Archives For cosmology

The central parts of our Galaxy, the Milky Way, as observed in the near-infrared with the NACO instrument on ESO’s Very Large Telescope. By following the motions of the most central stars over more than 16 years, astronomers were able to determine the mass of the supermassive black hole that lurks there. Credit: ESO/S. Gillessen et al.

Reinhard Genzel and Andrea Ghez have jointly been awarded the 2020 Nobel Prize in Physics for their work on the supermassive black hole, Sagittarius A*, at the center of our galaxy. Genzel, Director at the Max Planck Institute for Extraterrestrial Physics in Germany, and his team have conducted observations of Sagittarius A* for nearly 30 years using a fleet of instruments on European Southern Observatory (ESO) telescopes.

Genzel shares half of the prize with Ghez, a professor at the University of California, Los Angeles in the US, “for the discovery of a supermassive compact object at the center of our galaxy”, with the other half awarded to Roger Penrose, professor at the University of Oxford in the UK, “for the discovery that black hole formation is a robust prediction of the general theory of relativity.” 

“Congratulations to all three Nobel laureates! We are delighted that the research on the supermassive black hole at the center of our galaxy has been recognized with the 2020 Nobel Prize in Physics. We are proud that the telescopes ESO builds and operates at its observatories in Chile played a key role in this discovery,” says ESO’s Director General Xavier Barcons. “The work done by Reinhard Genzel with ESO telescopes and by Andrea Ghez with the Keck telescopes in Hawaii has enabled unprecedented insight into Sagittarius A*, which confirmed predictions of Einstein’s general relativity.”

ESO has worked in very close collaboration with Genzel and his group for around 30 years. Since the early 1990s, Genzel and his team, in cooperation with ESO, have developed instruments designed to track the orbits of stars in the Sagittarius A* region at the center of the Milky Way. 

They started their campaign in 1992 using the SHARP instrument on ESO’s New Technology Telescope (NTT) at the La Silla Observatory in Chile. The team later used extremely sensitive instruments on ESO’s Very Large Telescope (VLT) and the Very Large Telescope Interferometer at the Paranal Observatory, namely NACO, SINFONI and later GRAVITY, to continue their study of Sagittarius A. 

In 2008, after 16 years of tracking stars orbiting Sagittarius A*, the team delivered the best empirical evidence that a supermassive black hole exists at the center of our galaxy. Both Genzel’s and Ghez’s groups accurately traced the orbit of one star in particular, S2, which reached the closest distance to Sagittarius A* in May 2018. ESO undertook a number of developments and infrastructure upgrades in Paranal to enable accurate measurements of the position and velocity of S2.

The team led by Genzel found the light emitted by the star close to the supermassive black hole was stretched to longer wavelengths, an effect known as gravitational redshift, confirming for the first time Einstein’s general relativity near a supermassive black hole. Earlier this year, the team announced they had seen S2 ‘dance’ around the supermassive black hole, showing its orbit is shaped like a rosette, an effect called Schwarzschild precession that was predicted by Einstein.

Genzel and his team are also involved in the development of instruments that will be installed on ESO’s Extremely Large Telescope, currently under construction in Chile’s Atacama Desert, which will enable them to probe the environment even closer to the supermassive black hole.

Photo: Hubble Space Telescope image of distant galaxy cluster and gravitational lensing. Credit: NASA/ESA

This image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACSJ0717.5+3745. This is one of six being studied by the Hubble Frontier Fields programme, which together have produced the deepest images of gravitational lensing ever made. Due to the huge mass of the cluster it is bending the light of background objects, acting as a magnifying lens. It is one of the most massive galaxy clusters known, and it is also the largest known gravitational lens. Of all of the galaxy clusters known and measured, MACS J0717 lenses the largest area of the sky.

Observations by the NASA/ESA Hubble Space Telescope have taken advantage of gravitational lensing to reveal the largest sample of the faintest and earliest known galaxies in the Universe. Some of these galaxies formed just 600 million years after the Big Bang and are fainter than any other galaxy yet uncovered by Hubble. The team has determined, for the first time with some confidence, that these small galaxies were vital to creating the Universe that we see today.

An international team of astronomers, led by Hakim Atek of the Ecole Polytechnique Fédérale de Lausanne, Switzerland, has discovered over 250 tiny galaxies that existed only 600 to 900 million years after the Big Bang — one of the largest samples of dwarf galaxies yet to be discovered at these epochs. The light from these galaxies took over 12 billion years to reach the telescope, allowing the astronomers to look back in time when the universe was still very young.

Although impressive, the number of galaxies found at this early epoch is not the team’s only remarkable breakthrough, as Johan Richard from the Observatoire de Lyon, France, points out, “The faintest galaxies detected in these Hubble observations are fainter than any other yet uncovered in the deepest Hubble observations.”

By looking at the light coming from the galaxies the team discovered that the accumulated light emitted by these galaxies could have played a major role in one of the most mysterious periods of the Universe’s early history — the epoch of reionization. Reionization started when the thick fog of hydrogen gas that cloaked the early Universe began to clear. Ultraviolet light was now able to travel over larger distances without being blocked and the Universe became transparent to ultraviolet light.

By observing the ultraviolet light from the galaxies found in this study the astronomers were able to calculate whether these were in fact some of the galaxies involved in the process. The team determined, for the first time with some confidence, that the smallest and most abundant of the galaxies in the study could be the major actors in keeping the Universe transparent. By doing so, they have established that the epoch of reionization — which ends at the point when the Universe is fully transparent — came to a close about 700 million years after the Big Bang.

Lead author Atek explained, “If we took into account only the contributions from bright and massive galaxies, we found that these were insufficient to reionize the Universe. We also needed to add in the contribution of a more abundant population of faint dwarf galaxies.”

To make these discoveries, the team utilized the deepest images of gravitational lensing made so far in three galaxy clusters, which were taken as part of the Hubble Frontier Fields program. These clusters generate immense gravitational fields capable of magnifying the light from the faint galaxies that lie far behind the clusters themselves. This makes it possible to search for, and study, the first generation of galaxies in the Universe.

Jean-Paul Kneib, co-author of the study from the Ecole Polytechnique Fédérale de Lausanne, Switzerland, explains, “Clusters in the Frontier Fields act as powerful natural telescopes and unveil these faint dwarf galaxies that would otherwise be invisible.”

Co-author of the study Mathilde Jauzac, from Durham University, UK, and the University of KwaZulu-Natal, South Africa, remarks on the significance of the discovery and Hubble’s role in it,“Hubble remains unrivaled in its ability to observe the most distant galaxies. The sheer depth of the Hubble Frontier Field data guarantees a very precise understanding of the cluster magnification effect, allowing us to make discoveries like these.”