Hα and 4000 Å Break Measurements for ~3500 K-selected Galaxies at 0.5 < z < 2.0

Abstract

We measure spectral features of ~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—Hα measurements for a large magnitude-limited sample of distant galaxies. The line widths indicate a photometric redshift uncertainty of Δz < 0.02 × (1 + z). The composite SEDs also show the Balmer and 4000 Å breaks, Mg II absorption at ~2800 Å, the dust absorption feature at 2175 Å, and blended [O III]+Hβ emission. We compare the total equivalent width of Hα, [N II], and [S II] (W _{Hα +}) with the strength of the 4000 Å 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 W _{Hα +} 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 W _{Hα +} and D(4000) at 0.5 < z < 2.0 is similar to z ~ 0 and that the suppression of star formation in galaxies at z < 2 is generally not abrupt, but a gradual process.