Obtaining good stellar parameters or even chemical element abundances with Gaia is limited to much brighter objects and here 4MOST will provide the complimentary measurements in the form of radial velocities with ~1km/s accuracy to the depth of Gaia’s astrometric limits and detailed abundances to mV~16 mag for millions of stars. These kind of measurements allows one to determine the three-dimensional Galactic potential, measure the influence of the Milky Way’s bar and spiral arms on disk dynamics, determine the Galactic assembly history through chemo-dynamical substructure and abundance pattern tagging, and find 1000s of extremely metal-poor stars to constrain earliest galaxy formation and stellar evolution.
Large area surveys of faint Galactic stellar objects will enable us to elucidate the formation history of the Milky Way. Models of hierarchical galaxy formation predict large amounts of dynamical substructure in the Milky Way halo that 4MOST can detect through measuring radial velocities of Red Giant Stars. With the same data set we will be able to determine the three-dimensional Galactic potential and constrain the mass power spectrum of the dark matter halos by measuring the kinematics of the streams of stars that are the remnants of tidally disrupted dwarf galaxies surrounding the Milky Way.
A comparison between a range of dynamical models and the observed velocity field for an unprecedentedly large disk area will allow us, for the first time, to unambiguously quantify the bar and spiral structure dynamical parameters, as well as the disk merger history. Loss of information due to stellar diffusion processes will be recovered by accurate metallicity and a-element measurements, thus allowing us to distinguish substructure both dynamically and chemically.
The Gaia satellite will provide distances from parallaxes and space kinematics from proper motions for more than one billion Milky Way stars down to mV ~ 20 mag. Gaia will also provide radial velocities and astrophysical characterisation for about 150 million stars, but its sensitivity is limited to mV ~ 12–16 mag, strongly dependent on stellar spectral type, because its spectrograph only covers the Ca II-triplet region at 847–874 nm, as shown by the figure below of the Gaia end-of-live predicted radial velocity accuracy.
By covering the full optical wavelength region, the 4MOST instrument complements Gaia where it lacks spectroscopic capabilities, so that full 6D-space coordinate information can be obtained and objects throughout the Milky Way chemically characterised. 4MOST sensitivity will be such that large-area surveys of faint Galactic stellar objects will enable us to elucidate the formation history of all components of the Milky Way as indicated by the figure to the right.
In this Hertzsprung–Russell diagram we show limiting distances for radial velocity measurements with Gaia (maroon diagonal) and 4MOST (black horizontal). 4MOST can measure Sun-like stars to nearly the centre of the Milky Way, RGB stars to 100 kpc, and massive stars throughout the Local Group, thereby substantially expanding on Gaia’s spectroscopic view (volume increase a factor thousand in the red to 1 million in the blue). Distance limits for the 4MOST high-resolution spectroscopy are about four times smaller.
Models of hierarchical galaxy formation predict large amounts of dynamical substructure in the Milky Way halo that 4MOST can detect through measuring red giant branch (RGB) stars. Furthermore, with 4MOST we will determine the three-dimensional Galactic potential and its substructure, discern the dynamical structure of the Milky Way disc and measure the influence of its bar and spiral arms, measure the Galactic assembly history through chemo-dynamical substructure and abundance pattern labelling, and find thousands of extremely metal-poor stars to constrain early galaxy formation and the nature of the first stellar generations in the Universe.
For more information about the Gaia project, please click on Gaia-Homepage (external website).
Extragalactic Science and Cosmology
eROSITA will perform all-sky X-ray surveys in the years 2017 to 2021 to a limiting depth that is a factor of 30 deeper than the ROSAT all-sky survey, with broader energy coverage, better spectral resolution and better spatial resolution. We will use 4MOST to survey the >50,000 southern X-ray galaxy clusters that will be discovered by eROSITA, measuring 3–30 galaxies in each cluster. These galaxy cluster measurements will determine the evolution of galaxy populations in clusters, yield the cluster mass evolution, and provide highly competitive constraints on the evolution of dark energy. 4MOST will enable us to determine the nature of >1 million active galactic nuclei, thus constraining the cosmic evolution of active galaxies to z = 5. With 4MOST we will characterise several hundreds of thousands of dynamo and accretion-powered Galactic X-ray emitters, thereby uncovering the active Milky Way and constraining evolutionary channels of stellar populations.
Constraining the origin of the accelerating universe is expected to be a significant driver of the observations that are going to be done with 4MOST. Depending on the point of view, the accelerating universe can be interpreted as a form of dark energy or as modified gravity. 4MOST will provide constraints on the models by measuring the cosmic expansion history and the growth of structure using several different probes:
- Baryonic Acoustic Oscillations (BAO) and Redshift Space Distortions (RSD): By carrying out a large redshift survey (>10 million galaxies) across a large area of the southern sky (>12,000 deg2) 4MOST will measure the rate of expansion and structure growth of the universe.
- Weak Lensing: Weak lensing studies being carried out by imaging surveys like KiDS, DES, LSST and Euclid will be supported by providing large spectroscopic redshifts samples of galaxies to calibrate their photometric redshift techniques. Furthermore, by performing a redshift survey in the same area one can constrain the intrinsic alignment of galaxies that biases the weak lensing measurements.
- Galaxy Clusters: As a highly biased population, galaxy clusters provide a strong constraint on the growth rate of structure through measurements of the evolution of the galaxy cluster mass function. In addition to a redshift, 4MOST will provide velocity dispersions of a large fraction of the detected clusters to provide an independent cluster mass calibration.
- Type Ia supernovae (SNe Ia): By using large numbers of standardized candles in the form of SNe Ia strong constraints can be obtained on the expansion rate of the universe. Several tens of LSST transients can be followed up per 4MOST pointing, resulting in >25,000 active transients followed up per year.
For more information please consult the 4MOST Publications.