The Luminosity - Metallicity Relation of distant luminous infrared galaxies ⋆

One hundred and five 15μm selected objects in three ISO (Infrared Space Observatory) deep survey fields (CFRS 3, UDSR and UDSF) are studied on the basis of their high quality optical spectra with resolution R>1000 from VLT/FORS2. Ninety two objects (88%) have secure redshifts, ranging from 0 to 1.16 with a median value of zmed=0.587. Considerable care is taken in estimating the extinction property of individual galaxy, which can seriously affect diagnostic diagrams and estimates of star formation rates (SFRs) and of metal abundances. Two independent estimates of the extinction have been made, e.g. Balmer line ratio and energy balance between infrared (IR) and Hβ luminosities. For most of the sources, we find a good agreement between the two extinction coefficients (within ±0.64 rms in AV , the extinction in V band), with median values of AV (IR) = 2.36 and AV (Balmer)= 1.82 for z>0.4 luminous IR galaxies (LIRGs). At z>0.4, our sample show many properties (IR luminosity, continuum color, ionization and extinction) strikingly in common with those of local (IRAS) LIRGs studied by Veilleux et al. (1995). Thus, our sample can provide a good representation of LIRGs in the distant Universe. We confirm that most (>77%) ISO 15μm selected galaxies are dominated by star formation. Oxygen abundances in interstellar medium in the galaxies are estimated from the extinction corrected “strong” emission line ratios (e.g. [O ii]/Hβ, [O iii]/Hβ and [O iii]/[O ii]). The derived 12+log(O/H) values range from 8.36 to 8.93 for the z>0.4 galaxies with a median value of 8.67. Distant LIRGs present a metal content less than half of that of the local bright disks (i.e. L). Their properties can be reproduced with infall models though one has to limit the infall time to avoid overproduction of metals at late time. The models predict that total masses (gas + stars) of the distant LIRGs are from 10M⊙ to ≤ 10 M⊙. A significant fraction of distant large disks are indeed LIRGs. Such massive disks could have formed ∼50% of their metals and stellar masses since z∼1.