  The Planck satellite has identified more than 2000 protocluster candidates
with extreme star formation rates (SFRs). Here, we present the spectroscopic
identification of a Planck-selected protocluster located in the Cosmos field,
PHz G237.01+42.50 (G237). G237 contains a galaxy overdensity of 31
spectroscopically identified galaxies at z~2.16 (significant at 5.4 sigma) in a
10'x11' region. The overdensity contains two substructures or protoclusters at
<z>~2.16 and 2.195 with estimated halo masses at z=0 of ~(5-6)x10^14 Msun. The
overdensity total SFR, ~4000 Msun/yr, is higher than predicted by simulations
but much smaller than the SFR derived from the Planck data. The analysis of the
Herschel data, in combination with the available ancillary data, shows that
such a difference is due to an effect of source alignment along the line of
sight that produces a 5 sigma overdensity of red Herschel sources in the field.
We analyze the members' UV spectra and UV-far-infrared spectral energy
distributions to derive their SFR, stellar mass, and metallicity. Galaxy
members include blue star-forming galaxies and AGN with SFRs and stellar masses
consistent with the main sequence. AGN, identified through optical spectroscopy
or X-ray data, represent a significant fraction (20+/-10%) of all members of
the protocluster at z=2.16, and they are powerful enough to produce radiative
feedback. The core of this protocluster, besides being denser, includes members
that are, on average, more massive and star-forming and contains a larger
fraction of AGN and Herschel-detected galaxies than the full sample, suggesting
an environmental effect on galaxy growth. A comparison between G237 and other
protoclusters in the literature at similar redshifts reveals some common traits
and differences that reflect both observational biases and a diversity in
intrinsic properties that is not yet fully understood.
