Galaxies like ours are the result of many, many mergers. Exactly how galaxies form is a mystery, but we know these vast seas of stars often bump into each other and mix to form something new, and bigger.
Just last week scientists developed the first family tree of our home galaxy, but another paper published this week in the Monthly Notices of the Royal Astronomical Society claims to have discovered a hitherto unknown “fossil galaxy” hidden in the inner depths of our Milky Way.
It’s thought to have collided with the Milky Way about 10 billion years ago. The Milky Way is 13.5 billion years old, but this collision appears to have been responsible for adding a lot of stars.
The fossil galaxy, the remains of which were found in the dense halo of stars at the center of our galaxy, has been named “Heracles” by the discoverers. Heracles was an ancient Greek hero who in legend received the gift of immortality when the Milky Way was created.
Heracles is figured to be twice the mass of the recently discovered Gaia-Enceladus-Sausage galaxy, which merged with the Milky Way some 9 billion years ago, and was thought to have been the biggest collision event.
How was this new fossil galaxy found? If it’s all around us then why didn’t we see it before?
It all comes down to new data on what the stars are made of, and how they’re moving through the Milky Way.
“To find a fossil galaxy like this one we had to look at the detailed chemical make-up and motions of tens of thousands of stars,” said Ricardo Schiavon, a member of the research team, from Liverpool John Moores University. “That is especially hard to do for stars in the center of the Milky Way because they are hidden from view by clouds of interstellar dust.”
They did it by looking at the entire Milky Way at once in infra-red light.
That was done by using two telescopes in both hemispheres. The Apache Point Observatory Galactic Evolution Experiment (APOGEE), a program of the Sloan Digital Sky Survey III, has telescopes at Apache Point Observatory in New Mexico, USA and at Las Campanas Observatory in Chile.
APOGEE looks at stars in near-infrared light, which doesn’t get obscured by dust, and in the last decade has compiled data on both the chemical make-up and the velocities of half a million stars right across the Milky Way.
Including, crucially, stars in our galaxy’s previously dust-obscured, and densely populated, center of the Milky Way. “APOGEE lets us pierce through that dust and see deeper into the heart of the Milky Way than ever before,” said Schiavon.
“Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities,” said lead author Danny Horta, a graduate student at Liverpool John Moores University. “These stars are so different that they could only have come from another galaxy. By studying them in detail, we could trace out the precise location and history of this fossil galaxy.”
Finding the evidence for this ancient galaxy buried within the Milky Way was “like finding needles in a haystack,” added Horta.
Tantalizing though it may be, the collision between a smaller, younger Milky Way and this “Heracles Galaxy” must have been a major event since stars originally belonging to Heracles account for about a third of the mass of the entire Milky Way halo.
The stars of Heracles are hence now thought to be a major building block of our Galactic halo.
The results, of course, was the massive spiral galaxy we know today—our home.
Wishing you clear skies and wide eyes.