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recent observational data of the deep surveys ( canada - france redshift survey , lyman - break galaxy surveys , the hubble deep field ) have provided information on the astrophysical properties of galactic objects at different stages of evolution of the universe , making possible to carry out a suitable confrontation of different scenarios of structure formation and galaxy chemical evolution .
analytical models of chemical evolution are a very powerful tool to study galaxy formation ( e.g. , chiappini et al . 1997 and references therein ) .
however , they are restricted by several hypothesis ( no inflows or outflows , instantaneous recycling , etc . ) , and can not include dynamical and kinematical evolution of the matter according to its nature ( dark matter , baryons ) in consistency with a cosmological model .
steinmetz & muller ( 1994 ) have implemented chemical enrichment in a code based on sph for the first time ( see also raiteri , villata & navarro 1996 ) .
these works run prepared - cosmological initial conditions where the formation and evolution of one single object is studied .
fully - consistent cosmological simulations have the advantage of providing a coherent well - described environment for all objects and a complete record of their formation and evolution .
we report here results of a chemical model implemented in a fully cosmological sph code in hierarchical clustering scenarios .
we have developed a model to implement metal enrichment in a cosmological context based on the ap3msph code described by tissera et al .
( 1997 ) .    a star formation ( sf ) algorithm has been included based on the smidth law .
cold and dense gas particles that satisfy the jean s instability criterium are eligible to form stars . each star cluster formed in a given baryonic particle at a sf episode
is given by @xmath0 , where @xmath1 is the integration time - step , @xmath2 the gas density of the particle and @xmath3 the sf efficiency parameter .
baryonic particles carry out the information of the different stellar populations formed and the remanent gas mass ( hybrid particles ) .
when the gas reservoir of a particle is depleted , it is transformed in a star particle .
particles are initially formed by hydrogen and helium in primordial abundances ( @xmath4 , @xmath5 ) .
metals are produced and ejected to the interstellar medium at the end of the life of stars .
most of the elements are produced by type ii supernovae ( sniie ) , except for the iron that is mainly produced by type i supernovae ( snie ) .
each @xmath6 formed can be followed up in time and the number of stars of a given mass estimated by assuming an initial mass function ( imf ) .
we adopted a salpeter imf with a lower and upper mass cut - off of @xmath7 and @xmath8 @xmath9 , respectively .
we resort to woosley & weaver ( 1995 ) metal ejecta tables for sneii .
the adopted nucleosynthesis prescriptions for type ia sne are taken from thielemann , nomoto & hashimoto ( 1993 ) .
type ib sne are assumed to be half the total number of snei and to produce only iron ( @xmath10 per explosion ) .
we assume that the life time of binary stars that finish their lives as sni ( @xmath11 ) is @xmath12 gyr .
ejected metals are distributed within the neighboring sphere of the particle where a @xmath6 is formed according to the sph technique .
hydrogen and helium are proportionally decreased according to the metal mass received by the particle ( see for details mosconi et al . 2000 ) .    given an imf ,
the free parameters of our chemical model are @xmath3 , the relative rate of different types of supernovae ( @xmath13=snrii / snri ) and @xmath11 .
the effects of thermal or kinetic energy injection into the ism due to sn explosions are not included in this work .
we performed sph simulations consistent with a cold dark matter ( cdm ) spectrum with @xmath14 , @xmath15 , @xmath16 , and @xmath17 .
we used @xmath18 particles ( @xmath19 ) in a comoving box of length @xmath20 mpc ( @xmath21 , @xmath22 ) .
the simulations performed ( s1 , s2 , s3 , s4 ) share the same initial conditions but have different sf and sn parameters ( see figure 1 for values of these parameters ) .      in hierarchical clustering scenarios , the sf process in glos is affected by mergers and interactions ( tissera 2000 , see also observational evidence in barton et al .
1999 ) . in our simulations
, we found that sf rate histories can be described as series of starbursts supperposed to a continuous component .
for each glo we can also follow the evolution of the metallicity of the stellar component and calculate the age - metallicity relation ( amr ; e.g. rocha - pinto et al .
the mean amrs estimated for our glos show the expected trend with high metallicity stars forming at more recent times in agreement with observations .
the dispersion found in the simulations indicates the existence of coeval sf sites of different metallicities at a given time .
the values and trend of these relations depend on the particular evolutionary history of each glo , and the sf and sn model parameters ( see tissera et al
. 2000 for details ) .
the chemical abundances in the ism in gas - rich galaxies allows to trace the evolution of individual galaxies , being hii regions and early b - type main sequence objects the most accesible probes of interstellar abundances .
fairly steep negative gradients are found : @xmath25 dex kpc@xmath26 within @xmath27 kpc for oxygen ( smartt & rolleston 1997 ) .
figure 2 shows abundance gradients of the oxygen for glos 596 and 325 as examples .
the curves represent the mass - weighted averages of the metal abundances at each particle position : they clearly show negative abundance gradients .
the slopes of the calculated gradients range from @xmath28 to @xmath29 dex kpc@xmath26 , in agreement with observations .
the differences in the abundance gradient slopes for a given glo are due to the sf mechanism and sn parameters , and the fact that we are only including gas in hybrid particles .
the slopes of glo 325 are less pronounced than those of glo 596 .
the different behaviour may be due to the fact that the gas in glo 596 forms a well - defined disk , while glo 325 is a clear spheroid , indicating that their histories of formation and evolution have been very different .    to sum up , in our simulations
, glos have different evolutionary history in consistency with a hierarchical clustering scenario that affect their sf rates and chemical evolution .
this chemical model can take all these physical processes into account , resulting in a powerful tool to study galaxy formation .
barton , e. j. , geller , m. j. , & kenyon , s. j. 1999 , , 195 , 2104 chiappini , c. , matteucci f. , & gratton , r. 1997 , , 477 , 765 gratton , r. , cannetta , e. , matteucci , f. , & sneden , c. 1996 , in formation of the galactic halo , inside out , ed .
h. morrison & a. sanajedini , 92 , 307 mosconi , m. b. , tissera , p. b. , lambas , d. g. , & cora , s. a. 2000 , , submitted raiteri , c. m. , villata , m. , & navarro , j. f. 1996 , , 315 , 105 rocha - pinto , h. j. , maciel , w. j. , & flynn , c. 2000 , , submitted ( astro - ph/0001383 ) smartt , s. j. , & rolleston , w. r. 1997 , , 481 , l47 steinmetz , m. , & mller , e. 1994 , , 281 , l97 thielemann , f. k. , nomoto , k. , & hashimoto , m. 1993 , in origin and evolution of the elements , ed .
n. prantzos , e. vangoni - flam , & m. cass ( cambridge university press ) , 299 tissera , p. b. , lambas , d. g. , & abadi , m. g. 1997 , , 286 , 384 tissera , p. b. 2000 , , 534 , 636 tissera , p. b. , mosconi , m. b. , cora , s. a. , & lambas , d. g. 2000 , in preparation woosley , s. e. , & weaver , t. a. , 1995 , , 101 , 181
we report first results of an implementation of a chemical model in a cosmological code , based on the smoothed particle hydrodynamics ( sph ) technique . 
 we show that chemical sph simulations are a promising tool to provide clues for the understanding of the chemical properties of galaxies in relation to their formation and evolution in a cosmological framework .    
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