MOSAIC is a versatile multi-object spectrograph that will use the widest possible field-of-view provided by the Extremely Large Telescope (ELT). It will have three operating modes that cover observations in visible and infrared light for more than a hundred sources simultaneously.
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.