European
Southern
Observatory

ELT Instruments
MOSAIC
Multi-Object Spectrograph

By peering at objects from stars at the heart of the Milky Way to the most distant galaxies at the very edge of the observable Universe, MOSAIC will enable astronomers to trace the growth of galaxies and the distribution of matter from the Big Bang to the present day.

In a nutshell

By peering at objects from stars at the heart of the Milky Way to the most distant galaxies at the very edge of the observable Universe, MOSAIC will enable astronomers to trace the growth of galaxies and the distribution of matter from the Big Bang to the present day.

Multi-Object Spectrograph

By peering at objects from stars at the heart of the Milky Way to the most distant galaxies at the very edge of the observable Universe, MOSAIC will enable astronomers to trace the growth of galaxies and the distribution of matter from the Big Bang to the present day.

Astronomers face a major challenge: the Universe contains hundreds of billions of galaxies, each of which contains hundreds of billions of stars. Surveying all of these and unravelling the mysteries behind the evolution of the Universe calls for a multi-object spectrograph that can measure the light from many different objects at the same time. MOSAIC is being built to do just this, as well as to serve as a follow-up machine of future space-based missions, such as the James Webb Space Telescope, as well as the European Space Agency's Euclid, and Athena missions. 

Science with MOSAIC

MOSAIC will conduct the first exhaustive inventory of matter in the early Universe. This will lift the veil on how matter is distributed in and between galaxies, resulting in a tremendous leap forward in our understanding of how present-day galaxies formed and evolved.

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. 

Instrument Design

To satisfy MOSAIC's science cases and requirements, the Phase A design includes three observation modes:

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

Wavelength

0.47 − 1.80 µm (baseline)

Spectral resolution

 5,000 − 20,000

Field-of-view

40 square arcmin

High Multiplex Mode

200 in VIS and 80+80 in NIR (baseline)

High Definition Mode

8 (baseline)

Tools and Documents

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

Instrument Consortium and Contacts

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. 

Principal Investigator

François Hammer (GEPI – Observatoire de Paris, France)

Project Scientist

Mathieu Puech (GEPI – Observatoire de Paris, France)

Ruben Sanchez-Janssen (UK Astronomy Technology Centre, UK)

Project Manager

Pascal Jagourel (GEPI – Observatoire de Paris, France)

ESO Project Manager

Patrick Cailler

ESO Project Scientist

Vincenzo Mainieri

ESO Project Engineer

Antonio Manescau