The Andromeda Galaxy’s Cannibalistic Feasts Revealed by a Dark Stream

Illustration depicting Dulais Structure globular clusters strewn through Andromeda. Image supplied by Prof. Geraint Lewis/ University of Sydney

An international team of scientists, led by an astrophysicist from the University of Sydney, has found the leftovers of a colossal cannibalistic feeding event in the far outskirts of the Andromeda galaxy.

Honor students Tim Adams from Sydney University and Yuan Li from the University of Auckland discovered a structure of stars, known as a globular cluster (GC), inside the Andromeda galaxy. The structure originated outside of Andromeda and represents remnants of a huge feeding event, providing evidence that galaxies fatten by consuming smaller systems.

It’s not known if the structure is an object like a galaxy because the observed signature could be the result of disruptions from one big object or seven smaller objects.

Astrophysicist Prof. Geraint Lewis from the University of Sydney named the GC, The Dulais Structure, after the Welsh word for black stream. The Dulais Structure looks like a dark stream that is illuminated by star clusters, which orbit differently to all others in the Andromeda galaxy. It’s not known if the structure is an object like a galaxy because the observed signature could be the result of disruptions from one big object or seven smaller objects.

“We’ve come to realise over the last few decades that galaxies grow by eating smaller systems—so little galaxies fall in, they get eaten—it’s galactic cannibalism,” Lewis said in a Sydney university news release.

Previously it was thought that galaxies grew by grazing, consuming small quantities of constellation over a large period of time. However, the Dalais Structure suggests another type of more vigorous growth.

“There will be grazing between the binge eating as we expect a rain of dwarf galaxies from the local universe,” Lewis said in an email to The Epoch Times.

“But instead of a steady rain over billions of years, it looks like it is more bursts of heavy rain between the drizzle,” he said.

Illustration depicting the movement of the Dulais Structure within the Andromeda galaxy. Image supplied by Prof. Geraint Lewis/ University of Sydney

Understanding the Growth of the Universe

Lewis said that this finding provides a clearer picture of how the universe has assembled itself and demonstrates that at least one of the large galaxies in this universe has been sporadically feeding on small galaxies.

“It’s opened a new door in terms of our understanding. But exactly what it’s telling us, I think we still have to work that one out,” he said.

“From our observations, we can deduce that Andromeda has had two bouts of binge eating.”

Lewis said that the most recent binge, they believe, occurred within the last few billion years. But he noted that the older feeding, although hard to date, is estimated to have occurred about eight billion years ago, give or take.

Lewis also noted that researchers hadn’t discerned any particular pattern to the feeding so they hypothesize that the event was a bit more opportunistic, something like groups of smaller galaxies falling in from the larger scale distribution of mass and being captured by the Andromeda galaxy.

The universe is around 13.7 billion years old and is constantly expanding. The timing of the feeding events suggests that they may have occurred when the matter was closer together and more condensed.

“But we are keen to try and work out some detailed ages as this will help reveal just what fell in and where it all fell from, ” he said.

Lewis said that the next question is what was consumed in the feeding events because it doesn’t appear to be a singular thing. He said that it looked like Andromeda had consumed a collection of things, which are all now being slowly torn apart.

Understanding the Lives of Galaxies

Astrophysicists like Lewis observe the Andromeda galaxy to better understand our own home, the Milky Way galaxy, which researchers have trouble observing due to Earth’s vantage point in the Milky Way.

Considering the similar sizes of the Andromeda and Milky Way galaxies, as well as their shared spirality, researchers hypothesize that this discovery may also illustrate what the Milky Way galaxy has done to achieve its large size.

“What we want to know is, has the Milky Way done the same, or is it different? Both of those have interesting consequences for the overall picture of how galaxies form,” Lewis said.

He said that lenticular galaxies, elliptical galaxies, and irregular galaxies all grow in a similar fashion by eating smaller systems. However, the similarities and differences aren’t known.

Lewis said that the feeding habits and maybe even the form of a galaxy might be shaped by its environment, and he added that some elliptical galaxies are thought to have started off as spirals but, after a vigorous bout of feeding, changed in form.

He said at present, researchers have noted that while Andromeda and the Milky Way are similar, they are not identical.

“Andromeda seems larger, with more stuff about it, and it has been proposed that this is precisely due to feeding habits,” he said.

He added that the team of scientists would like to find out whether these binge-feeding events were completely random or if Andromeda was just in the right place at the right time while food fell in.

“Like every new discovery, we end up with more questions than answers!” he said.

Lewis said that this new result provides a clearer picture of how the universe has assembled. Illustration - MENAHEM KAHANA/AFP via Getty Images

Creating Two- and Three-Dimensional Views

Lewis noted that researchers would like to devise a more accurate clock that can ascertain the timing of these massive feeding events, which he argues is necessary to include in models of how galaxies evolve.

Through analyses of data on the speeds and chemistries of the globular clusters that comprise The Dulais Structure, Lewis and his colleagues were able to produce a two-dimensional view of the history.

“The speeds and chemistries of the GCs give us their personality—in images, they just appear as groups of roughly a million stars each, and it can be hard to see if any are related,” Lewis said.

Lewis said that the chemistries could offer information on which objects might have been born together based on the similarity of their chemistry fingerprints, and their speeds can give details on their orbits. He said that knowledge of the GC’s orbits could be used to deduce whether they came from the same direction.

“Just like people, the more information you can gain, the more you can learn how things are related to each other,” he said.

Furthermore, to construct a three-dimensional view of history, the researchers need to understand distances.

“Distances in astronomy are hard because there are no rulers scattered throughout the universe,” Lewis said.

However, Lewis said that there are characteristics in the populations of stars that can be used like a standard light bulb. He said that should the true brightness of these stars, which can be calibrated from nearby stars, be known, distances can be accurately determined.

“However, whilst these stars are intrinsically quite bright, they are faint in the sky, and so we need to call on the power of the Hubble Space Telescope to do these accurately,” he said.

He said that with this information, scientists could begin to run the clock backwards in the hopes of achieving a coherent picture of when things fell in.