After the Big Bang, the Universe cooled down and grew dark. Only once dark matter and gas had clumped together did stars and galaxies begin to shine. Energetic radiation from those stars reionised (ripped electrons from the atomic nuclei) the remaining neutral gas, but exactly when and how this all occurred remains a mystery that MOSAIC will contribute to solving. Identifying the main — and elusive — ionising sources requires detailed observations of a specific hydrogen emission line (the Lyman-alpha line) of very distant objects. 

Using MOSAIC to make these observations will enable astronomers to find out the ionisation state of the intergalactic medium (IGM),   in the first few billion years of the universe (from redshift 5 to 13), allowing them to reconstruct the timeline of reionisation. MOSAIC will provide the largest observational sample of the very first galaxies at sufficient spectral resolving power to determine the properties of their stellar populations, as well as of the interstellar medium (ISM), the matter that exists in between the stars. The instrument will also be used to search for the presence of gas flowing out from these galaxies. 

Furthermore, MOSAIC will be used to identify the "missing" baryons in the circumgalactic medium (CGM), the gas surrounding galaxies. Taking advantage of pairs of galaxies where one is in front of the other from our perspective, astronomers will observe light from the background galaxy passing through gas surrounding the foreground galaxy. This foreground gas leaves absorption lines features in the spectrum of the background galaxy that will allow astronomers to find out what elements are present in the gas, and what their ionisation states are. Such studies have already been conducted on 8-metre telescopes using quasars as the background object. With the ELT, astronomers will be able to conduct the same analysis using dimmer and more widespread Lyman break galaxies as background sources instead of quasars, which will provide many more galaxy pairs to be studied, giving statistics at an epoch which remains out of reach of current facilities. 

• High multiplex mode in the visible (HMM-VIS): Simultaneous integrated-light observations of almost 200 objects, with the possibility of using both medium and high spectral resolving power (R~5000 and R~20,000).
• High multiplex mode in the near-infrared (HMM-NIR): Simultaneous integrated-light observations of 80 objects (goal: 100) each of them with dedicated sky fibres to subtract the strong sky background present in the near-infrared. This mode will cover simultaneously the wavelength range 0.8–1.8 µm with the possibility to observe in medium and high spectral resolving power.
• High-definition mode (HDM): Simultaneous observations of eight integral field units (IFUs) (goal: 10 IFUs) deployed within a ~40 arcmin patrol field. Each IFU will cover a 2.5 arcsec hexagon with ~200 milliarcseconds spaxels, sharing the same spectrograph as for the HMM-NIR, with same wavelength coverage and spectral resolving power.

MOSAIC has just completed the initial project stage known as Phase A.

Simulator

Tool to simulate instrument observations

Science Case

Description of the scientific motivations for the instrument, as initially submitted by the Instrument Consortium

Top Level Requirements

Description of the characteristics of the instrument required by the science case

The MOSAIC project is managed by an international consortium composed of research institutes from European and overseas countries.

MOSAIC is being designed and built under the leadership of the National Institute for Earth Sciences and Astronomy at the National Centre for Scientific Research (CNRS-INSU, France) by a consortium of partners in France, the United Kingdom, the Netherlands, Brazil, Germany, Austria, Finland, Italy, Portugal, Spain, Sweden, Switzerland. In addition to CNRS-INSU, the MOSAIC consortium consists of Durham University (United Kingdom), the Galaxies, Stars, Physics and Instrumentation Department at Paris Observatory (France), the Institute for Research in Astrophysics and Planetology at the University of Toulouse (France), the Marseille Astrophysics Laboratory (France), the National Physics Laboratory (Brazil), the Leibniz Institute for Astrophysics Potsdam (Germany), the LESIA Space and Astrophysics Instrumentation Research Laboratory at Paris Observatory (France), the Netherlands Research School for Astronomy (Netherlands), the University of São Paulo (Brazil), the University of Amsterdam (Netherlands), the University of Oxford / RAL Space (United Kingdom), the UK Astronomy Technology Centre (United Kingdom), University of Helsinki (Finland), the University of Michigan (US), Stockholm University (Sweden), LSW Heidelberg (Germany), the University of Geneva (Switzerland), the University of Vienna (Austria), IACE – Universidade de Lisboa (Portugal), Space Telescope Science Institute (US), Universidad Complutense de Madrid (Spain), INAF Roma (Italy). ESO also contributes to the project development.