Astronomers have excavated a galactic tomb: new discoveries, the most recent by the Sloan Digital Sky Surveys’ Apache Point Observatory Galactic Evolution Experiment (APOGEE) that observes the “archaeological” record embedded in hundreds of thousands of stars to explore the assembly history and evolution of the Milky Way Galaxy, have unveiled a stunning picture of our home galaxy’s successive mergers with neighboring galaxies.
“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,” said astronomer Danny Horta-Harrington from Liverpool John Moores University (LJMU) in the UK, about the effects of a fossil galaxy, Heracles, named after the ancient Greek hero who received the gift of immortality, that may have collided with the Milky Way ten billion years ago, when our galaxy was still in its infancy, changing understanding of how it evolved into the galaxy we see today.“
The Galactic Tomb
An all-sky image above shows the stars in the Milky Way as seen from Earth. The colored rings show the approximate extent of the stars that came from the fossil galaxy known as Heracles. The small objects to the lower right of the image are the Large and Small Magellanic Clouds, two small satellite galaxies of the Milky Way. Because galaxies are built through mergers of smaller galaxies across 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 since our Galaxy built up from the inside out, finding the earliest mergers requires looking at the most central parts of the Milky Way’s halo, which are buried deep within the disc and bulge.
The remnants of Heracles account for about one third of the Milky Way’s spherical halo. But if stars and gas from Heracles make up such a large percentage of the galactic halo, why didn’t we see it before, asks the researchers at APOGEE, who discovered the “fossil galaxy” hidden in the depths of our own Milky Way. The answer, they say, lies in its location deep inside the Milky Way.
A barred galaxy like the Milky Way. Just left of center, extending about 1/5 of the way out, is a series of red ellipses marking the location of Heracles. The location of the Sun is marked about halfway out, with a scale bar to the Sun saying 26,000 light-years.
An artist’s impression of what the Milky Way might look like seen from above. The colored rings show the rough extent of the fossil galaxy known as Heracles. The yellow dot shows the position of the Sun.
The Chemical Makeup and Motions of Tens of Thousands of Stars
“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,” says Ricardo Schiavon from LJMU, a key member of the research team. “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.”
APOGEE does this by taking spectra of stars in near-infrared light, instead of visible light, which gets obscured by dust. Over its ten-year observational life, APOGEE has measured spectra for more than half a million stars all across the Milky Way, including its previously dust-obscured core.
Needles in a Haystack
Horta, the lead author of the paper announcing the result, explains, “examining such a large number of stars is necessary to find unusual stars in the densely-populated heart of the Milky Way, which is like finding needles in a haystack.” To separate stars belonging to Heracles from those of the original Milky Way, the team made use of both chemical compositions and velocities of stars measured by the APOGEE instrument.
Stars originally belonging to Heracles account for roughly one third of the mass of the entire Milky Way halo today – meaning that this newly-discovered ancient collision must have been a major event in the history of our Galaxy. That suggests that our Galaxy may be unusual, since most similar massive spiral galaxies had much calmer early lives.
And this new age of discovery will not end with the completion of APOGEE observations. The fifth phase of the SDSS has already begun taking data, and its “Milky Way Mapper” will build on the success of APOGEE to measure spectra for ten times as many stars in all parts of the Milky Way, using near-infrared light, visible light, and sometimes both.
Image credit: Danny Horta-Darrington (Liverpool John Moores University), ESA/Gaia, and the SDSS