The first building blocks of the universe
The first galaxies evolved only a few hundred million years after the Big Bang. But why do they have such a great variety of shapes and structures? How did the universe evolve as a whole? Two German-Chinese Partner Groups at the Max Planck Institute for Astrophysics in Garching are using observations and simulations to investigate how the early universe evolved: Cheng Li and Guinevere Kauffmann, as well as Liang Gao and Simon White.
Text Alexander Stirn
They can be large or small, red or blue, extremely massive or just bright, can be individuals or simply follow the crowd: The galaxies in the universe come in almost all conceivable shapes and sizes. The cosmological standard model, which describes the evolution of the universe, does not really provide for such a variety, however. The theory states only that minute density fluctuations shortly after the Big Bang must have been responsible for distributing the mass and energy in the universe.
“This results in a very useful, very simple picture of how the universe evolved,” says Cheng Li, professor at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences. “But in reality, we come across, in this simple and beautiful universe, a surprisingly high number of different types of galaxy.”
Within the framework of a Partner Group together with Guinevere Kauffmann from the Max Planck Institute for Astrophysics, Li wants to resolve this apparent contradiction. The most important question here is: How did the different galaxies form and what were their fundamental building blocks?
In the search for answers, so-called surveys play an important role – sky surveys where a telescope focuses its sights on a large region of the universe step by step. The Sloan Digital Sky Survey (SDSS), for example, has observed and mapped more than one million galaxies in the vicinity of the Milky Way. This involved dispersing the light captured from each object into its different wavelengths.
In these spectra, lines show up – fingerprints that disclose, among other things, which elements are in the galaxy, how much metal the stars contain, how old they are and how quickly they are forming. “A great deal of information on the properties of a galaxy can be gained from one of these spectra,” says Cheng Li.
Li began to analyze these data together with Guinevere Kauffmann back in 2005, when he was still a postdoctoral student at the Max Planck Institute for Astrophysics. They concentrated on searching for correlations with the environment in which the galactic systems are found. One of their findings was that galaxies with a large number of stars often turned up in a so-called galaxy cluster – a region with a particularly high density of galaxies.
The SDSS spectra have one disadvantage, however: They have all been recorded in the range of visible light and thus show only stars. “Although galaxies are made up of these stars, the stars themselves form from gas,” says Li. It is not possible to make out this cold gas in the visible spectra, however. This is a problem for Li and his colleagues: “We still know very little about the gas, yet it is an important factor for the formation of galaxies.”
When the cosmologist returned to China in 2010, this was an important motivation for establishing the Partner Group. “We didn’t just want to keep up the collaboration, we also wanted to move our focus from the visible range to wavelengths where the gas shows up,” says Guinevere Kauffmann.
The Partner Group eventually started work in January 2011 – the third such Group between the Max Planck Institute for Astrophysics and the Shanghai Astronomical Observatory. Max Planck researcher Gerhard Börner had already laid the foundation stone of the collaboration in 2000 when he formed the first Partner Group ever, which was established in Garching and Shanghai between the Max Planck Gesellschaft and the Chinese Academy of Science.
The new Group, with its concentration on wavelengths beyond the visible range, has already been able to clarify initial contradictions. At the center of most galaxies is an extremely massive black hole that attracts, accelerates and swallows matter from its immediate vicinity – a process which is noticeable as a giveaway signal in the light from the galaxies. Theoreticians are convinced, not least on the basis of simulations, that such an active galactic nucleus must emit significantly more radiation as soon as two galaxies collide. The images of the SDSS show no trace of this, however. “It didn’t matter whether a galaxy was close to another one or not, the activity always remained the same,” remembers Li.