The history of the Milky Way continues to be completed.
A huge galaxy hidden within our Milky Way and we had not seen it until now? That’s how it is. Thanks to the analysis of data from the Apache Point Observatory’s Galactic Evolution Experiment (APOGEE), within the Sloan Digital Sky Survey project, a team of astronomers has discovered in the depths of our galaxy the remains of a galaxy that would have been absorbed some time ago. 10,000 million years.
This cosmic remnant, nicknamed ‘Heracles’ after the hero of ancient Greek mythology, represents one third of the Milky Way’s spherical ‘halo’ of stars and gas. And, although we had already found remains of other ancient galaxies, it had always been in the outer halo of the Milky Way. However, the remains of this merger between Heracles and our galaxy are found in the heart of the Milky Way, as it is the oldest and most massive “galactic fossil” of any previously discovered.
This galaxy would have collided with ours, according to the study published in the Monthly Notices of the Royal Astronomical Society magazine, about 10 billion years ago, which gives us a new clue about how our own galaxy grew and developed during its first years of cosmic life.
Hard to find
We have already commented that it is not the first fossil galaxy that we discovered in the Milky Way, but it is the most elusive: it is deeply embedded in our own galaxy and it is difficult to analyze the native stars of our galaxy, especially when they are hidden in the center of the same.
“To find a fossil galaxy like this one, we had to look at the detailed chemical composition and motions of tens of thousands of stars. That’s especially difficult for stars in the center of the Milky Way to do because they are hidden from view by clouds of interstellar dust. APOGEE allows us to go through that dust and see more deeply than ever the heart of the Milky Way ”, explains Ricardo Schiavon of the John Moores University of Liverpool (United Kingdom) and co-author of the work.
In a cosmological context there are barely 100 million stars, but it represents almost half the mass of the entire Milky Way halo. It’s huge.
“It is necessary to examine such a large number of stars to find unusual stars in the densely populated heart of the Milky Way, which is like finding needles in a haystack,” says Danny Hort, leader of the study.
That’s when they discovered that of the tens of thousands of stars observed, a few hundred had strikingly different chemical compositions and speeds. The stars in question were so low in metals compared to their neighbors and were moving so fast that the authors concluded that they must have originated in another galaxy.
“By studying them in detail, we were able to trace the precise location and history of this fossil galaxy,” the authors state.
The stars in the Heracles galaxy had a combined mass about 500 million times that of the Sun. That’s twice the stellar mass of the next largest consumed galaxy, known as the Gaia Sausage (which merged with the Milky Way ago). about 8,000-11,000 million years).
Considering that the stars that originally belonged to Heracles represent a third of the mass of the entire Milky Way halo , this collision had to be a very important event in our history, hence this new discovery is so special.
Referencia: “Evidence from APOGEE for the Presence of a Major Building Block of the Halo Buried in the Inner Galaxy,” Danny Horta et al., 2020 Nov. 20, Monthly Notices of the Royal Astronomical Society: arxiv.org/abs/2007.10374].
