Hidden ‘fossil’ galaxy is missing link in Milky Way formation

Much of the Milky Way was formed 10 billion years ago by a massive collision with a relatively small galaxy dubbed Heracles, according to scientists in the UK.

And they say it is this merger which endows our galaxy with its distinctive ‘halo’.

For the first time detailed chemical compositions and precision velocities have been obtained for ‘hidden’ stars in the central Milky Way, which have guarded one of the last secrets of the Milky Way’s origins.

“Most of the Milky Way mergers can be spotted in the halo of the Milky Way, a huge thin cloud of stars enveloping the entire galaxy,” explains Dr Ricardo Schiavon, of Liverpool John Moores University’s Astrophysics Research Institute.

Buried from sight

“However, the very earliest episodes of galaxy merging cannot easily be picked out, because the physical clues are buried in the centre of the halo and extremely difficult to observe.”

He said they believe Heraklis is what is known as a fossil galaxy, a fossil left over from the early universe.

Dr Schiavon’s team, along with the Royal Astronomical Society, have been conducting a project called the Apache Point Observatory Galactic Evolution Experiment (APOGEE), an exhaustive survey of data focussed on around half a million stars in this ‘shadow zone’.

Using state-of-the-art infrared, APOGEE has been able to pierce the dust and see deeper than ever into the heart of the Milky Way. To separate stars belonging to Heraklis from those of the original Milky Way, the team made use of both chemical compositions and velocities of stars measured by the APOGEE instrument.

Needles in haystack

PhD researcher Danny Horta from LMJU, the lead author of the paper, explains: “Examining such a large number of stars is necessary to find unusual stars in the heart of the Milky Way, which are like needles in a haystack.”

“Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities,” Horta said. “They are so different that they could only have come from another galaxy. By studying those stars in detail, we could trace out the precise location and history of this fossil galaxy.”

“To ‘catch sight’ of that galaxy is awesome,” added Dr Schiavon. “It is really small in the cosmological context - only 100 million stars - but accounts for almost half the mass of the entire Milky Way halo today.”

Sky survey

Karen Masters, at the Sloan Digital Sky Survey IV, which runs APOGEE said: “APOGEE is one of our flagship surveys and this result is an example of the amazing science that anyone can do now that we have almost completed our six-year mission.”

The result is published today, November 20, 2020 in The Monthly Notices of the Royal Astronomical Society.