Halpha and 4000 Angstrom Break Measurements for ~3500 K-selected
  Galaxies at 0.5<z<2.0
  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 -- Halpha measurement for a large magnitude-limited sample of
distant galaxies. The linewidths indicate a photometric redshift uncertainty of
dz<0.02x(1+z). The composite SEDs also show the Balmer and 4000 Angstrom
breaks, MgII absorption at ~2800 Angstrom, the dust absorption feature at 2175
Angstrom, and blended [OIII]+Hbeta emission. We compare the total equivalent
width of Halpha, [NII], and [SII] (W_Halpha+) 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 W_Halpha+ and Dn(4000)
provides interesting clues to the SFHs of galaxies, as these features are
sensitive to different stellar ages. We find that the correlation between
W_Halpha+ 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.
