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We present high-spatial-resolution ($\sim 0.12''$ or $\approx 800 \, {\rmpc}$ at $z = 4.5$) ALMA $870\,\mu$m dust continuum observations of a sample of44 ultrared dusty star-forming galaxies (DSFGs) selected from the H-ATLAS andHerMES 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 photometricredshifts in the range $z \sim 4$-6, our sample includes the most luminousstarbursting systems in the early Universe known so far, with total obscuredstar-formation rates (SFRs) of up to $\sim 4,500 \, M_\odot \, {\rm yr}^{-1}$,as well as a population of lensed, less intrinsically luminous sources. Thelower limit on the number of ultrared DSFGs at 870 $\mu$m (with flux densitiesmeasured from the ALMA maps and thus not affected by source confusion) derivedin this work is in reasonable agreement with models of galaxy evolution,whereas there have been reports of conflicts at 500 $\mu$m (where fluxdensities are derived from SPIRE). Ultrared DSFGs have a variety ofmorphologies (from relatively extended disks with smooth radial profiles, tocompact sources, both isolated and interacting) and an average size,$\theta_{\rm FWHM}$, of $1.46 \pm 0.41\, {\rm kpc}$, considerably smaller thanthe values reported in previous work for less-luminous DSFGs at lowerredshifts. The size and the estimated gas-depletion times of our sources arecompatible with their being the progenitors of the most massive, compact,red-and-dead galaxies at $z \sim 2$-3, and ultimately of local ultra-massiveelliptical galaxies or massive galaxy clusters. We are witnessing the birth ofthe high-mass tail of the red sequence of galaxies.

Witnessing the birth of the red sequence: the physical scale and morphology of dust emission in hyper-luminous starbursts in the early Universe