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Galactic Winds in Irregular Starburst Galaxies

F. MATTEUCCI Department of Astronomy,Trieste University, Via G.B. Tiepolo 11, 34100 Trieste, Italy E-mail: matteucci@ts.astro.it S. RECCHI Department of Astronomy,Trieste University, Via G.B. Tiepolo 11, 34100 Trieste, Italy E-mail: recchi@ts.astro.it
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 e?ciently than those produced by type II SNe. We also ?nd that one single burst is able to enrich chemically the surrounding region in few Myr. Both these results are the e?ect of the assumed e?ciency 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 su?ered two bursts in its life, with the previous being less intense than the last one.

arXiv:astro-ph/0101284v1 17 Jan 2001


One and two bursts of star formation

We assume a single instantaneous burst occurring at the center of a gas-rich dwarf surrounded by a dark matter halo. The mass of stars formed is M? = 6 · 105 M⊙ , the gas mass is Mgas = 1.7 · 107 M⊙ and the mass of the dark halo is Mdark = 6.5 · 108 M⊙ . These parameters are chosen to reproduce, for what possible, the characteristics of IZw18 (unevolved local galaxy). The galactic region extends for 700 pc in the vertical (z) direction and for 1kpc in the radial direction (R). In order to study the chemo-dynamical evolution consequent to the burst, we adopt a 2-D hydrocode coupled with detailed chemical yields from: type II SNe, type Ia SNe and low and intermediate mass stars (see Recchi et al. 2001 for more details about this model). We follow the evolution of the abundances of H, He, C, N, O, Mg, Si, Fe in the gas. The evolution is followed for 375 Myr since the burst. We also consider a case with a second burst occurring after 300 Myr from the ?rst one The mass in stars formed during the ?rst burst is M? = 105 M⊙ , whereas the mass in stars formed in the second burst is M? = 5.8 · 105 M⊙ . The initial gas mass and dark matter 1

halo are the same as in the one-burst case. The simulation lasts 450 Myr in total since the ?rst burst. The SN e?ciencies of energy transfer into the ISM from SNe is the same in both cases, in particular: for type II SNe we assume that the e?ciency is η = 0.03 and for type Ia SNe η = 1. This choice is due to results from Bradamante et al. (1998) for type II SNe indicating that the ?rst SNe to explode lose a large fraction of their initial energy by radiation due to the cold and dense ISM in their surroundings, whereas for type Ia SNe the e?ciency is maximum due to the fact that they explode into an ISM already hot and rare?ed (see Recchi et al. 2001, for details). 2 Dynamical Results

We ?nd that the starburst triggers indeed a galactic wind and the metals leave the galaxy more easily than the unprocessed gas con?rming previous results (e.g. McLow and Ferrara, 1999). We ?nd that SNe Ia eject their metals more e?ciently than SNe II since they inject all of their initial energy into the ISM. This is a new result relative to previous studies since it is the ?rst time that type Ia SNe are taken into account. At variance with previous studies (see e.g. Tenorio-Tagle 1996) we ?nd that most of the metals are already in the cold gas phase after 8-10 Myr from the beginning of the burst, due to the fact that the superbubble created by the SNe does not break immediately and thermal conduction can act e?ciently. 3 Chemical results

We ?nd that one single instantaneous burst, occurring in a primordial gas (no metallicity), at an age of ? 31 Myr can reasonably reproduce the abundances measured in IZw18 (see ?gure 1). From this one would conclude that perhaps this galaxy is experiencing its ?rst burst of SF, although one cannot exclude a previous burst which enriched the gas no more than 1/50 Z⊙ . However, as evident in ?gure 1, the correct N/O ratio would last only for a very short time since for t > 31 Myr the N/O ratio will start to increase outside the permitted observational range. In addition, Color Magnitude Diagram studies (e.g. Aloisi et al. 1999) indicate the presence of an old underlying stellar population in IZw18, thus suggesting that the two-burst case is more realistic. An interesting result is that the [α/Fe] ratios in the gas outside the galaxy (i.e. in the galactic wind) are lower than inside, due to the larger ejection e?ciency by type Ia SNe (more iron is lost than α-elements). This creates an interesting dichotomy which can have important consequences on the evolution of the ICM, especially if this e?ect will be found also in elliptical galaxies. 2

Figure 1: Evolution of O, C, N for the single-burst model (left panels) and the doubleburst model (right panels). The superimposed grids represent the observative values found in literature for IZw18

In the two-burst case, at the onset of the second burst the metallicity in the burst region is ? Z⊙ /50, and a very good agreement with the measured abundances in IZw18 is obtained in this case, especially for the C/O and N/O ratios and for a larger range of burst ages (see ?gure 1). Our results suggest that a ?rst weak burst occurred more than 300 Myr ago and followed by a more intense one with an age between 20 and 70 Myr can reproduce most of the properties of this galaxy. The [α/Fe] ratios in the two burst case are always lower outside than inside the galaxy, but the e?ect is less evident than in the one-burst case. Acknowledgments We are gratefull to Annibale D’Ercole and Monica Tosi for their help in this work. References 1. 2. 3. 4. 5. Aloisi A., Tosi M. and Greggio L., AJ 118, 302 (1999). Bradamante F., Matteucci F. and D’Ercole A., A&A 337, 338 (1998). Mac Low M.-M. and Ferrara A., ApJ 513, 142 (1999). Recchi S., Matteucci F. and D’Ercole A., MNRAS in press (2001). Tenorio-Tagle G., AJ 111, 1641 (1996) 3

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