A dominant population of optically invisible massive galaxies in the
  early Universe
  Our current knowledge of cosmic star-formation history during the first two
billion years (corresponding to redshift z >3) is mainly based on galaxies
identified in rest-frame ultraviolet light. However, this population of
galaxies is known to under-represent the most massive galaxies, which have rich
dust content and/or old stellar populations. This raises the questions of the
true abundance of massive galaxies and the star-formation-rate density in the
early universe. Although several massive galaxies that are invisible in the
ultraviolet have recently been confirmed at early epochs, most of them are
extreme starbursts with star-formation rates exceeding 1000 solar masses per
year, suggesting that they are unlikely to represent the bulk population of
massive galaxies. Here we report submillimeter (wavelength 870um) detections of
39 massive star-forming galaxies at z > 3, which are unseen in the spectral
region from the deepest ultraviolet to the near-infrared. With a space density
of about $2 \times 10^{-5}$ per cubic megaparsec (two orders of magnitudes
higher than extreme starbursts) and star-formation rates of 200 solar masses
per year, these galaxies represent the bulk population of massive galaxies that
have been missed from previous surveys. They contribute a total
star-formation-rate density ten times larger than that of equivalently massive
ultraviolet-bright galaxies at z >3. Residing in the most massive dark matter
halos at their redshifts, they are probably the progenitors of the largest
present-day galaxies in massive groups and clusters. Such a high abundance of
massive and dusty galaxies in the early universe challenges our understanding
of massive-galaxy formation.
