4MOST Consortium Survey 5: eROSITA Galaxy Cluster Redshift Survey
  Groups and clusters of galaxies are a current focus of astronomical research
owing to their role in determining the environmental effects on galaxies and
the constraints they provide to cosmology. The eROSITA X-ray telescope on board
the Spectrum Roentgen Gamma observatory will be launched in 2019 and will have
completed eight scans of the full sky when 4MOST starts operating. The
experiment will detect groups and clusters of galaxies through X-ray emission
from the hot intergalactic medium. The purpose of the 4MOST eROSITA Galaxy
Cluster Redshift Survey is to provide spectroscopic redshifts of the optical
counterparts to the X-ray emission from 40,000 groups and clusters of galaxies
so as to perform dynamical estimates of the total mass and to measure the
properties of the member galaxies. The survey aims to obtain precise redshift
measurements of the photometrically identified brightest cluster galaxies at
redshift $z > 0.7$. At lower redshifts ($z < 0.7$), the programme aims to
sample over 15 member galaxies per cluster and enable dynamical mass
measurements to calibrate the clusters for cosmological experiments. At $z <
0.2$, eROSITA will also detect X-ray emission from galaxy groups and filaments.
4MOST spectroscopic data from the survey will be used for optical
identification of galaxy groups down to eROSITA's mass detection limits of
$10^{13} M_\odot$, as well as the detection of the largest filaments for
pioneering studies of their X-ray emission.
