  Galaxy clusters at high redshift are key targets for understanding matter
assembly in the early Universe, yet they are challenging to locate. A sample of
>2000 high-z candidate structures has been found using Planck's all-sky submm
maps, and a sub-set of 234 have been followed up with Herschel-SPIRE, which
showed that the emission can be attributed to large overdensities of dusty
star-forming galaxies. In order to resolve and characterise the individual
galaxies we targeted the eight brightest SPIRE sources in the centre of the
Planck peak PLCK G073.4-57.5 using ALMA at 1.3 mm, and complemented these
observations with data from IRAC, WIRCam J,K, and SCUBA-2. We detected a total
of 18 millimetre galaxies brighter than 0.3 mJy in 2.4 arcmin^2. The ALMA
source density is 8-30 times higher than average background estimates and
larger than seen in typical 'proto-cluster' fields. We were able to match all
but one of the ALMA sources to a NIR counterpart. The most significant (four)
SCUBA-2 sources are not included in the ALMA pointings, but we find an 8sigma
stacking detection of the ALMA sources in the SCUBA-2 map at 850 um. We derive
photo-z, L_IR, SFR, stellar mass, T_dust, M_dust for all of the ALMA galaxies;
the photo-zs identify two groups each of five sources, at z~1.5 and 2.4. The
two groups show two 'red sequences' (i.e. similar NIR [3.6]-[4.5] colours and
different J-K colours). The majority of the ALMA-detected galaxies are on the
SFR versus stellar mass main sequence, and half of the sample is more massive
than the characteristic stellar mass at the corresponding redshift.
Serendipitous CO line detections in two of the galaxies appear to match their
photometric redshifts at z~1.54. We performed an analysis of star-formation
efficiencies and CO- and mm-continuum-derived gas fractions of our ALMA
sources, combined with a sample of 1<z<3 cluster and proto-cluster members.
