Hα AND 4000 Å BREAK MEASUREMENTS FOR ∼3500 K-SELECTED GALAXIES AT 0.5 < z < 2.0

We measure spectral features of similar to 3500 K-selected galaxies at 0.5 < z < 2.0 from high-quality medium-band photometry using a new technique. First, we divide the galaxy sample in 32 subsamples based on the similarities between the full spectral energy distributions (SEDs) of the galaxies. For each of these 32 galaxy types we construct a composite SED by de-redshifting and scaling the observed photometry. This approach increases the signal-to-noise ratio and sampling of galaxy SEDs and allows for model-independent stellar population studies. The composite SEDs are of spectroscopic quality and facilitate-for the first time-H alpha measurements for a large magnitude-limited sample of distant galaxies. The line widths indicate a photometric redshift uncertainty of Delta z < 0.02 x (1 + z). The composite SEDs also show the Balmer and 4000 angstrom breaks, Mg II absorption at similar to 2800 angstrom, the dust absorption feature at 2175 angstrom, and blended [O III]+H beta emission. We compare the total equivalent width of H alpha, [N II], and [S II] (WH alpha+) with the strength of the 4000 angstrom break (D(4000)) and the best-fit specific star formation rate, and find that all these properties are strongly correlated. This is a reassuring result, as currently most distant stellar population studies are based on just continuum emission. Furthermore, the relation between WH alpha+ and D(4000) provides interesting clues to the star formation histories of galaxies, as these features are sensitive to different stellar ages. We find that the correlation between WH alpha+ and D(4000) at 0.5 < z < 2.0 is similar to z similar to 0 and that the suppression of star formation in galaxies at z < 2 is generally not abrupt, but a gradual process.