Name a galaxy ‘Milky Way’ and every time the moniker is heard it brings up pleasant thoughts of candy bars with chewy, creamy centers. We already know that the center of the Milky Way is actually a supermassive black hole (not something one wants to find in a candy bar) and a new reports brings another unpleasant fact – the galaxy grew in its early days by consuming other galaxy and the ancient fossil remains of one still exist deep inside it like a ghost haunting a cannibal. Time for a new name … or a new candy bar?
"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. 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. APOGEE lets us pierce through that dust and see deeper into the heart of the Milky Way than ever before."
As the world mourns the imminent demise of the Arecibo telescope, others are quickly steeping into the spotlight. Liverpool John Moores University’s Ricardo Schiavon, a co-author of a new study on the ancient fossil galaxy, is referring to the Sloan Digital Sky Surveys' Apache Point Observatory Galactic Evolution Experiment (APOGEE) in New Mexico, whose mission is to chart a historical archeological record of the Milky Way by studying the motions and chemical compositions of its stars. That study, published in the Monthly Notices of the Royal Astronomical Society, has upended the accepted history of the early days of the galaxy with the discovery of the remains of Heracles, a galaxy that appears to have collided with the Milky Way 10 billion years ago.
"Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities. 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."
Likening the search to finding a needle in a haystack (is that old idiom still valid?), lead author Danny Horta, an LJMU graduate student, describes the subtle hints that suggested the existence of the fossil galaxy – named for the ancient Greek hero who was said to have received the gift of immortality when the Milky Way was created – but in a place where these ancient galaxies are never found … in the center of the Milky Way. Most galaxies that old have their remains dispersed among the outer rings of the galaxy that assumed/consumed them, and indeed the stars that were once Heracles make up one-third of Milky Way’s outer halo. But the discovery of so many odd stars in the center contradicted the accepted theory that spiral galaxies like the Milky Way have peaceful births – not violent collisions, explosions and consumptions of nearby neighbors. The press release calls this “a major event in the history of our galaxy.”
"APOGEE is one of the flagship surveys of the fourth phase of SDSS, and this result is an example of the amazing science that anyone can do, now that we have almost completed our ten-year mission."
Karen Masters, spokesperson for SDSS-IV, points out the less obvious but very significant other aspect of this discovery – it came at the end of the 10-year-life of the project, justifying the money and time invested – an important milestone that will help future projects.
So, the discovery of ancient fossilized Heracles in the center of the Milky Way conjures what one might find when biting into a stale candy bar – that tooth-shattering jolt that says you shouldn’t eat what you find under the sofa cushion. Stick with fresh Milky Ways – like the fresh news about the early days of our home galaxy.