On the evolution of galaxies

Scientists have investigated the properties of globular clusters (GCs) to identify those belonging to the tidal stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph), a satellite galaxy to our Milky Way, deepening understanding of how galaxies form.

It is believed that Dwarf galaxies are the first that formed, many of them merging afterwards to form bigger galaxies like the Milky Way, with the process leaving behind tidal debris such as GCs in the halo of the host galaxies.

However, how GCs formed and the role they play in the development of galaxies is still unknown.

Now, Naira Arakelyan and colleagues from the Russian Academy of Sciences have developed a method for identifying GCs belonging to the Sgr dSph tidal stream, shedding further light on their relationship to the formation of the Milky Way.

“GCs in our galaxy are divided into two groups — clusters that formed within the galaxy and clusters that are of extragalactic origin,” explained Arakelyan. “It is necessary to understand which GCs belonged to satellite galaxies like Sgr dSph to understand how the Milky Way formed and evolved. Furthermore, since some of these GCs are comparable in age to the Universe, they can provide important information about its formation and evolution too.”

Through its long interaction with the Milky Way, the Sgr dSph tidal stream has had many of its GCs stripped away through the gravitational pull of the Milky Way, where they lay scattered throughout its halo. “Therefore, the probability that the stream contains GCs that contributed to the formation of the Milky Way is high,” she said.

To search for GCs belonging to the Sgr dSph tidal stream, the researchers analysed the spatial positions, radial velocities relative to the Galactic Standard of Rest, the proper motions, and the ratio of iron-to-hydrogen, called the ‘age-metallicity,’ of GCs in the stream.

According to Arakelyan, the advantage of this approach “is that it allows us to say with certainty that the selected cluster is of extragalactic origin as it must correspond to all the parameters.”

The work has led to the identification of three categories of GCs, namely, A, B, and C, with GCs in category A proven to be extragalactic in origin, confirming that the Milky Way evolved through the accretion of GCs from the Sgr dSph tidal stream.

“We now plan to investigate the spatial orientation of GCs belonging to tidal streams that lie outside the galaxy and that were involved in the formation of the Milky Way,” Arakelyan said.