The Luminosity-Metallicity relation of distant luminous infrared galaxies

One hundred and five 15 \mum-selected objects in three ISO (Infrared Space Observatory) deep survey fields (CFRS 3^h, UDSR and UDSF) are studied on the basis of their high-quality optical spectra with resolution R>1000 from VLT/FORS2. ∼92 objects (88%) have secure redshifts, ranging from 0 to 1.16 with a median value of z_med=0.587.

Considerable care is taken in estimating the extinction properties 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 H\beta luminosities. For most of the sources, we find a good agreement between the two extinction coefficients (within ±0.64 rms in A_V, the extinction in V band), with median values of A_V(IR) = 2.36 and A_V(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. (\cite{Veilleux1995}). Thus, our sample can provide a good representation of LIRGs in the distant Universe.

We confirm that most (>77%) ISO 15 μm-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. \ion{[O}{ii]}/Hβ, \ion{[O}{iii]}/Hβ and \ion{[O}{iii]}/\ion{[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 although one has to limit the infall time to avoid overproduction of metals at late times. The models predict that total masses (gas + stars) of the distant LIRGs are from 10¹¹ M_⊙ to ≤10¹² 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.

Based on observations collected with the ESO Very Large Telescope at the Paranal Observatory (66.A-0599(A)) and with ISOCAM Deep Survey and ISO-CFRS follow-up.

Table \ref{tab1}-\ref{tab7} are only available in electronic form at http://www.edpsciences.org