Southward ho!

When astrophysicist Roelof de Jong joined the Leibniz Institute for Astrophysics Potsdam in Germany some 16 years ago, he found that researchers were interested in developing instruments to gain deeper insights into celestial objects. Bringing together what was then a small group of astronomers, Jong envisioned a more ambitious project: to build an instrument dedicated to galaxy studies of the southern sky. In October 2010, he submitted a proposal to the European Southern Observatory (ESO) for the development of an advanced spectroscopic instrument, to be known as the 4-metre Multi-Object Spectroscopic Telescope, or 4MOST. The group now includes hundreds of member-astronomers.

On October 18, 2025, the telescope saw its 'first light': a bunch of photons from the distant past. "Seeing this 'first light' after 15 years of design, planning, and building, seeing the instrument finally collect spectra was deeply rewarding," says Jong, now the principal investigator of 4MOST, a project comprising over 25 survey groups. The instrument is mounted on VISTA — the Visible and Infrared Survey Telescope for Astronomy — located in the Atacama Desert of northern Chile. It can capture light from 2,400 stars and galaxies — objects ranging from a few light-years away, located within the Milky Way, to those a few billion light-years away. Unlike telescopes that only take images, VISTA with 4MOST splits the incoming light from these cosmic objects simultaneously into their component wavelengths and analyses them using its spectrograph. This helps astronomers probe deeper into the objects that are imaged.

By examining how the brightness changes across wavelengths, astronomers can determine what each object is made of, how fast it is moving, and how far away it lies. For example, a spectrum from an active galactic nucleus, the bright region at the centre of a galaxy around a supermassive black hole, shows distinct emission lines of magnesium, oxygen, and hydrogen.

"4MOST had a unique approach to using instruments, particularly because it could observe many targets at the same time. To address this, the team had combined many different science cases, ranging from nearby objects such as white dwarfs or stars with exoplanets, to the most distant galaxies in the universe — and observed them simultaneously," Jong explains. Using this instrument, researchers will study topics ranging from the formation of stars and white dwarfs to the structure and evolution of galaxies and the universe. Many of these programmes, Jong adds, address fundamental questions about dark matter and dark energy, the physics that underpins the universe.

The project also seeks a deep survey of the Magellanic Clouds — two small galaxies orbiting the Milky Way — and to explore black holes. It is expected to complement and add to the work carried out by the Dark Energy Spectroscopic Instrument, attached to the Mayall 4-meter Telescope, at the Kitt Peak National Observatory in the U.S. 4MOST's vantage is its view of the southern sky; most instruments are located in the Northern Hemisphere.

As a consortium survey to be carried over five years, 4MOST is divided into two main categories: galactic surveys, which focus on the Milky Way and its immediate neighbourhood, and extragalactic surveys, says Shadab Alam, an astrophysicist at the Tata Institute of Fundamental Research, Mumbai, and co-lead of an infrastructure working group.

Twenty-five scientific working groups will focus on different aspects of galactic and extragalactic surveys. To develop the instrument and related software and calibrations, researchers formed Infrastructure Working Groups, each group handling a specific part of the project. "Our team developed methods and software to distinguish the instrument's fingerprints from real astrophysical signals," says Alam.

The instrument's uniqueness lies in its breadth of vision. It is designed as a survey instrument, capable of scanning large areas of the sky. Along with this, it has the advantage of offering both medium- and high-resolution spectroscopy, allowing for precise measurements of redshift and radial velocity.

The instrument is still in its commissioning phase. "Commissioning will continue through the end of the year, with the five-year survey expected to begin in the first half of 2026," ESO states. The survey will provide astronomers with a wealth of data to uncover the mysteries hidden in the stars and galaxies.