF, the stellar yields, and the percentage of Type la SNe progenitors are the same than those that reproduce the chemical history of the Solar Vicinity and the Galactic disk. 

Fig. 1. Evolution of baryonic mass, gaseous mass, O/H, C/O and Fe/O predicted by a model that considers an early galactic wind. Observations from Peimbert et al. (2005) and Venn et al. (2001).

The star formation lasts for several Gyrs (7-8), then a powerful galactic wind is produced owing to the energy injected into the ISM by SN explosions. At this point the dwarf spheroidal (dSph) ejects most of its residual gas. 

The energetics from type II and la SNe is taken into account for the development of a galactic wind (the star formation does not halt during the wind but lowers its intensity due to the gas loss). 

The sharp decrease of the [a/Fe] ratios observed in dSphs is due to the combined effect of a slow star formation regime and a strong galactic wind. The... 

Show that, in the Simple model (no inflow and no Galactic wind), the evolution of deuterium abundance in the interstellar medium is given by... 

Fig. 11.13. Left panel projected heavy-element abundance Zp along lines of sight as a function of galactocentric distance, for differing values of the mass fraction in the form of stars at the onset of a terminal galactic wind. Right panel (mass-weighted) mean abundance as a function of final total stellar mass, for two different assumptions as to the dependence of the amount of gas lost on the initial mass. The assumed bulk yield is 0.02 and the trend along the linear part of the curves is approximately Z a M3/8. Adapted from Larson (1974b).

Assuming a continuous and homogeneous galactic wind with a mass flux rja, iff (rj = const.), combined with a selective (metal-enhanced) wind r/ x the mass of Type II supernova ejecta (tj = const. < 1), show that Eq. (8.6) for the Simple model is replaced by... 

During the propagation phase, reacceleration of particles has been suggested at shock fronts in the galactic wind. Also this mechanism yields a rigidity dependent cut-off Volk Zirakashvili 2003. 

Supplementary Parameters- Infall of extragalactic gas, radial flows and galactic winds are important ingredients in building galactic chemical evolution models. 

Early monolithic collapse of a gas cloud or early merging of lumps of gas where dissipation plays a fundamental role (Larson 1974 Arimoto Yoshii 1987 Matteucci Tornambe 1987 ). In this scenario the star formation stops soon after a galactic wind develops and the galaxy evolves passively since then. 

Different amounts and/or concentrations of dark matter as functions of Ml I n particular, less dark matter should be present in the most massive systems (Matteucci, Ponzone Gibson, 1998). As a consequence of this, again galactic winds occur earlier in more massive objects. 

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