Extragalactic astrophysics: Scaling the Universe

Ever heard that our Milky Way weighs about a trillion times the mass of the Sun? How can we know that? Did we put our Galaxy on a scale? Of course, there is no such scale, yet there are ways to weigh galaxies. It was the encounter with these methods that took Prof. Bodo Ziegler onto the path of observational extragalactic astronomy more than twenty years ago. He started with the FORS instrument (a spectrograph used to image emitted light of celestial bodies) at the Very Large Telescope (VLT) looking at the kinematics and dynamics which address the movement behaviour according to the gravitational force of galaxies. Now he leads the Extragalactic Group at the Department for Astrophysics at the University of Vienna.

The group’s research delves into the nature of galaxies. Were there many stars all at once and galaxies formed immediately or were there phases of strong star formation, the so-called starbursts? Furthermore, galaxies are never still. They move, evolve and change their appearance. Galaxies exist in different environments, they can be lonely with no neighbours or they can huddle together in groups and clusters. Depending on their surroundings, they develop differently. They can merge, fly-by each other tearing gas away, fall into a cluster and lose their gas and hence not be able to form stars anymore. Galactic evolution takes billions of years, so we can never see a full transformation.

Which methods allow us to observe processes that take place on astronomical timescales? There are a gazillion galaxies in our Universe and high-end telescopes take snapshots of galaxies in different evolutionary stages. The latter can be observed by extracting data from the picture itself with so-called photometry and direct imaging. Pictures taken by the optical telescopes on Earth – such as the VLT – or in space, like the Hubble Space Telescope give snapshots at different evolutionary stages. The composition of stars, gas and dust changes and the measurements hint to the transformations and the ongoing processes, respectively. Was it violent, how violent, did two galaxies interact, was it in a cluster or a group? Additionally, just as twenty years ago, when Prof. Ziegler started studying galaxies, spectra from new state-of-the-art spectrographs give us more precise information for many kinds of astronomical objects every time. By analysing the emitted light from galaxies, it is possible to investigate the movements of their components, how stars form and the properties of the gas present in them. These observations help to paint a clearer picture on the structure, the amount of stars, the kinematics and dynamics that govern a galaxy. These observations help astronomers, such as the Extragalactic group in Vienna, to better understand the ongoing processes.

For their observations, the group has high requirements on technology, namely the telescopes, satellites and the instruments attached to them. Their results provide insight into the evolutionary processes taking place in galaxies and how they affect the stars they harbour. Just as they provide new insights on the difference of galaxies transforming in clusters or groups, they build upon findings from other fields of astronomy such as star formation and the interstellar medium.

Apart from studying large galaxies such as our own, the Milky Way, Prof. Ziegler’s group aims to study smaller, less massive dwarf galaxies, for which we have less information. An interesting property of dwarf galaxies is their high amount of dark matter, which we cannot see directly, only its gravitational effects. It plays a role in the evolution of the Universe as it accounts for most of its mass. Consequently, it has an impact on galaxies. While we have a clear picture on the present-day structure of the Universe, it gets fuzzier when we look farther back in time. Additionally to examine topics like dwarf galaxies and dark matter, the Extragalactic group wants to expand its field of research to earlier times approaching the Big Bang.

Author: Sanje Fenkart