A unique instrument for the biggest telescope


The warm and hot gas between galaxies and within their halo is a reservoir of matter from which proto-galaxies can form. MOSAIC will provide an unprecedented map of the distant 3D structures of this gas as well as evaluating for the first time the distribution of the different baryonic components of the matter.

Detailed simulations show that MOSAIC will play an important role in the mapping of the intergalactic medium. An ambitious galaxy survey with MOSAIC will provide a 3D map of the IGM at z > 3, complementing similar surveys that will focus on lower redshifts. In synergy with the missions like Euclid and JWST, MOSAIC will enable us to probe the full redshift evolution of galaxy growth in the cosmic web throughout the cosmic period of intense star formation.


The oldest and lowest metallicity stars that exist today carry the imprint of the first massive stars that ended their lives as supernovae. We can constrain star formation in very metal-poor environments by analysing the metallicity distribution functions (MDFs) of stellar populations in the local Universe. Specifically, we are interested in probing the MDFs in a variety of environments, from the outer stellar halo in the Galaxy to the bulge, as well as in other galaxies in the Local Group (mostly dwarf spheroidals and ultra-diffuse dwarfs).

The characterization of primordial stars in the Galaxy has great scientific potential. These stars are the long-lived descendants from the earliest stellar generations and will have formed from a (near-)pristine ISM, which would have only been weakly enriched in metals from the first supernovae. Their atmospheres therefore give us a fossil record of the ISM from which they were formed, corresponding to redshifts of z ≥ 10. Having a direct tracer of chemical abundances at such an early time can provide fundamental constraints on the properties of the first generations of stars. The limit of the current observations do not allow to discriminate between different theoretical models on e.g. the need and the level of a “critical metallicity” for the formation of low mass stars. MOSAIC will allow to greatly increase the constraints on the low-metallicity tail of the MDF which would result in stronger constraints on the formation models.

The stellar populations of the Galactic bulge are a template for studies of ellipticals and bulges of spirals. The formation history of the bulge can give hints on proto-galaxy counterparts observed at high-redshift, providing strong motivation for the detailed study of this component of our Galaxy. The formation of our bulge is still a controversial issue, and it is probably a combination of a pseudo-bulge population, and an old, spheroidal, true bulge. Studies of the chemical compositions and kinematics of its stellar populations will be the key to disentangle its formation mechanism(s). Current high-spectral resolution observations in the optical (e.g. with VLT/FLAMES) are limited to giant stars, but observations of dwarfs will be important to disentangle the complex mix of stellar populations in the bulge. Currently, few dwarfs have been studied via microlensing techniques, but with MOSAIC a significantly larger population of dwarf will be accessible in the Galactic bulge.

As for extragalactic studies, the current limiting factor is that only giant stars are bright enough to have high quality, high-resolution spectroscopy with an 8-m class telescope. Unfortunately, the statistics available from the analysis of extragalactic red giant branch (RGB) stars is not sufficient to determine the metal-poor tail of the MDF robustly in their host galaxies. There are simply not enough giant-branch stars in most of the LG dwarf galaxies to sample these rare populations and to observe these faint stars in extragalactic systems we need the sensitivity of the E-ELT. One then needs large samples of stars at the main-sequence turn-off (MSTO), in multiple nearby galaxies.


Resolved stellar populations enable us to explore the star-formation and chemical-enrichment histories of galaxies, providing direct constraints on galaxy formation and evolution models. Current facilities limit precise chemical abundances and stellar kinematics measurements to the Local Group. However, to fully assess the diversity of galaxy populations we need to move to a broader range of galaxies in the Local Volume, from the edge of the Local Group out to Mpc distances.

Metallicities studies of large number of stars in the outer halo of external galaxies (e.g. the Sculptor Group) would provide an extraordinary tool to characterize their past star formation and evolution history. The CaT will likely be the most used metallicity diagnostic similarly to what done with Gaia, but also the Mg I b triplet or the G-band could be valid alternatives, but it would require to go down at least to 0.41 mm. For basic metallicities measurements a R³5000 should be sufficient, given an adequate S/N (³20).

A powerful tool to reconstruct the formation of a given stellar system consist in determining the evolution of the chemical enrichment of that system. In order to obtain it once needs to measure chemical abundance of many elements (alpha-elements, s- and r- process, etc…). The goal is to perform a detailed chemical tagging (i.e., identifying peaks in the abundances space), which would require accessing to as many elements as possible, which translates into a wide spectral coverage from the optical to the NIR (Bland-Hawthorn et al. 2010, ApJ, 721, 582, Ting et al. 2012, MNRAS, 421, 1231, Hogg et al 2016, ApJ, 833, 262). For example,  the 0.8-1.8 μm range can provide direct abundance estimates of elements such as iron-peak (Fe, Mn, Cr, Co, Ni), α-group (Mg, Si, Ti, Ca), other light elements (atomic C, S, Na, Al, K) and Sr (an s-process element). These are critical diagnostics to reconstruct the chemical-enrichment history of stellar systems, in both dwarf and red giant/supergiant stars (e.g. Allende-Prieto et al. 2008). In particular, the simultaneous observation of CO, CN and OH lines in the H-band can provide robust abundance estimates of C, N and O in cool stars.

In addition, a high spectral resolution will be required to enable abundaces measurement using lines that at lower resolution would be blended. High-resolution spectroscopic surveys as APOGEE (IR, R~20,000) and GALAH (optical, R=28,000) have enabled the analysis of over 15 elements. Recent simulations for MOONS of quantitative analysis in the H-band, using the latest model-atmosphere-fitting techniques, have concluded that R~18,000 is sufficient for the global abundance precision required for each species (with R ≥ 20,000 still preferable/desirable).

With current and upcoming instrumentation at 8-10m telescopes, precise chemical tagging is limited to stars on the upper red-giant branch in the Magellanic Clouds, and to even more luminous red super-giants or stars on the asymptotic giant branch in Local Group galaxies. MOSAIC at the ELT would significantly extend these studies to larger distances and less luminous stars.

In the present Universe, the Inter-Galactic Medium (IGM) is fully ionized and maintained so from the integrated ultraviolet emission from stars and Active Galactic Nuclei (AGN). Some 380,000 years after the Big Bang the temperature of the Universe was low enough that the IGM was neutral. Exactly when and how the Universe has been re-ionised is still unknown and a debated topic in modern astrophysics. From the Planck 2018 results (Aghanim et al. 2020) we have indication that the IGM may have been already half-ionized by z~9, and the spectra of high redshift quasars suggest that the re-ionization process was concluded already around z~6 (e.g. Fan et al. 2006).

The identification of the main ionizing sources has been elusive until know due to their faintness. Thanks to its larger collecting area, the ELT will allow us to push the search of these sources to much fainter magnitudes and higher redshifts, compared to the currently achieved with 8-10 m telescopes. The overarching goal is to derive a precise characterization of the ionisation state of the IGM during the first Gyr of the life of the Universe, to construct the timeline and topology of reionisation, and to observe the formation and growth of the first galaxies.

Very deep optical-to-near-IR spectroscopy to probe the UV rest-frame emission of galaxies in the early Universe is crucial to study these questions. Some of the fundamental questions and hot topics are:

  • Measurements of Ly-alpha emission in galaxies (i.e., fraction of Ly-alpha emitters in the Lyman break galaxy population, evolution of the Ly-alpha luminosity function with redshift, Ly-alpha equivalent width measurements and information from the detailed Ly-alpha line profile, see Dijkstra et al. 2013, Mesinger et al. 2015, Mason et al. 2018) provide crucial information on the ionization state of the IGM;
  • Overdensities of Ly-alpha emitters at z~7-8 (Castellano et al. 2016, Tilvi et al. 2020) providing the first evidence for overlapping reionized bubbles in the IGM, i.e. a hint of topology. Also, the detailed Ly-alpha line profile of z>>6 galaxies can constrain the properties of reionized bubbles in the early Universe (Haiman 2002, Mason & Gronke 2020);
  • One of key quantities to establish if galaxies reionized the Universe, is the measurement of escape fraction of Lyman continuum photons, fesc, which relies on measurements of the Ly-alpha line profile (121.6 nm), the MgII doublet line (280.7,280.9 nm), and UV low ionization absorption lines (e.g. SiII 126.0 nm) (Izotov et al. 2018, Verhamme et al. 2017, Reddy et al. 2018, Gazagnes et al. 2018, Chisholm et al. 2018, 2020). In contrast, no reliable method has yet been found to determine fesc from rest-frame optical spectra (see e.g. Plat et al. 2019, Wang et al. 2020);
  • UV absorption lines and nebular emission lines also provide fundamental information on the ISM of high-z galaxies, such as the geometry and properties of the gas (outflows, the presence of channels allowing LyC photon escape, gas density, and others), chemical abundances (C, O, and Si, e.g);
  • Spectroscopy of distant galaxies with 8-10m class telescopes reveal UV emission lines that require moderately metal-poor gas and a harder radiation field than is seen in galaxies at lower redshift (cf. review by Stark 2016). Several z>6 galaxies show the presence of CIV 154.8nm emission, others HeII 164 nm, [OIII] 166.7 nm, [CIII] 190.9 nm with varying intensities. However, both nebular CIV and HeII emission, requiring higher energies, is challenging to produce and their origin is intensely debated (e.g. Schaerer et al. 2018, Saxena et al. 2019). Spatially resolved studies in, e.g., HeII in emission will be essential to constrain the ionizing sources;
  • Structure formation can also be investigated at the epoch of reionisation by looking for proto-clusters (Toshikowa et al. 2014; Overzier et al. 2016). Spatially-resolving the galaxies and AGN, and studying the circum-galactic and inter-galactic medium (CGM and IGM) within these proto-clusters provides us a unique opportunity to witness the early formation, growth, and co-evolution of these structures on a huge range of spatial scales.
Many of these questions cannot be addressed with the JWST, e.g. since they require a) measurements of very faint emission lines (in the rest-optical), which will only be accessible for relatively bright sources with the JWST (i.e. not for the bulk of the population, see Vanzella et al. 2014), b) UV absorption line observations with R>~2000, c) faint rest-UV emission lines (with EWrest~1-10A), d) R>~2000 Lya line profile measurements.

mass assembly

A large (hundreds to thousands of galaxies) and representative survey of spatially-resolved galaxies selected homogeneously over the redshift range z=2-4 is an important scientific goal for the ELT. This will probe the mass assembly of galaxies by disentangling the different physical processes at work as a function of time and mass (Puech et al. 2010; 2018). When combined with deep imaging in the rest-frame near-IR from, e.g., the JWST, the evolution of their dynamical state as a function of time and mass can be investigated. Amongst the whole mass spectrum, low mass galaxies represent a unique niche for the large integrated area of the ELT as we detail below.

Dwarf galaxies are expected to play a key role in galaxy formation and evolution. In hierarchical models they are thought to be the first structures to form in the Universe and are believed to have an important contribution to the reionisation process. Investigating the detailed properties of dwarf galaxies around the peak of cosmic star formation history (z ~ 2) is therefore an important test of structure formation in ΛCDM. Spatially resolved studies of the star formation activity and the metal enrichment provide powerful diagnostic tools: metallicity gradients are thought to be highly sensitive to the gas surface density, its kinematic structure (coherent rotation vs unordered motions), and the prevalence of inflows and outflows. Distant sub-M* galaxies have faint apparent magnitudes and we still have limited knowledge of their morphological and chemodynamical properties (e.g., Kassin et al. 2012; van der Wel et al. 2014; Kartaltepe et al. 2015; Whitaker et al. 2015; Simons et al. 2015). The samples for which spatially-resolved kinematics can be obtained remain small and the integration times very large (e.g., Contini et al. 2016).

Simulations show that mapping the properties of the ionised haloes of starbursting dwarfs similar to Haro11 at z ~ 2 should be feasible in ~10hr with multi-IFU observations. Typical logM* = 9 galaxies at z = 2 have internal velocity dispersions 𝜎 < 50 km/s (Mason et al. 2016), which MOSAIC will be able to resolve but JWST/NIRSpec will struggle with. In addition to enabling the measurement of (spatially-resolved) internal motions, resolving emission lines down to 𝜎 ~ 10 km/s will be critical to unveiling the presence of the narrow kinematical subcomponents that are characteristic of rapidly assembling dwarfs (Amorin et al. 2012)--and which indicate that star formation typically proceeds in an ensemble of several compact and turbulent clumps.