Groups and clusters of galaxies are a current focus of astronomical research owing to their role in determining the environmental effects on galaxies and their constraining power in cosmology. The eROSITA X-ray telescope on board the Spectrum Roentgen Gamma observatory will be launched in 2019 and will have completed 8 scans of the full sky by the start of 4MOST operations. The experiment will detect groups and clusters of galaxies through the X-ray emission of the hot intergalactic medium. The purpose of the 4MOST eROSITA Galaxy Clusters Survey is to provide the spectroscopic redshifts of the optical counterparts to the X-ray emission coming from 40,000 groups and clusters of galaxies, to perform dynamical estimates of the total mass and to enable exploration of properties of the member galaxies. At redshift z > 0.7 the main goal of the survey is the precise redshift measurement for the photometrically identified brightest cluster galaxies. At lower redshift (z < 0.7) the program aims to sample over 15 member galaxies per cluster and to enable dynamical mass measurements to calibrate the clusters for the cosmological experiments. At z < 0.2, eROSITA will also detect X-ray emission from galaxy groups and filaments. Their optical identification requires spectroscopic data and this 4MOST survey is designed to statistically enable spectroscopic identification of galaxy groups down to the mass limits of group detection by eROSITA of 10¹³ M_⊙, and the detection of the largest filaments for pioneering studies of their X-ray emission.

X-ray and mid-IR emission are signposts of accretion of matter onto the supermassive black holes (SMBHs) that reside at the centres of most galaxies. As a major step towards our understanding of accreting SMBHs and their role in the evolution of galaxies, we will use 4MOST to provide a highly complete census of Active Galactic Nuclei (AGN) over a large fraction of the eROSITA extragalactic sky visible to 4MOST. We will systematically follow-up all eROSITA point-like extragalactic X-ray sources (mostly AGN), and complement them with a heavily obscured AGN selection approach using WISE mid-IR data. The X-ray and mid-IR flux limits of eROSITA and WISE are well matched to the spectroscopic capabilities of a 4-meter class telescope, allowing us to reach completeness levels of ~80-90% for all X-ray selected AGN with fluxes f_0.5-2 keV > 10^-14 erg/s/cm², about a factor of 30 deeper than the ROSAT all-sky survey. With these data we will determine the physical properties (redshift, luminosity, line emission strength, masses, etc.) of up to one million supermassive black holes, constrain their cosmic evolution and clustering properties, and explore the connection between AGN and large scale structure from z ~ 0 up to z ~ 6.

WAVES is designed to study the growth of structure, mass and energy on scales of ~1 kpc to ~10 Mpc over a 7 Gyr timeline. On the largest length scales (1 – 10 Mpc) WAVES will measure the structures defined by groups, filaments and voids, and their emergence over recent times. Comparisons to bespoke numerical simulations will be used to confirm, refine or refute the Cold Dark Matter (CDM) paradigm. At intermediate length scales (10 kpc – 1 Mpc) WAVES will probe the size and mass distribution of galaxy groups, as well as the galaxy merger rates, in order to directly measure the assembly of dark matter halos and stellar mass. On the smallest length scales (1 – 10 kpc) WAVES will provide accurate distance and environmental measurements to complement high-resolution space-based imaging to study the mass and size evolution of galaxy bulges, discs and bars. In total, WAVES will provide a panchromatic legacy dataset of ~1.6 million galaxies, firmly linking the very low (z < 0.1) and intermediate (z ≈ 0.8) redshift Universe.

This survey will perform stringent cosmological tests via spectroscopic clustering measurements that will complement the best lensing, cosmic microwave background (CMB) and other surveys in the southern hemisphere. The combination of carefully selected samples of bright galaxies, luminous red galaxies, emission line galaxies, and quasars, totalling ~8 million objects over the redshift range 0.15 to 3.5, will allow definitive tests of gravitational physics. Many key science questions will be addressed by combining CRS spectra of these targets with data from current or future facilities such as LSST, Euclid and SKA.

TiDES is focused on the spectroscopic follow-up of extragalactic optical transients and variable sources selected from forthcoming large sky surveys such as that from the Large Synoptic Survey Telescope (LSST). TiDES contains three sub-surveys: (i) spectroscopic observations of supernova-like transients, (ii) comprehensive follow-up of transient host galaxies to obtain redshift measurements for cosmological applications, and (iii) repeat spectroscopic observations to enable the reverberation mapping of Active Galactic Nuclei (AGN) and supermassive black hole binary candidates. Our simulations predict we will be able to classify transients down to r = 22.5 mag (AB), and, over five years of 4MOST operations, obtain spectra for up to 30,000 live transients to redshift z ~ 0.5, measure redshifts for up to 50,000 transient host galaxies to redshift z ~ 1, and monitor up to 1,000 AGN, including binary candidates, to redshift z ~ 2.5.