We present high-spatial-resolution ( \sim 0.12 ^ { \prime \prime } or \approx 800 { pc } at z = 4.5 ) ALMA 870 \mu m dust continuum observations of a sample of 44 ultrared dusty star-forming galaxies ( DSFGs ) selected from the H -ATLAS and Her MES far-infrared surveys because of their red colors from 250 to 500 \mu m : S _ { 500 } / S _ { 250 } > 1.5 and S _ { 500 } / S _ { 350 } > 1.0 .
With photometric redshifts in the range z \sim 4 –6 , our sample includes the most luminous starbursting systems in the early Universe known so far , with total obscured star-formation rates ( SFRs ) of up to \sim 4 , 500 M _ { \odot } { yr } ^ { -1 } , as well as a population of lensed , less intrinsically luminous sources .
The lower limit on the number of ultrared DSFGs at 870 \mu m ( with flux densities measured from the ALMA maps and thus not affected by source confusion ) derived in this work is in reasonable agreement with models of galaxy evolution , whereas there have been reports of conflicts at 500 \mu m ( where flux densities are derived from SPIRE ) .
Ultrared DSFGs have a variety of morphologies ( from relatively extended disks with smooth radial profiles , to compact sources , both isolated and interacting ) and an average size , \theta _ { FWHM } , of 1.46 \pm 0.41 { kpc } , considerably smaller than the values reported in previous work for less-luminous DSFGs at lower redshifts .
The size and the estimated gas-depletion times of our sources are compatible with their being the progenitors of the most massive , compact , red-and-dead galaxies at z \sim 2 –3 , and ultimately of local ultra-massive elliptical galaxies or massive galaxy clusters .
We are witnessing the birth of the high-mass tail of the red sequence of galaxies .