Dusty starburst galaxies in the early Universe as revealed by
  gravitational lensing
  In the past decade, our understanding of galaxy evolution has been
revolutionized by the discovery that luminous, dusty, starburst galaxies were
1,000 times more abundant in the early Universe than at present. It has,
however, been difficult to measure the complete redshift 2 distribution of
these objects, especially at the highest redshifts (z > 4). Here we report a
redshift survey at a wavelength of three millimeters, targeting carbon monoxide
line emission from the star-forming molecular gas in the direction of
extraordinarily bright millimetrewave-selected sources. High-resolution imaging
demonstrates that these sources are strongly gravitationally lensed by
foreground galaxies. We detect spectral lines in 23 out of 26 sources and
multiple lines in 12 of those 23 sources, from which we obtain robust,
unambiguous redshifts. At least 10 of the sources are found to lie at z > 4,
indicating that the fraction of dusty starburst galaxies at high redshifts is
greater than previously thought. Models of lens geometries in the sample
indicate that the background objects are ultra-luminous infrared galaxies,
powered by extreme bursts of star formation.
