Dawn of a New Era of Interferometry with GRAVITY+
Four powerful Lasers enhance Observational Power of ESO’s Paranal Observatory
Last week, four lasers were set off into the skies above the European Southern Observatory’s (ESO’s) Paranal site in Chile. The lasers are each used to create an artificial star, which astronomers use to measure and then correct the blur caused by Earth's atmosphere. The striking launch of these lasers, one from each of the 8-metre telescopes at Paranal, is a significant milestone of the GRAVITY+ project — a major upgrade to ESO’s Very Large Telescope Interferometer (VLTI) which was designed and developed by a European consortium led by the Max Planck Institute for Extraterrestrial Physics (MPE). GRAVITY+ allows observing fainter objects than before and vastly increases the sky coverage for the VLTI.
The Very Large Telescope Interferometer (VLTI) combines the light from four individual telescopes of the Very Large Telescope (VLT), utilizing either the 8-meter Unit Telescopes or the smaller Auxiliary Telescopes through interferometry. GRAVITY+, developed by a consortium of European institutes and led by the Max Planck Institute for extraterrestrial physics (MPE), serves as an upgrade to the VLTI and GRAVITY, an instrument, which revolutionized high angular resolution astronomy in recent years. GRAVITY has facilitated precision tests of general relativity with the black hole at the center of the Milky Way, imaged exoplanets and young stellar objects, and determined the masses of supermassive black holes across the universe. Now, with GRAVITY+, the telescopes and underground beam combiner obtained many upgrades. The recent installation of lasers at each of the previously unequipped 8m telescopes is a - brightly visible - cornerstone of the project. The lasers further enhance the worldwide most powerful optical interferometer.
“The VLTI with GRAVITY has already enabled so many unpredicted discoveries, we are excited to see how GRAVITY+ will push the boundaries even further,” says GRAVITY+ principal investigator and director at MPE, Frank Eisenhauer.
Animation of the light path through the VLTI's GRAVITY+ instrument
Project Lead at MPE
MPE played a pivotal role in the development of GRAVITY+, leading the overall design. The institute also developed and installed four new, state-of-the-art wavefront sensors. These sensors are used to observe the artificial stars created by the newly installed laser guide stars, yielding an advanced adaptive optics correction for the VLTI. This technique compensates for the blurring effects caused by Earth’s atmosphere, and GRAVITY+ brings cutting-edge sensors, lasers and deformable mirrors to the VLTI.
Before the installation of the laser, the correction of the atmospheric seeing at the VLTI had to rely on bright natural reference stars. But in most cases, there is no suitable star located near the target of interest, which severely limited the number of observable objects. With the new lasers, artificial stars can now be created anywhere in the sky. The laser light excites a small spot in a layer of atmospheric sodium atoms, approximately 90 kilometers above the Earth’s surface, creating a laser guide star. This significantly expands the VLTI’s observational reach, granting access to the entire southern sky.
From the Galactic Center to the Edge of the Universe
The MPE Infrared Group plans to harness the new adaptive-optics systems with laser guide stars for pioneering studies across two major research areas: the Galactic Center and early Universe quasars.
In the Galactic Center, the improved sharpness and sensitivity will enable the team to detect and track ever fainter stars orbiting the supermassive black hole at the heart of the Milky Way — paving the way for a direct measurement of the black hole’s spin. According to the theory of general relativity by Albert Einstein, the space-time around the black hole is rotating with these mysterious objects, affecting notably the orbits of stars that come close enough – a regime only the VLTI can probe.
The enhanced capabilities will further allow researchers to spatially resolve the gas fastly swirling around supermassive black holes in galaxies far away. The method works across cosmic time and leads to a direct measurement of the black hole mass. “These upgrades open up the instrument to observations of objects in the early distant Universe, such as the quasar we observed in the second night where we resolved the hot, oxygen emitting gas very close to the black hole. We will be able to measure black hole masses more precisely than ever before and at a critical time when black holes and their host galaxies were rapidly evolving", says Taro Shimizu, an MPE astronomer who is part of the instrument consortium.
A first laser target for the GRAVITY+ and ESO teams at Paranal performing test observations was a cluster of massive stars at the centre of the Tarantula Nebula, a star-forming region in the Large Magellanic Cloud, a neighbour galaxy of the Milky Way. „Already these very first observations revealed that a bright object in the nebula, thought to be the most massive single star known, actually is a binary of two stars close together“ explains Guillaume Bourdarot from MPE. This showcases the stunning resolving power and scientific potential of the upgraded VLTI.
The background image is a wide-field view of the Tarantula nebula taken with the 1.5 Danish telescope at ESO’s La Silla Observatory. The first inset shows a closeup of the central stellar cluster obtained with ESO’s Very Large Telescope at Paranal Observatory. We then see an even closer look taken with the GRAVITY+ acquisition camera, and finally the binary star itself. The small ellipse in this last inset represents the resolution of GRAVITY+.
Overall, this improvement is much more than an update and was first envisioned decades ago. The laser system was suggested already in 1986, before the VLT and VLTI even existed. “If it could work in practice, it would be a breakthrough,” stated the final report of the “Very Large Telescope Project”, which was coauthored by MPE director and Nobel prize winner Reinhard Genzel. Now this breakthrough is a reality.
Laser-Show at Paranal
Credits: A. Berdeu/ESO
The GRAVITY+ consortium consists of the following partners:
- Max Planck Institute for Extraterrestrial Physics (MPE); Max Planck Institute for Astronomy; University of Cologne (Germany)
- Institut National des Sciences de l'Univers, French National Centre for Scientific Research; Institut de Planétologie et d'Astrophysique de Grenoble; Laboratoire d’instrumentation et de recherche en astrophysique (LIRA); Lagrange Laboratory; Centre de Recherche Astrophysique de Lyon (France)
- Instituto Superior Técnico’s Centre for Astrophysics and Gravitation (CENTRA); University of Lisbon; University of Porto (Portugal)
- University of Southampton (UK)
- Katholieke Universiteit Leuven (Belgium)
- University College Dublin (Ireland)
- Instituto de Astronomia – Universidad Nacional Autónoma de México (Mexico)
- European Southern Observatory.
