ALMA Deep Field in SSA22: A concentration of dusty starbursts in a
  z=3.09 protocluster core
  We report the results of $1^{\prime}.5 \times3^{\prime}$ mapping at 1.1~mm
with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the central
region of the $z=3.09$ SSA22 protocluster. By combining our source catalog with
archival spectroscopic redshifts, we find that eight submillimeter galaxies
(SMGs) with flux densities, $S_{\rm 1.1~mm}=0.7-6.4$~mJy ($L_{\rm
IR}\sim10^{12.1}-10^{13.1}L_\odot$) are at $z=3.08-3.10$. Not only are these
SMGs members of the protocluster but they in fact reside within the node at the
junction of the 50 Mpc-scale filamentary three-dimensional structure traced by
Lyman-$\alpha$ emitters (LAEs) in this field. The eight SMGs account for a star
formation rate density (SFRD) $\sim$10 $M_\odot$ yr$^{-1}$ Mpc$^{-3}$ in the
node, which is two orders of magnitudes higher than the global SFRD at this
redshift. We find that four of the eight SMGs host a X-ray luminous active
galactic nuclei (AGN). Our results suggest that the vigorous star formation
activity and the growth of super massive black holes (SMBHs) occurred
simultaneously in the densest regions at $z\sim3$, which may correspond to the
most active historical phase of the massive galaxy population found in the core
of the clusters in the present universe. Two SMGs are associated with
Lyman-$\alpha$ blobs (LABs), implying that the two populations coexist in high
density environments for a few cases.
