One of the smaller galaxies that went into making up our galaxy has been found hiding near the core of the Milky Way. The discovery suggests our galaxy had an unusually turbulent start to its life by the standards of spiral galaxies.
Astronomers have known for a long time that large galaxies like the Milky Way are built out of smaller ones. However, it is only recently we have been able to identify traces of former galaxies within our own. Now technology has evolved to the point where we can track the movements and spectra of vast numbers of stars at the same time, results in this field are coming thick and fast. Just last week a study of globular clusters provided evidence for the size of the mysterious Kraken Galaxy, and the time at which the Milky Way consumed it.
Now another component galaxy has been added to the story, but this one has a major difference from the others we have seen, with many of its stars located within 13,000 light-years of the Milky Way's core, rather than dispersed across the halo. The obscuring clouds of dust between us and the center of the galaxy have helped prevent us from seeing these stars clearly before, and therefore identifying their origins.
“To find a fossil galaxy like this one, we had to look at the detailed chemical makeup and motions of tens of thousands of stars,” said Dr Ricardo Schiavon of Liverpool John Moores University in a statement. To do that, looking towards the Milky Way's center was once impossible, but the Apache Point Observatory Galactic Evolution Experiment (APOGEE) changed that, using infrared observations to peer through the obscuring dust.
“Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities,” graduate student Danny Horta said. The stars in question were so low in metals compared to their neighbors, and so fast-moving, the authors concluded they must have originated in another galaxy.
In Monthly Notices of the Royal Astronomical Society Horta and co-authors have named the former galaxy Heracles, a nod to the Greek myth where the Milky Way was formed when the demigod (Hercules is his Roman incarnation) was pushed away from Hera's breast.
Heracles is thought to have been absorbed into our galaxy around 10 billion years ago, close in time to the capturing of several other small galaxies, suggesting an unusually active period for a galaxy of this type. The paper proposes its stars had a combined mass around 500 million times that of the Sun. That's twice the stellar mass of the next largest consumed galaxy, known as Gaia-Enceladus/Sausage.
Former galaxies make up much of the Milky Way's halo, and the authors estimate remnants of Heracles account for a third of the halo's stars. Unlike the other small galaxies, however, many Herculean stars have remained together, concentrated into part of the bulge that sits above and below the core.