Astronomers discover new ‘fossil galaxy’ buried deep in the Milky Way

Scientists working with data from the Galactic Evolution Experiment (APOGEE) from Sloan Digital Sky Surveys’ Apache Point Observatory have discovered a “fossil galaxy” hidden deep within our own Milky Way.

This result, published today in Monthly notices from the Royal Astronomical Society, can shake our understanding of how the Milky Way came to be the galaxy we see today.

The proposed fossil galaxy may have collided with the Milky Way ten billion years ago, when our galaxy was still in its infancy. Astronomers named him Heracles, after the name of the ancient Greek hero who received the gift of immortality during the creation of the Milky Way.

Heracles’ remains make up about a third of the spherical halo of the Milky Way. But if Heracles’ stars and gas make up such a large percentage of the galactic halo, why haven’t we seen it before? The answer lies in its location deep in the Milky Way.

“To find a fossil galaxy like this, we had to look at the detailed chemical makeup and motions of tens of thousands of stars,” says Ricardo Schiavon from John Moores University in Liverpool (LJMU) in the UK, a key research member. team. “This is particularly difficult to do for the stars in the center of the Milky Way, because they are hidden from view by clouds of interstellar dust. APOGEE allows us to pierce this dust and see more deeply than ever the heart of the Milky Way. “

APOGEE does this by taking spectra of stars in near infrared light, instead of visible light, which is obscured by dust. During its ten-year observational life, APOGEE has measured the spectra of more than half a million stars throughout the Milky Way, including its previously obscured core with dust.

LJMU graduate student Danny Horta, lead author of the article reporting the result, explains: “It is necessary to examine so many stars to find unusual stars in the densely populated core of the Way. milky, which is like finding needles in a haystack. “

To separate stars belonging to Heracles from those in the original Milky Way, the team used both chemical compositions and star velocities measured by the APOGEE instrument.

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

Because galaxies are built through mergers of smaller galaxies over time, the remnants of older galaxies are often spotted in the Outer Halo of the Milky Way, a huge but very sparse cloud of stars enveloping the main galaxy. . But because our galaxy has been built from the inside out, finding the first mergers requires examining the more central parts of the Milky Way’s halo, which are buried deep within the disk and bulge.

The stars originally belonging to Heracles today make up about a third of the mass of the entire Milky Way halo – which means that this recently discovered ancient collision must have been a major event in our galaxy’s history. . This suggests that our galaxy may be unusual, as most similar massive spiral galaxies have had much calmer first lives.

“As a cosmic home, the Milky Way is already special to us, but this ancient galaxy buried within makes it even more special,” says Schiavon.

Karen Masters, spokesperson for SDSS-IV comments: “APOGEE is one of the flagship surveys of the fourth phase of SDSS, and this result is an example of the incredible science that anyone can do, now that we have almost completed our ten – year mission. “

And this new era of discovery will not end with the completion of the APOGEE observations. The fifth phase of SDSS has already started taking data, and its “Milky Way Mapper” will build on APOGEE’s success to measure spectra for ten times as many stars in all parts of the Milky Way, including using near infrared light, visible light, and sometimes both.

Provided by Sloan Digital Sky Survey