Until recently , only a handful of dusty , star-forming galaxies ( DSFGs ) were known at z > 4 , most of them significantly amplified by gravitational lensing .
Here , we have increased the number of such DSFGs substantially , selecting galaxies from the uniquely wide 250- , 350- and 500- \mu m Herschel -ATLAS imaging survey on the basis of their extremely red far-infrared colors and faint 350- and 500- \mu m flux densities – ergo they are expected to be largely unlensed , luminous , rare and very distant .
The addition of ground-based continuum photometry at longer wavelengths from the James Clerk Maxwell Telescope ( JCMT ) and the Atacama Pathfinder Experiment ( APEX ) allows us to identify the dust peak in their spectral energy distributions ( SEDs ) , better constraining their redshifts .
We select the SED templates best able to determine photometric redshifts using a sample of 69 high-redshift , lensed DSFGs , then perform checks to assess the impact of the CMB on our technique , and to quantify the systematic uncertainty associated with our photometric redshifts , \sigma = 0.14 ( 1 + z ) , using a sample of 25 galaxies with spectroscopic redshifts , each consistent with our color selection .
For Herschel -selected ultrared galaxies with typical colors of S _ { 500 } / S _ { 250 } \sim 2.2 and S _ { 500 } / S _ { 350 } \sim 1.3 and flux densities , S _ { 500 } \sim 50 mJy , we determine a median redshift , \hat { z } _ { phot } = 3.66 , an interquartile redshift range , 3.30–4.27 , with a median rest-frame 8–1000- \mu m luminosity , \hat { L } _ { IR } , of 1.3 \times 10 ^ { 13 } L _ { \odot } .
A third lie at z > 4 , suggesting a space density , \rho _ { z > 4 } , of \approx 6 \times 10 ^ { -7 } Mpc ^ { -3 } .
Our sample contains the most luminous known star-forming galaxies , and the most over-dense cluster of starbursting proto-ellipticals yet found .