GOODS-ALMA: Optically dark ALMA galaxies shed light on a cluster in
  formation at z = 3.5
  In this paper we study the properties of the six optically dark galaxies
detected in the 69 arcmin^2 GOODS-ALMA 1.1mm continuum survey. While none of
them are listed in the deepest H-band based CANDELS catalog in the GOODS-South
field down to H=28.16AB, we were able to de-blend two of them from their bright
neighbor and measure an $H$-band flux for them. We note that AGS4 and AGS15
have H=25.23, 27.11AB respectively. Their extreme proximity (0.50", 0.27") to a
bright optical source and their extreme faintness prevented them from being
included in the H-band catalog. We present the spectroscopic scan follow-up of
five of the six sources with ALMA band 4. All are detected in the 2mm continuum
with signal-to-noise ratios higher than eight. One emission line is detected in
AGS4 (\nu_{obs} =151.44GHz with a S/N=8.58) and AGS17 (\nu_{obs} =154.78GHz
with a S/N=10.23), which we interpret in both cases as being due to the CO(6-5)
line at z^{AGS4}_{spec}=3.556 and z^{AGS4}_{spec}=3.467, respectively. These
redshifts match both the probability distribution of the photometric redshifts
derived from the UV to near-infrared spectral energy distributions (SEDs) and
the far-infrared SEDs for typical dust temperatures of galaxies at these
redshifts. We present evidence that nearly 70% (4/6 of galaxies) of the
optically dark galaxies belong to the same overdensity of galaxies at z~3.5.
overdensity The most massive one, AGS24 (M_{\star} = 10^{11.32^{+0.02}_{-0.19}}
M_{\odot}), is the most massive galaxy without an active galactic nucleus (AGN)
at $z$\,>\,3 in the GOODS-ALMA field. It falls in the very center of the peak
of the galaxy surface density, which suggests that the surrounding overdensity
is a proto-cluster in the process of virialization and that AGS24 is the
candidate progenitor of the future brightest cluster galaxy (BCG).
