Astronomers have excavated a galactic tomb –a stunning picture of our home galaxy’s successive mergers with neighboring galaxies. Recent discoveries by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) revealed the “archaeological” record embedded in hundreds of thousands of stars that unveiled the assembly history and evolution of our Milky Way Galaxy.
“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 video below shows a computer simulation of a galaxy like the Milky Way. The movie fast-forwards through simulated time from 13 billion years ago to today. The main galaxy grows as many small galaxies merge with it. Heracles resembles one of the smaller galaxies that merged with the Milky Way early in the process. (Created by Ted Mackereth based on the EAGLE simulations).
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.
The Last Word –“The Alpha Elements”
The main difference in chemical composition between Heracles and the stars that were formed in the Milky Way regards mostly the abundances of iron and those of so-called “alpha elements”, wrote Ricardo Schiavon in an email to The Daily Galaxy. Those are elements whose nuclei are made of the gluing together of a number of Helium nuclei (aka, an “alpha particle”, thus the terminology). Oxygen and magnesium are good examples of alpha elements. When we compare the abundance of alpha elements with that of iron in stars from a galaxy we can get a sense of how the galaxy evolved, in particular how many stars it formed, and how quickly it did so. When we compare the alpha and iron abundances of Heracles with those of the Milky Way stars, we find that star formation in Heracles was brought to an end abruptly, which we interpret as being due to the collision with the Milky Way. Galaxies form stars out of the gas that is bound gravitationally to them. When a dwarf galaxy such as Heracles collides with a massive galaxy such as the Milky Way, it loses its gas and as a result stops forming stars. The stars that already lived in Heracles are then mixed with those formed in the Milky Way, and we have to use their chemical compositions (and in some cases there orbital properties) to distinguish them today.”
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