4MOST - 4-metre Multi-Object Spectroscopic Telescope
4-m Multi-Object Spectroscopic Telescope
4MOST

Extragalactic Community Surveys

The nine Extragalactic Community Surveys target a broad range of extragalactic sources, ranging from galaxies in the local Universe to quasars at redshift 4 and beyond, and beautifully complement the science goals of the Consortium Surveys. Several provide vital spectroscopic followup of sources detected by other major new facilities, such as eROSITA galaxy clusters, strong lens systems discovered by the Euclid and Rubin observatories, radio continuum detections from the MeerKAT SKA pathfinder, and quasars detected from lack of proper motion by Gaia. Others investigate the detailed astrophysics of the baryon cycle in the circumgalactic medium, or the connection between stellar populations, gas properties and environment. All fully exploit the unique capabilities of 4MOST and its synergy with other leading southern hemisphere facilities to further our understanding of the extragalactic Universe.

Below we provide more details on each of the nine surveys (click on the survey titles).

CHANCES is designed to study the evolution of galaxies in and around 150 of the most massive galaxy clusters in the local Universe and out to z~0.4. It will provide legacy spectroscopic support for the eROSITA X-ray mission, complementing that from the 4MOST Galaxy Clusters Survey, by obtaining spectroscopic redshifts confirming membership for >1000 galaxies per cluster. CHANCES will provide comprehensive spectroscopic coverage of cluster galaxies both within and well beyond the virial radius, covering the surrounding infall regions out to 5r200 from the cluster. This will permit us to map the hierarchical assembly of the clusters in detail, and measure the importance of pre-processing, where galaxies are transformed within X-ray groups and filaments prior to their arrival into the clusters themselves. For local clusters (z < 0.07), CHANCES will push cluster galaxy evolution studies well into the dwarf galaxy regime (108-109 M) where the environment is expected to play a dominant role in their evolution. CHANCES will also probe the effect of clusters on the gaseous halos of member galaxies by obtaining spectra of QSOs behind z > 0.35 clusters to look for MgII absorption systems linked to the galaxies.

ChANGES aims to acquire 4MOST spectroscopy for a large, representative sample of AGN, selected primarily on optical variability and opt/NIR/MIR SEDs over ~18,000 deg2. We will also investigate the time domain by obtaining multiple visits for a random subset of AGN plus target-of-opportunity observations for strongly variable AGN and newly discovered Tidal Disruption Event hosts.

The Circumgalactic Medium (CGM) is central to our understanding of the baryon cycle, including the connection between gas, stars and metals. Absorption lines detected against background quasars offer a compelling way to study this CGM. Given that absorption measurements are limited to pencil-beams along the sightline, only a large sample of quasar absorbers will enable a major breakthrough. This survey aims at using the powerful synergy of absorption and emission diagnostics by observing a sizeable sample of background quasars and foreground galaxies in the same fields. It will cross-correlate 4MOST high-resolution observations of the quasars with low-resolution galaxy surveys (including WAVES and the Cosmology Redshift Survey) over the same area. It will allow us to measure the radial profile, covering fraction and optical depth of the neutral hydrogen and metals in their CGM. At high redshift, it will build on an unprecedented catalogue of Lyα absorbers (Damped Lyman-α absorvers and Lyman Limit systems), to measure their dust-free and ionization-corrected metallicity to re-appraise the missing metals problem. Together, these results will redefine our view of the CGM and provide a definitive census of the cosmic metals. This long-lasting legacy dataset will bring high-resolution quasar spectroscopy to a new level by providing ~250,000 quasar absorbers to be correlated with 1 million foreground galaxies, and a grand total of 1 million R=20,000 quasar spectra — a 3 orders of magnitude increase over currently available samples.

4HS will obtain spectra and redshifts for ~6 million galaxies with high and unbiased completeness across the full southern hemisphere, to define the benchmark reference sample of galaxies at z < 0.15. The result is a dataset with exceptional legacy value with wide applicability. As examples: 4HS will provide the z~0 anchor for evolutionary studies with WAVES; the critical field sample for cluster surveys like CHANCES; and the mass-limited comparison sample for HI or X-ray selected samples from WALLABY or eROSITA. In combination with other flagship surveys including VRO-LSST, VHS, WISE, Euclid, ASKAP/MeerKAT/SKA, 4HS will establish a transformative laboratory for studying the baryon cycle within and around galaxies as a function of galaxy/halo mass, as well as local/large-scale environment. At the same time, 4HS cosmology will measure the cosmic velocity field over ~1 Gpc from the peculiar velocities of ~500,000 massive early type galaxies. By mapping mass and motion on the largest possible scales, 4HS will reveal a complete picture of gravitational collapse and cosmic structure formation in the Local Volume.

Strong gravitational lensing is on the cusp of a revolution, with Euclid and LSST each forecast to discover over 100,000 strong lenses. Strong gravitational lenses are powerful probes of both galaxy formation and cosmology. However, without spectroscopic redshifts, only minimal science can be done despite the huge new samples that will be discovered in the current decade. Therefore, we propose to observe Euclid and LSST lens candidates with a source magnitude R < 24 and a source photometric redshift below zphot < 1.8, yielding 10,000 spectroscopic source redshifts. We also propose to obtain the velocity dispersion for the 5000 brightest of these lenses. These data will enable a range of legacy science, including: the stellar Initial Mass Function, constraining dark energy, testing general relativity, and highly magnified source studies.

4C3R2 will observe a subset of galaxies out to twice the redshift and down to half the stellar mass of the WAVES samples, representatively selected from ugriZYJHK colour-magnitude space over the WAVES survey regions. This will provide a complete calibration of the ugriZYJHK colour-redshift relation, augmented by the abundance, luminosity function, and halo occupation distribution at each point in colour-magnitude space from photometric and lensing data from KiDS/DES/Euclid/LSST/WFIRST. 4C3R2 provides a complete census of galaxies out to z~1.5, and 60% of the redshift calibration urgently required by future lensing surveys. It triples the photo-z impact of WAVES alone, at the unique combination of depth and volume enabled by 4MOST, complementary to the ongoing C3R2 VLT/Keck/LBT programs.

Galaxy spectra encode in their continuum shape and absorption/emission features a wealth of information on galaxy physics, mass build-up history and chemical enrichment history. 4MOST-StePS will collect deep, high-S/N spectra for a sample of ~3000 galaxies brighter than IAB~20.5 in the redshift range 0.3 < z < 0.8. The sample falls within the ~66 deg2 WAVES-Deep footprint and 4MOST-StePS will re-observe the same sources multiple times during repeated visits. Pushing beyond 4MOST’s “redshift machine” capabilities, our spectra — at a typical S/N per Å of ~30, far above all existing/ongoing surveys at intermediate redshifts — will provide a precise empirical description of the evolutionary path of massive galaxies in the as yet unexplored intermediate redshift range between LEGA-C and SDSS. The location of these galaxies within the cosmic web, unveiled in sharp detail by WAVES-Deep, will allow us to explore, for the first time at these redshifts, the connection between stellar population, gas properties and environment, down to scales of galaxy pairs.

Galaxy evolution is regulated by the continuous cycle of gas accretion, consumption and feedback. Crucial in this cycle is the availability of neutral atomic (HI) and molecular hydrogen. Our current inventory of HI, however, is limited to the local Universe (z < 0.25), resulting in a very incomplete picture. ORCHIDSS will address this critical challenge with a spectroscopic survey of ~200,000 radio continuum detected objects selected from the MeerKAT deep extragalactic surveys. Using the powerful combination of radio and 4MOST spectroscopy, ORCHIDSS will: i) provide the most reliable measurement of the evolution of the cosmic HI density and HI mass function since z=1.4; ii) measure the fundamental relations between HI and galaxy properties across a wide range of redshifts and environments for the first time; and iii) directly measure the effects of AGN and star-formation driven feedback on regulating the neutral gas reservoirs of galaxies across cosmic time. With a target selection based on activity (both star-formation and accretion), the resulting spectra will also offer significant legacy value for tackling a broad range of scientific challenges.

The main goals of this survey are to provide the first large-scale and systematic analysis of quasar feedback through broad absorption line outflows from redshift 0.8 to 4, and to quantify the effect of dust-bias in intervening absorption systems at 2 < z < 3. Both broad absorption line quasars and metal-strong intervening absorption systems are currently under-represented by optical color-selection methods for quasar identification. The cornerstone of our survey is therefore a color-unbiased approach where we select ~200,000 quasar candidates purely based on astrometry from Gaia with no assumption on redshift or spectral shape.