Astronomers have modeled the motions of stars in hundreds of galaxies in the local universe and they are using this valuable resource to better understand how these objects evolved since their formation billions of years ago.
As reported in Nature Astronomy, an international team of researchers looked at 300 nearby galaxies from the CALIFA (Calar Alto Legacy Integral Field Area) survey – a project using the Calar Alto telescope in Spain – and from the survey, were able to work out if the stars were moving in regular settled orbits or in a more chaotic way. By observing the stars' orbits they could "read" the history of the galaxies.
All stars in every galaxy are moving in multiple different ways, but by comparing how many stars are experiencing each type of motion, astronomers can reconstruct what galaxies have experienced in the past. A galaxy merger, for example, would lead to a more erratic movement of stars while a circular orbit indicates a more serene history. Being able to work out the orbits of stars is not straightforward though. We can work out the motion for many stars inside our own galaxy but when it comes to galaxies millions of light-years away, this approach is not feasible.
“The motion of every single star is not directly observable in external galaxies. External galaxies are projected on the observational plane as an image and we cannot resolve the discrete stars in it,” lead author Ling Zhu, from the Max Planck Institute for Astronomy, said in a statement. “The CALIFA survey uses a recently developed technique, integral field spectroscopy, which can observe the external galaxies in such a way that it provides the overall motion of stars. Thus, we can get kinematic maps of each galaxy.”
As stars move, their visible light can be blue-shifted or red-shifted depending if they are coming toward us or going away from us. This shift, called the Doppler effect, is measurable and the CALIFA survey is capable of doing so in multiple regions of each galaxy. This is the base for the kinematic maps that the team has produced.
The team was able to show a clear relationship between mass and type of orbits. They discovered that galaxies that have about 10 billion solar masses worth of stars tend to be dominated by the more ordered orbits, while much bigger galaxies have more chaotic orbits. This independently confirms an expectation in galaxy evolution. Bigger galaxies have increased their size through mergers so they must have more chaotic orbits.
“This is the first orbit-based mass sequence across all morphological types," added co-author Glenn van de Ven, form the European Southern Observatory. "It includes flourishing information of a galaxy's past, basically whether it had been a quiet succession of only smaller mergers or shaped by a violent major merger. Further studies are needed to understand the details.”
This new way of reading the history of galaxies could be very useful for testing against cosmological simulations of galaxies.