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many dwarf irregular galaxies ( dig ) are known to be in a starburst phase , or are believed to have experienced periods of intense star formation in the recent past .
these galaxies are generally called blue compact dwarfs ( bcd ) .
energetic events associated with star formation ( stellar winds and supernova explosions ) may sweep the interstellar gas out of the region actively forming stars , thus creating galactic winds .
observational evidences of outflows have been found recently in many edge - on starburst galaxies , like ngc1705 , ngc1569 and ngc3628 .
martin ( 1998 ) found that large expanding supershells are common byproducts of massive star formation in dwarf galaxies .
both dynamical and chemical simulations of these galaxies have suggested the existence of a ` differential galactic wind ' , in the sense that , after a starburst event , these objects would loose mostly metals ( pilyugin 1992 , 1993 ; marconi , matteucci & tosi 1994 ; maclow & ferrara 1999 ; dercole & brighenti 1999 ) . however in none of these studies , detailed chemical and dynamical evolution was taken into account at the same time .    among bcd , the galaxy izw18 ( the most metal - poor galaxy locally known ) constitutes the best candidate for a truly `` young '' galaxy .
however , there is still a debate in literature on whether izw18 is experiencing the first burst of star formation or not .
evolutionary population synthesis models by mas - hesse & kunth ( 1999 ) show that the present burst is very young ( between 3 and 13 myr ) and the contribution of older stars , if any , is negligible .
dynamical arguments ( martin 1996 ) suggest a single burst with an age between 15 and 27 myr . recent color magnitude diagram ( cmd ) studies of izw18 , both in the optical ( aloisi , tosi & greggio 1999 ; hereafter atg ) and in the infrared ( stlin 2000 ) , revealed the presence of an underlying older population , with an age of some 10@xmath0 yr .
legrand ( 2000 ) and legrand et al . ( 2000 ) proposed instead a low and continuous star formation regime for izw18 .
we study , through numerical simulations , the dynamical and chemical evolution of a gas - rich dwarf galaxy whose structural parameters resembles izw18 .
we consider single instantaneous starburst or a couple of starburst separated by a quiescent period .
we include effects ( both energetical and chemical ) of type ii and type ia sne in a detailed way .
the aim of this work is to test the ` differential wind ' hypothesis with an hydrodynamical approach and to find constraints for the number and for the age of the starburts in izw18 .
we consider a gaseous component in hydrostatic isothermal equilibrium with the centrifugal force and a potential well .
the potential well is the sum of a quasi - isothermal dark halo ( @xmath1 m@xmath2 ) and an oblate king profile .
the mass of gas inside the galactic region ( an ellipsoid with dimensions 1 kpc @xmath3 730 pc ) is @xmath4 m@xmath2 .    to describe the evolution of the ism
, we adopt a 2-d hydrocode , with source terms describing the rate of mass and energy return from the starbursts , taking into account sne of type ii and ia and low and intermediate - mass stars ( ims ) . by using passively evolving tracers
, we are able to follow the evolution , in space and time , of some chemical elements of particular astrophysical interest .
the production of these elements are obtained following the nucleosynthesis prescriptions from various authors : woosley & weaver ( 1995 ) for sneii , nomoto , thielemann & yokoi for sneia and renzini & voli ( 1981 ) , case r , or van den hoek & groenewegen ( 1997 ) , case v , for ims .
see recchi , matteucci & dercole ( 2001 ) for more details about model prescriptions .    in the single - burst model , the mass of stars formed is @xmath5 m@xmath2 , whereas in the two - burst model we consider a weaker first burst ( the mass of stars is @xmath6 m@xmath2 ) .
after 300 myr ( model m300 ) or 500 myr ( model m500 ) we consider that 10% of cold gas inside a central region ( a sphere of 200 pc of radius ) is turned into stars .
the initial abundances of this second stellar generation are simply the metallicity of the cold gas in the central region at the onset of the burst . according to the results of atg , we consider also a flatter imf ( model m300f ) , with a slope @xmath7 .
more details about these two - burst models are in recchi et al .
( 2001b ) .
the efficiency of sn heating is assumed to be very low for type ii sne . in particular ,
following results of bradamante , matteucci & dercole ( 1998 ) , we consider a thermalization efficiency of @xmath8 for the single - burst model and @xmath9 for the two - burst model , namely only 3% ( or 5% ) of the explosion energy is able to thermalize the ism , while the rest is radiated away .
type ia sne instead explode in an already heated and diluted medium , thus their thermalization efficiency is assumed to be @xmath10 .
there is debate in literature about the correct value of thermalization efficiency .
in particular , strickland & stevens ( 1999 ) assume a value @xmath11 ( see also the contribution of strickland in this volume ) .
the reason of our choice is that the number of sneii exploding in our model is rather low , thus the time interval between single explosions ( @xmath12 yr ) is larger than the typical cooling time - scale , thus remnant of type ii sne evolve as single snr .
in addition , we tried to run simulations with an efficiency @xmath11 also for type ii sne and the galaxy is quickly devoided of gas , thus the actual gas content in izw18 rules out the possibility of an high @xmath13 .
owing to the energy released by type ii sne , a galactic wind develops .
it expands faster along the z direction , where the ism density gradient is steeper .
sneii activity lasts for only 29 myr ( the lifetime of a 8 m@xmath2 star ) , then is replaced by a weaker snia wind , not strong enough to sustain the galactic outflow ( fig .
1 ) . after @xmath14 300 myr ,
the expanding ism is diluted enough and the hot bubble finally breaks out through a funnel .
most of the sneii ejecta reamins locked into the cold and dense shell , whereas metals ejected by sneia are easily channelled along the funnel .
iron , mostly produced by type ia sne , is thus easily lost by the galaxy and the [ @xmath15/fe ] ratios ouside the galaxy are lower than inside ( see section 4 ) . owing to the low evolution of the superbubble ,
the internal cavity becomes soon radiative ( i.e. radiates an energy comparable to the thermal energy content of the shocked wind ) thus after @xmath14 10 myr most of metals are in a cold phase .
owing to the low luminosity of the first burst , after @xmath14 300 myr a galactic wind still does not develop .
for the model m300 a cold , dense shell of dimensions 200 @xmath3 100 pc forms and outside this region the ism is almost unperturbed .
the impact of the second generation of stars on the ism dynamics is rather vigorous .
already after @xmath14 30 myr after the onset of the second burst a breakout occurs ( see fig . 1 ) and
the gas produced during this second burst is easily lost along the galactic chimney .
the hypothesis of a differential wind is substantially confirmed in these simulations : metals are ejected more easily than pristine ism . also for the two - burst model the ejecta of type ia sne are lost more easily , but this effect is less evident compared to single - burst model .
the consequence of this selective losses of metals is that [ @xmath15/fe ] ratios outside the galaxy are lower than inside .      the evolution of oxygen abundance and c / o and n / o ratios for the single - burst model , cases r and v ( left panels ) and for the two - burst models m300 , cases r and v and m300f , case v only ( right panels ) are shown in fig .
2 . shaded areas represents the observed values found in literature for izw18    single - burst model reproduces the observed abundances of izw18 only for a very short time , at an age of @xmath14 31 myr . after this time , n / o ratio begins to increase over the permitted range , owing to the n produced by intermediate - mass stars .
model m300 is able to reproduce observed abundances of izw18 for a wider range of times : between 25 and 40 myr after the onset of the second burst ( case r ) and between 50 and 70 myr ( case v ) .
the m300f model produces too much oxygen during the first burst of star formation and does not fit the observed abundance ratios , unless the age of the second burst is extremely short ( around 4 myr ) .
model m500 ( not shown here ) is able to fit abundances found in literature for an evolutionary time between 40 and 80 myr .
our main conclusions can be summarized as follows :    * energetic events associated with the starbursts , are able to trigger a galactic wind and the metals produced in the burst leave the galaxy more easily thaan the unprocessed gas . * in particular ,
the ejecta of type ia sne are lost more efficiently than type ii sne , because this kind of explosions occur in a hot and rarefied medium .
the consequence is that [ @xmath15/fe ] ratios outside the galaxy are lower than inside .
this effect is more evident in the single - burst model . *
single - burst model reproduces the observed abundances in izw18 after @xmath14 31 myr , while for the two - burst model we obtain agreement with the data found in literature for wider ranges of time , when the second starburst has an age of some tenth of myr , depending on the adopted model and nucleosynthesis prescriptions . * the classical salpeter imf should be preferred over a flatter one which would predict a too high oxygen abundance . *
finally , we can suggest that a first , weak burst of star formation , occurred more than 300 myr ago , followed by a stronger one , having an age of some tenth of myr , with a salpeter imf , best reproduces the properties of izw18 .
aloisi , a. , tosi , m. & greggio , l. 1999 , , 118 , 302 ( atg ) bradamante , f. , matteucci , f. & dercole , a. 1998 , , 337 , 338 dercole , a. & brighenti , f. 1999 , , 309 , 941 legrand , f. 2000 , , 354 , 504 legrand , f. , kunth , d. , roy , j .-
r . , mas - hesse , j.m .
& walsh , j.r .
2000 , , 355 , 891 maclow , m .-
& ferrara , a. 1999 , , 513 , 142 martin , c.l .
1996 , , 465 , 680 martin , c.l .
1998 , , 506 , 222 marconi , g. , matteucci , f. & tosi m. 1994 , , 217 , 391 mas - hesse , j.m .
& kunth , d. 1999 , , 349 , 765 nomoto , k. , thielemann , f.k . &
yokoi , k. 1984 , , 286 , 644 stlin , g. 2000 , , 535 , l99 pilyugin , l.s .
1992 , , 260 , 58 pilyugin , l.s .
1993 , , 277 , 42 recchi , s. , matteucci , f. & dercole , a. 2001 , , 322 , 800 recchi , s. , matteucci , f. , dercole , a. & tosi , m. 2001b , submitted renzini , a. & voli , m. 1981 , , 94 , 175 strickland , d.k . & stevens , i.r . , 306 , 43 van den hoek , l.b . & groenewegen , m.a.t .
1997 , , 123 , 305 woosley , s.e . & weaver , t.a .
1995 , , 101 , 181
in this paper we present some results of numerical simulations concerning the development of galactic winds in starburst galaxies . 
 in particular , we focus on a galaxy similar to izw18 , the most metal - poor galaxy locally known . 
 we compute the chemo - dynamical evolution of this galaxy , considering the energetic input and the chemical yields originating from supernovae ( sne ) of type ii and ia and from intermediate - mass stars . 
 we consider both single , instantaneous starburst and two starburst separated by a quiescent period . in all considered cases a metal enriched wind develops and in particular the metals produced by type ia sne are ejected more efficiently than the other metals . 
 we suggest that two bursts of star formation , the first being weaker and the last having an age of some tenth of myr , can satisfactorily reproduce the abundances and abundance ratios found in literature for izw18 .    
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