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Luminous Gas Halos – Astrophysicists research galaxy envelopes in the young universe

It is a journey to the beginning of our being. The hunt for galaxies in the young universe and for what surrounds these heavenly worlds is in full swing. Science still knows very little about them. The space around stellar systems is not empty at all; it contains large amounts of so-called circumgalactic gas. But how is this gas distributed? How does it move around? What are the differences between the galaxies? Lutz Wisotzki and his teams at the Leibniz Institute for Astrophysics Potsdam (AIP) and at the University of Potsdam investigate such phenomena. Deciphering them is important to better understanding how galaxies are born and evolve.

They are invisible to the naked eye. Even the pictures taken by large telescopes reveal them only as tiny specks of light: galaxies at unimaginable distances. They are stellar systems in the young universe, whose light has taken billions of years to reach us. A new door to them has been opened now by a technological innovation called the Multi Unit Spectroscopic Explorer (MUSE), which was installed in 2014 at the Very Large Telescope (VLT) in the Atacama Desert in Chile. This 3D integral-field spectrograph – equipped with 24 identical spectrographs – makes it possible to look back almost to the origin of the universe. The telescope delivers much more than just beautiful pictures of the sky, though. With a single exposure, MUSE records more than 90,000 spectra of astronomical objects in a single go. It disperses the light into colors or wavelengths so that its intensity can be simultaneously measured at all frequencies. Wisotzki is particularly interested in the spectra, especially the so-called Lyman-alpha line, which reveals a lot about the circumgalactic medium surrounding young galaxies. This gas emits a substantial portion of its light in certain spectral lines. The Lyman-alpha line originates from hydrogen atoms, i.e. the most common element in the cosmos.

In collaboration with a consortium of six European research institutes, Wisotzki and his Potsdam colleagues at the AIP have developed a "magical machine". They provided the data reduction software that filters the scientific signal from the noise, eliminates the influence of instrument properties and the atmosphere, and then compiles the complex data, the AIP also developed and built the calibration unit as a hardware contribution. "The performance of MUSE is unique and unrivaled," Wisotzki enthuses about it. The spectrograph combines detailed images, spectra over a wide range, and high sensitivity. "Instead of creating a picture of the region and then selecting objects that were extensively and individually spectroscopically reobserved as had been done in the past, the device delivers data of high spectral quality for all objects in the field of view in a single step," Wisotzki explains. It offers an observing method that allows a very deep look into space and a journey through time to galaxies shortly after the Big Bang. For the first time, galaxies previously only visible to the Hubble Space Telescope can now be studied in much greater detail from the VLT with MUSE. And MUSE has even revealed galaxies so small and faint that they had escaped Hubble.

The observed young galaxies are surrounded by hydrogen halos

Lutz Wisotzki, also Professor for Observational Cosmology at the University of Potsdam, and his team have studied the spectra of many galaxies in the young universe in the so-called Hubble Deep Fields, regions where the Hubble Space Telescope had previously obtained supremely sharp images. "We expose for as long as possible," Wisotzki reports. "The final result is then assembled on the computer." Two of the pointings alone combined data from 30 hours exposure time per field. The effort was worth it. In each of these fields, the researchers were able to measure redshifts from nearly 300 galaxies – a huge step forward, as only 10-20 redshifts per field had been previously known. Since redshifts can be converted directly into distances, it became possible for the first time to reconstruct these fields in three dimensions. While the images deliver the positions of the galaxies in the sky, the spectra provide redshifts and thus distances. Moreover, based on the redshifts the astrophysicists at the AIP and the University of Potsdam were able to classify the galaxies into cosmic epochs, thanks to MUSE.

The researchers also achieved a breakthrough by observing the so-called Lyman-alpha spectral line of the most distant galaxies. They were able to demonstrate that all observed stellar systems of this cosmic age are surrounded by very extended hydrogen envelopes, many times larger than the galaxies themselves. No astronomical instrument before MUSE had been able to detect these Lyman-alpha envelopes; there had only been statistically averaged values. How did this discovery happen? "We had previously found that the systems observed with MUSE in the Lyman-alpha light were slightly diffuse and definitely larger than in the Hubble images. After this initial visual impression, we analyzed the light distribution and applied statistical methods that confirmed this impression," Wisotzki explains. "We realized that we had seen not only radiation from inside the galaxies but also from the circumgalactic medium." This discovery will likely have consequences for astrophysics. Conventional wisdom on the ecosystem of galaxies must now be put to the test.

According to Wisotzki, the fact that the glow of the gaseous halos is detectable despite their extremely low density can only be explained if standard assumptions concerning the structure of the circumgalactic gas are revised. "Presumably, the gas in the vicinity of galaxies is much less uniformly distributed than previously believed," says the Potsdam astrophysicist. The circumgalactic gas appears to consist of many small clumps or filaments reprocessing the radiation from the central galaxy. Whether this is actually the case will become clear once these effects have been more accurately calculated.

The insights gained will be incorporated into the next generation of numerical simulations, which need to take into account the newly discovered properties of circumgalactic nebulae. Possibly, theory and observations may get into alignment in about five years. It would be a big step in better understanding the environmental properties of young galaxies – and in understanding the evolution of galaxies in general – because the circumgalactic gas is both reservoir and catch basin. On the one hand, it supplies new gas from the outside, which results in the evolution of new stars in the galaxy. On the other hand, it also absorbs material ejected from the galaxies, much of which then returns to the galaxies. One of the biggest unresolved questions in astrophysics is how this complicated chaos of so many processes is self-regulated. The new observation results from MUSE provide an essential component in solving this problem.

The Very Large Telescope in the Atacama Desert is a very special place

The research successes of Wisotzki and his team were the outcome of numerous observation nights at the VLT. While the property rights of the MUSE instrument have been transferred to the European Observatory (ESO), the Potsdam researchers and their colleagues received 255 nights of “Guaranteed Observing” for the project, as "remuneration" for their work on MUSE. The researchers still have about 100 nights remaining for their investigations. On average, three researchers travel to Chile for a week, several times a year. When in South America, not only their work hours change but also the duration, with a night length of 8-10 hours. "We start preparing in the afternoon," Wisotzki reports. Work at the telescope then starts as soon as it gets dark.

During these nights, the experts from Brandenburg are not alone in the control room on the edge of the platform. Astrophysicists from all over the world are observing the sky. "It is a strangely sober atmosphere with dim light and a strict ban on music," the much-traveled professor describes the situation. "Only low partition walls divide the work areas." The place is still very special for Wisotzki, especially at sunset. "When the sun is low and the landscape is glowing in an impressive orange light, it’s hard to believe that such a thing exists," he says. The area’s barrenness and solitude are sublime.

In all likelihood, Wisotzki will soon be traveling back to Chile. Even after the current project funding ends in 2019, the questions remain: Where do the hydrogen envelopes get the energy from that makes them glow? What are the mechanisms for the observed Lyman-alpha radiation? Do the young, hot stars play the dominant role, or are external influences more important? Each discovery raises new questions. "MUSE has catapulted us to the scientific forefront in research on the young universe," says Wisotzki. "We want to maintain this position." The hunt for the unknown continues.


A celestial body only shines when energy is generated inside of it or when it is illuminated. While low-density gas clouds reflect no light, they can be stimulated to be luminous by absorbing radiation from energy sources. Of particular interest for astronomers is the hydrogen in such gas clouds, by far the most common chemical element in the universe. In a hydrogen atom, the electron orbits the atomic nucleus, i.e. the proton, on the innermost orbit (the so-called ground state). If sufficiently high-energy radiation hits the electron, it is either transported to a higher orbital or completely ejected, in which case there is a free electron and a free proton. If in turn two such particles collide, an atom is created again and the electron moves from higher orbitals back to the ground state in a cascade-like movement and energy is emitted in the form of radiation. This creates the spectral lines of hydrogen that make gas nebulae glow. Experts refer to the emission that results from the electron falling from the second-innermost to the innermost orbit as the Lyman alpha line. It is usually the brightest of the spectral lines of a galaxy.


The distance to a galaxy cannot be measured directly but can only be indirectly determined or estimated. For distant galaxies, however, the universe provides the researchers with a "trick" based on a discovery by Edwin Hubble 90 years ago that allows them to infer distance from observations. Light waves passing through the expanding universe are involved in the expansion, which causes the radiation to shift to longer wavelengths. The further away a galaxy is, the faster it seems to be moving away from us. This is however no real movement in space, but only the immediate consequence of the expansion of the universe. Experts denote this effect as "cosmological redshift". In order to determine distances, astronomers first measure the redshift in a spectrum of the galaxy and then calculate the distance based on a relation established by Hubble.


Given the finite nature of the speed of light, researchers observe each a distant galaxy as it was at the time its light was emitted. The observable universe is, so to speak, a perfect time machine but only into the past. This phenomenon is the prerequisite for astrophysicists to explore the state of the universe at very different cosmic times with a single observation – all the way back to the early stages when a large part of today’s stars and galaxies did not exist or were only just coming into existence.


Prof. Dr. Lutz Wisotzki studied physics and astronomy at the University of Hamburg. Since 2009, he has been Professor of Observational Cosmology at the University of Potsdam, a joint chair with the Leibniz Institute for Astrophysics Potsdam (AIP). At the AIP, Wisotzki heads the “Galaxies and Quasars” program area. He is also scientific coordinator of the overall MUSE project.


The infancy of normal galaxies revealed with MUSEFunding: Leibniz AssociationFunds: approx. 1 million eurosDuration: 2015–2019

The MUSE consortium consists of 7 major European research institutes: The Centre de Recherche Astrophysique de Lyon (CRAL, France), the Leibniz Institute for Astrophysics Potsdam (AIP, Germany), the Göttingen Astrophysics Institute (AIG, Germany), NOVA/Leiden Observatory (NOVA, the Netherlands), the Laboratoire d’Astrophysique de Tarbes-Toulouse (LATT, France), the Department of Astrophysics at the Zurich Polytechnic Institute of Technology (ETH, Switzerland), and the European Southern Observatory (ESO).


Text: Petra Görlich 
Translation: Susanne Voigt
Published online by: Alina Grünky
Contact to the online editorial office: onlineredaktionuni-potsdamde