A COHERENT STUDY OF EMISSION LINES FROM BROADBAND PHOTOMETRY: SPECIFIC STAR FORMATION RATES AND [O III]/H beta RATIO AT 3 \textless z \textless 6

International audience; We measure the H alpha and [O III] emission line properties as well as specific star formation rates (sSFRs) of spectroscopically confirmed 3 \textless z \textless 6 galaxies in COSMOS from their observed colors versus redshift evolution. Our model describes consistently the ensemble of galaxies including intrinsic properties (age, metallicity, star formation history), dust attenuation, and optical emission lines. We forward-model the measured Ha. equivalent widths (EW) to obtain the sSFR out to z similar to 6 without stellar mass fitting. We find a strongly increasing rest-frame Ha. EW that is flattening off above z similar to 2.5 with average EWs of 300-600 angstrom at z similar to 6. The sSFR is increasing proportionally to (1 + z)(2.4) at z \textless 2.2 and to (1 + z)(1.5) at higher redshifts, indicative of a fast build-up of mass in high-z galaxies within e-folding times of 100-200 Myr at z similar to 6. The redshift evolution at z \textgreater 3 cannot be fully explained in a picture of growth driven by cold accretion. We find a progressively increasing [O III]lambda 5007/H beta ratio out to z similar to 6, consistent with the ratios in local galaxies selected by increasing H alpha EW (i.e., sSFR). This demonstrates the potential of using “local high-z analogs” to investigate the spectroscopic properties and relations of galaxies in the re-ionization epoch.