The velocities of the stars near the Sun vary with stellar age and typically correspond to galaxy evolution mechanisms characterized by astrophysical theories. The distributions of these velocities are quite consistent for different types of galaxies; however, any unexpected behavior of stars in this aspect may indicate ancient events of colossal scale, such as galaxy mergers. In fact, one of such disturbances has been detected in our Milky Way galaxy.



An international team of scientists from Germany, Australia, Slovenia, US, UK, France, Netherlands, Italy and Canada presented a study based on the analysis of velocity dispersion of a sample of giant stars from the spectroscopic RAdial Velocity Experiment (RAVE) survey. The researchers found that the mentioned velocity dispersion decreases significantly for the stars that had formed during the first billion years of galaxy’s life. The paper describing this study has been published at arXiv.org.

What do the outcomes of this observation mean? The authors of the study specify that the differences in the age-related stellar velocity characteristics of the stars residing in approximately the same region as our Sun may be explained by perturbations caused by massive mergers of galaxies which took place in the early Universe and which were significantly more frequent compared to nowadays.

The scientists note that the possible galaxy merger has had the strongest effect on the outer parts of the galaxy disc, and this event has also caused the radial migration of the stars with cooler kinematics (i.e. older stars) from the inner disc. The team also used a numerical simulation on the basis of a developed chemodynamical model to compare the results against the RAVE observations. The comparison makes it possible to predict that the last massive merger took place approximately 8-9 billion years ago and the mass ratio of the colliding galaxies should have been approximately 1:5.

The results of this study also provide a valuable glimpse into the formation and evolution of the Milky Way galaxy. Furthermore, the authors argue that it is possible to study older stars located in the outer galactic disc and in the vicinity of our Sun due to the merger-caused radial stellar migration, especially considering the fact that astronomers look for “suitable old age star” candidates in the inner parts of the Milky Way, where such observations are relatively difficult to implement.

The team hopes that their study would be confirmed with the availability of information characterizing stellar ages that could be obtained in the near future from the Gaia mission, which should also provide more data about the dark matter content inside and around our galaxy.

By Alius Noreika, Source: Technology.org