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since the discovery of an extragalactic component for the ir background a decade ago  @xcite , numerous studies have demonstrated the key importance of ir galaxies to reconstruct the history of galaxy star formation and stellar mass assembly .
recent surveys conducted with the _
spitzer space telescope _ have allowed for the resolution of more than 70% of the mid and far - ir background light  @xcite and a quite detailed understanding of the evolution of ir galaxies with cosmic epoch , e.g.  @xcite .
the ir galaxy luminosity function has a strong positive luminosity evolution from redshifts @xmath3 to 1 ( see  @xcite ) , implying that more luminous ir galaxies dominate the ir output with increasing redshift . beyond @xmath4 ,
basically all the ir light is produced by luminous and ultra - luminous ir galaxies ( lirgs and ulirgs , respectively ) with bolometric ir luminosities @xmath5 .
active galactic nuclei ( agn ) have some significant ( but not yet well - determined ) contribution to the ir background and govern the shape of the bright - end of the mid - ir lf at @xmath6  @xcite .
the study of the optical spectra of ir galaxies can reveal several aspects of the physical conditions in which star formation and agn activity take place .
what is the evolutionary stage of the hosts of ir activity ?
what is the relationship between dust emission and the observed reddening in optical bands ? does the ir phase accompany all the process of star formation or is it set at a particular time ?
the large sample of zcosmos spectra  @xcite being taken over the 2 deg@xmath7 cosmos field offers a perfect opportunity to address these questions .
the cosmos survey  @xcite is composed of multi - wavelength observations over 2 deg@xmath7 of the sky at nearly equatorial latitudes . among these observations , a large spectroscopic campaign is being carried out with the vimos spectrograph on the very large telescope ( vlt ) , namely the zcosmos survey  @xcite .
the zcosmos survey consists of two parts : 1 ) zcosmos - bright , whose aim is to obtain 20,000 optical spectra for a magnitude - limited @xmath8 ( ab mag ) source sample and which is now half - way complete ; 2 ) zcosmos - deep , which will yield 10,000 optical spectra in the central 1 deg@xmath7 of the field , for galaxies colour - selected to be at high redshifts .
the cosmos field is also completely covered by deep _ spitzer _
maps from 3.6 through 160 @xmath0  @xcite .
these data have been obtained as part of two _ spitzer _ legacy programs in the observation cycles 2 and 3 .
we present some of the first results of a series of studies we are conducting to analyse the optical spectral properties of 24 @xmath0-selected galaxies in the cosmos field .
we particularly focus here on the brightest ir galaxies ( @xmath1 ) found at redshifts @xmath2 , whereas a complete study of the zcosmos - bright spectra of more than 600 @xmath9 galaxies is presented elsewhere  @xcite .
the completed first half of the zcosmos - bright program provides us with good - quality redshifts and spectra for 24 ir galaxies with @xmath1 over 1.5 deg@xmath7 of the cosmos field .
these galaxies are tracers of the most intense star - formation and/or agn activity and , within zcosmos - bright , 22 out of 24 are found at redshifts @xmath2 . among these 24 brightest ir galaxies with @xmath8 ( ab mag ) ,
only 3 are identified with agn from their optical spectra and/or x - ray emission .
figure [ fig1 ] shows _ advanced camera for surveys ( acs ) _ @xmath10-band postage stamps and zcosmos spectra for four of these ir sources with @xmath1 .
the four examples correspond to galaxies at redshifts @xmath11 . in all these cases ,
the optical spectra display prominent emission lines , which are characteristic of actively star - forming galaxies .
in fact , the intrinsic emission is even larger in all these galaxies , as all of them have at least mildly important degrees of dust obscuration , with @xmath12-band extinctions ranging between @xmath13 and 2.5 ( as obtained by equating the [ ir+uv ] and the optically - derived star formation rates ) .
the _ acs _ stamps of the four example galaxies reveal compact nuclear morphologies , where two different objects can be resolved in each case .
these are probably galaxies in an advanced stage of a merger , as most of the brightest ir galaxies are thought to be .
_ acs _ @xmath10-band postage stamps and zcosmos spectra for four galaxies with @xmath1 in the cosmos field .
all these four galaxies are probably in the last stage of a merger.,width=453 ]    the equivalent widths of the emission lines provide information on the strength of the emission produced by the new generations of stars with respect to the underlying older stellar populations , which mostly contribute to the spectral continuum . figures [ fig2 ] and [ fig3 ] show the h@xmath14 and h@xmath15 equivalent widths , respectively , for the @xmath16 galaxies ( filled circles ) , in comparison to the equivalent widths of all the other @xmath9 sources ( cross - like symbols ) .
h@xmath14 measurements have been performed for galaxies at @xmath17 and h@xmath15 measurements correspond to galaxies at @xmath18 .
the equivalent widths are only shown in the cases that they are larger than 5 @xmath19 .
the h@xmath14 equivalent widths of the ir galaxies with @xmath16 as a function of redshift ( filled circles ) , in comparison to all the other galaxies with @xmath9 ( cross - like symbols).,width=302 ]     the h@xmath15 equivalent widths of the ir galaxies with @xmath16 as a function of redshift , in comparison to all the other galaxies with @xmath9 .
symbols are the same as in figure  [ fig2].,width=302 ]    inspection of figures [ fig2 ] and [ fig3 ] shows that some of the brightest ir galaxies display very large h@xmath14 or h@xmath15 equivalent widths .
however , in many other cases , the equivalent widths are much smaller and indistinguishable of those of fainter ir galaxies . thus , as other optical spectral characteristics  @xcite , the balmer line equivalent widths have little correlation with the ir photometric properties .
this simple comparison of equivalent widths illustrates the heterogeneity of the sources making the brightest ir - galaxy population at low redshifts .
the differences in equivalent widths are to some extent due to the different relative importance that the previously formed stars have in these galaxies .
some of the brightest ir galaxies are relatively young systems still in the process of chemical enrichment , while others are already - massive galaxies with strong stellar continua , which might possibly be forming their last generations of stars  @xcite .
with the availability of the deepest 24 @xmath0 images for the cosmos field down to flux limits @xmath20 , we will be able to explore the variations of optical spectral properties of mid - ir galaxies over a wide range of ir luminosities .
on the other hand , by next year , zcosmos - deep will already have produced one of the largest samples of optical spectra for galaxies at redshifts @xmath21 .
this will enable us to conduct an unprecedented study of the rest - frame uv spectral properties of ir galaxies and to compare them to the spectra of other known high-@xmath22 galaxy populations , as e.g. lyman - break galaxies .
we present here some of the first results we have obtained on the study of the optical spectra of _ spitzer / mips _ 24 @xmath0-selected galaxies in the cosmos field . 
 this is part of a series of studies we are conducting to analyse the optical spectral properties of mid - infrared ( mid - ir ) galaxies with different ir luminosities up to high redshifts . 
 the results shown here correspond to the brightest @xmath1 ir galaxy population at @xmath2 .    [ 
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