Recent observations of globular clusters with the VIRUS-W instrument at the McDonald observatory revealed a rotation signal at the centre of these huge agglomerations of stars.
This finding is very surprising, as the astronomers expected that any central rotation should have been erased by now due to the old age of these clusters.
The core of the globular cluster Messier 13 is just 25 000 light-years away and measures about 145 light-years in diameter. It lies in the constellation Hercules and sometimes can even be seen with small binoculars.
Credit: ESA / Hubble und NASA
In addition, the astronomers from the Max Planck Institute for Extraterrestrial Physics and the University of Texas found that the rotation axis agrees with the slight elongation found for some of the clusters, indicating that this flattening is caused by rotation.
Globular clusters are ancient formations orbiting most galaxies, including our own Milky Way: they consist of up to a million old, metal-poor stars, which are tightly bound by gravity. Due to their old age and fairly spherical shape, with a strong concentration of stars towards the centre, they have historically been viewed as simple systems. However, new observations keep revealing surprising results.
“For all globular clusters in our sample we find a rotation signal in the centre – an astonishing result,” says Maximilian Fabricius, lead scientists of this study at the Max Planck Institute for Extraterrestrial Physics (MPE). “We did not expect this; originally we observed these globular clusters to measure their central velocity dispersions.” The velocity dispersion is a measure of the amount of random stellar motion in a cluster. Rotation, on the other hand, means that the cluster has an actual rotation axis around which more stars revolve in one way than the other.
“Theory and numerical simulations of globular clusters indicate that any central rotation should be erased on relatively short timescales,” says Eva Noyola, co-author of this study at the University of Texas in Austin. “Because these globular clusters were formed billions of years ago, we would expect that any rotation signature would have been eradicated by now. Even though previous measurements showed some rotation in a handful of systems, they only probed the motion of stars in the outer regions.”
The new measurements of a dozen globular cluster cores were only possible with the help of the VIRUS-W instrument developed at MPE. This “integral field spectrograph” (IFU) allows the scientists to simultaneously measure more than 260 spectra in their field of view, determining the motion of stars to an accuracy of a few kilometres per second. That means that for a given globular cluster, one night at the Harlan J. Smith 2.7m Telescope of the McDonald observatory in Texas with an observing time of a few hours is enough to determine the velocity field at the core of the cluster. Such a project was not possible before VIRUS-W.
Traditionally, astronomers determined the line-of-sight velocities of cluster member stars one-by-one using high resolution spectrographs that can measure the Doppler shift of the stars. This, however, is painstakingly slow and very hard to do in the core of globular clusters. Therefore, so far, there have been no systematic studies of the kinematics of globular cluster cores. Even though the VIRUS-W instrument was originally developed to study the kinematics of nearby galaxies, the astronomers realized that the combination of a large field of view and a relatively high spectral resolution make this instrument a very efficient tool to also study stellar motions in globular clusters.
There are about 150 globular clusters in the Milky Way and the astronomers selected 27 of them, which are observable with the McDonald telescope (i.e. in the northern sky) during one night. Between August 2012 and August 2013 they observed the first 11 globular clusters and now present their surprising results: all clusters show signatures of rotation.
Furthermore, the new velocity measurements are actually in remarkably good agreement with the flattening measured for the globular clusters in this sample. This seems to indicate that the central rotation drives the flattening for these objects rather than the influence of the Milky Way tidal field.
These findings raise interesting questions about the formation history and evolution of globular clusters – none of the current theoretical models predict such a ubiquitous and strong rotation. However, it is worth noting that the present sub-sample does not include any “core-collapsed” globular clusters yet.
Core-collapse is a process that might eradicate rotation. Future observations of the remaining clusters in the full sample will shed light on additional questions such as a possible correlation between rotation and the position of a globular cluster inside our galaxy.
Dr. Hannelore Hämmerle | Max-Planck-Institute
Squeezed quantum cats
27.05.2015 | ETH Zurich
Supernovas help 'clean' galaxies
27.05.2015 | Michigan State University
The only professorship in Germany to date, one master's programme, one laboratory with worldwide unique equipment and the corresponding research results: The University of Würzburg is leading in the field of biofabrication.
Paul Dalton is presently the only professor of biofabrication in Germany. About a year ago, the Australian researcher relocated to the Würzburg department for...
Physicists have developed an innovative method that could enable the efficient use of nanocomponents in electronic circuits. To achieve this, they have developed a layout in which a nanocomponent is connected to two electrical conductors, which uncouple the electrical signal in a highly efficient manner. The scientists at the Department of Physics and the Swiss Nanoscience Institute at the University of Basel have published their results in the scientific journal “Nature Communications” together with their colleagues from ETH Zurich.
Electronic components are becoming smaller and smaller. Components measuring just a few nanometers – the size of around ten atoms – are already being produced...
Development and implementation of an advanced automobile parking navigation platform for parking services
To fulfill the requirements of the industry, PolyU researchers developed the Advanced Automobile Parking Navigation Platform, which includes smart devices,...
The world's first electrical car and passenger ferry powered by batteries has entered service in Norway. The ferry only uses 150 kWh per route, which...
On Tuesday, 19 May 2015 the research icebreaker Polarstern will leave its home port in Bremerhaven, setting a course for the Arctic. Led by Dr Ilka Peeken from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) a team of 53 researchers from 11 countries will investigate the effects of climate change in the Arctic, from the surface ice floes down to the seafloor.
RV Polarstern will enter the sea-ice zone north of Spitsbergen. Covering two shallow regions on their way to deeper waters, the scientists on board will focus...
20.05.2015 | Event News
18.05.2015 | Event News
12.05.2015 | Event News
27.05.2015 | Power and Electrical Engineering
27.05.2015 | Health and Medicine
27.05.2015 | Physics and Astronomy