A Coherent Study of Emission Lines from Broad-Band Photometry: Specific
  Star-Formation Rates and [OIII]/H{\beta} Ratio at 3 < z < 6
  We measure the H{\alpha} and [OIII] emission line properties as well as
specific star-formation rates (sSFR) of spectroscopically confirmed 3<z<6
galaxies in COSMOS from their observed colors vs. 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 H{\alpha} equivalent-widths (EW)
to obtain the sSFR out to z~6 without stellar mass fitting. We find a strongly
increasing rest-frame H{\alpha} EW that is flattening off above z~2.5 with
average EWs of 300-600A at z~6. The sSFR is increasing proportional to
(1+z)^2.4 at z<2.2 and (1+z)^1.5 at higher redshifts, indicative of a fast mass
build-up in high-z galaxies within e-folding times of 100-200Myr at z~6. The
redshift evolution at z>3 cannot be fully explained in a picture of cold
accretion driven growth. We find a progressively increasing
[OIII]{\lambda}5007/H{\beta} ratio out to z~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.
