Ruprecht-Karls-Universität Heidelberg
The ever-increasing thickness of our home galaxy

The ever-increasing thickness of our home galaxy

The ever-increasing thickness of our home galaxy

The vertical thickness of the Milky Way disc is constantly increasing due to heating mechanisms. Astronomers have been discussing the exact causes of this expansion for some time. In an elaborate simulation, Reza Moetazedian and Andreas Just from the Collaborative Research Centre (SFB) 881 "the Milky Way System" at the Centre for Astronomy in Heidelberg have now investigated one of these mechanisms with unprecedented accuracy. For the first time, they were able to quantify the influence of incident satellite galaxies on the stellar disk of our home galaxy.

View of NGC 4565, a typical spiral galaxy seen in profile. (Credit: Joseph D. Schulman). The Galaxy shape as a thin disc with a central bulge is clearly visible.

Our Milky Way is a fairly typical spiral galaxy: Most of the visible matter is distributed within a bright central bulge and a flat disk with spiral structures. The vast majority of its mass, however, is contained in the so-called halo, a spherical distribution of dark matter surrounding the Galaxy. This structure is by no means static. It can be demonstrated, for example, that the galactic disk is expanding in the vertical direction – the thickness of our galaxy is increasing.

A possible explanation for this is the impact of smaller satellite galaxies onto our own. The halo of our galaxy features many substructures that can be traced back to earlier incidences of such satellites. It is assumed that the gravitational influence of these structures is responsible for heating mechanisms and thus trigger a vertical expansion of the Galactic disc.

Recently, the astrophysicists Reza Moetazedian and Andreas Just from subproject A2 of the SFB 881 quantified the contribution of these substructures for the first time in great detail. The result of their complex simulations surprised both: "We have hardly seen any influence on the heating of the galactic disc," says Just. The contribution to the vertical expansion of the Milky Way that can be assigned to the accretion of satellites is only about 10 to 15 percent of the observed vertical heating rate in the solar neighbourhood.

The astronomers took great care to use particularly stable initial conditions as well as a high resolution of the simulated galaxy in their simulations. They thus ensured that the influence of the infalling satellite galaxies could be reliably quantified. In order to achieve a statistically relevant result, Moetazedian and Just ran seven different simulations with differing initial distributions of the substructures in the halo around the Galaxy. For all simulations, the researchers investigated the evolution of the galactic disk over two billion years.

IIn the simulations, a certain increase in the average disc thickness and the vertical velocity distribution of the stars could be observed. However, these were usually on the same order of magnitude as those of galaxies simulated without the influence of additional satellites. Only a simulation run featuring a particularly heavy substructure within the galactic halo resulted in a significantly larger influence on the vertical expansion of the galactic disc. This suggests that the Milky way would have been subject to regular encounters with such heavy galaxies comparable to the Large Magellanic Cloud in order to reach the observed heating rate. However, there is no indication for such a violent history of our galaxy in the observational data of astronomers. "Alternative mechanisms are more likely to dominate the disc heating," Just adds. These remaining processes are subject of their ongoing research.

Additional Information:

The work appeared in: Impact of cosmological satellites on the vertical heating of the Milky Way disc, R. Moetazedian, A. Just, Monthly Notices of the Royal Astronomical Society, Oxford University Press, vol. 459 (3): pp. 2905-2924.

This work was supported by the Collaborative Research Center (Sonderforschungsbereich) SFB 881 "The Milky Way System" at Heidelberg University. Collaborative Research Centers are long-term research projects funded by the Deutsche Forschungsgemeinschaft (DFG) for up to 12 years.

The research work of SFB 881 revolves around our own galaxy, the Milky Way, a typical spiral galaxy and hence a member of the most common class of massive galaxies in the universe. The scientists involved in the SFB investigate the origins and evolution of the Milky Way and its surroundings in order to clarify fundamental principles of galaxy formation.

The SFB 881 is located at the Zentrum für Astronomie der Universität Heidelberg (ZAH) and includes scientists from the Astronomisches Rechen-Institut (ARI), the Institute of Theoretical Astrophysics (ITA) and the Landessternwarte Königstuhl (LSW). The participating non-university research institutions are the Max Planck Institute for Astronomy (MPIA) and the Heidelberg Institute for Theoretical Studies (HITS). In addition, the Haus der Astronomie (HdA) helps to make research on the Milky Way accessible to the general public.


Dr. Renate Hubele
Sonderforschungsbereich 881 „Das Milchstraßensystem“
Public Outreach
Phone: +49 6221 528-291
Email: hubele(at)

Editor: T. Lisker
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