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Cosmic Choreography

Andromeda Galaxy

Thirteen dwarf galaxies are lying in an extremely thin plane, 45,000 light years thick, but immense in size.

By Geraint Lewis

A newly discovered form of circle dancing is perplexing astronomers, not due to its complex choreography but because it's unclear why the dwarf galaxies are dancing in a ring around the much larger Andromeda Galaxy.

In January I was an author of a paper that appeared in the journal Nature. This paper, which is the culmination of more than decade of study of our nearest large cosmic companion, the Andromeda Galaxy, uncovered something quite unexpected: that small galaxies accompanying Andromeda are dancing together in a vast plane.

While spectacular, the existence of this plane seems to present a major challenge to our ideas of how the universe actually works. But to understand this we need to begin with what we do know about the cosmos.

Deep images of the sky reveal that the universe contains billions of galaxies. Some, like our own Milky Way, are immense, containing hundreds of billions of stars. Most galaxies, however, are dwarfs, being much smaller and with only a few billion stars.

Modern cosmology has proved to be amazingly accurate in predicting how galaxies are scattered through the universe. Instead of being randomly thrown about, galaxies like to live together, some in clusters of 1000 individual systems, but most in groups of tens and hundreds.

The Milky Way is located in the “Local Group”, a small patch of the universe it inhabits with the similar-sized Andromeda Galaxy and a smaller spiral known as the Triangulum Galaxy. Accompanying these larger systems are almost 100 dwarf galaxies. Those of us lucky enough to be living in the Southern Hemisphere can clearly see two of these dwarfs in the night sky – the Large and Small Clouds of Magellan.

These many Local Group dwarfs tend to be grouped around the larger galaxies, precisely as predicted by our understanding of the expansion of the universe and the growth of cosmic structure. But it is precisely these dwarf galaxies that are proving to be a serious headache for the cosmological models they appear to be supporting.

The first problem, known as the “missing satellite problem”, has been known for more than a decade. While the almost 100 dwarf galaxies in the Local Group may sound impressive, this is far fewer than the several thousand predicted by cosmological theories.

Some believe that our diminutive dwarf galaxy population is a crippling blow to the prevailing “cold dark matter” model of galaxy formation. Others, however, think that there is no crisis, with the dwarfs being out there as starless dark matter halos, having lost their gas – the raw material for forming new stars – due to the explosions of super-stars in the very early universe.

This neatly brings us to the result in the Nature paper. For more than 10 years I have been part of a collaboration whose goal has been to map out the extensive stellar halo of Andromeda. This tenuous distribution of stars, which is hundreds of thousands of light years in extent, is the remnants of small galaxies that have strayed too close and been cannibalised by the larger galaxy.

Over the years the collaboration has called on larger telescopes and more sensitive instruments to map out the immense portion of the sky that encompasses Andromeda’s halo. Since 2008 we have used the 3.6 -metre Canada-France-Hawaii Telescope to undertake the Pan-Andromeda Archaeological Survey (PAndAS).

Now that the data collection and processing are complete, the scientific results from PAndAS are starting to flow. As well as the stellar halo, there are large, extended shreds of stars, the ongoing cannibalisation of other systems, and lots of globular clusters – small balls of a million stars living together.

Within PAndAS we found also almost 30 dwarf galaxies orbiting Andromeda, most of which were identified only in the past few years. With the quality of the PAndAS data, PhD student Anthony Conn was able to accurately measure the distances to each of the dwarf galaxies and, for the first time, we knew the three-dimensional distribution of dwarf galaxies surrounding Andromeda.

What do our theoretical models for the structure and evolution of galaxies tell us about the expected distribution of dwarf galaxies around large galaxies? While we know that there are not as many dwarfs as predicted, our models tell us that the ones we do see should be buzzing around randomly like a swarm of angry bees.

So, what do we see with the dwarfs orbiting Andromeda? On the face of it, it seems that the theoretical predictions are borne out, with the dwarfs seemingly distributed at random.

But we decided to look a little deeper and see if there was any underlying structure in the dwarf galaxy population. Instead of considering the entire dwarf population, we instead looked for structure subsamples of galaxies, comparing the actual distributions to many thousands of randomly generated samples.

What we found surprised us, with 13 dwarf galaxies lying in an extremely thin plane, 45,000 light years thick, but immense in size: 1.2 million light years in diameter. Such a configuration would appear extremely rarely in a randomly distributed population of dwarfs. Rather strangely, the edge of this plane points towards the Milky Way.

However, there were more surprises to come. As the dwarfs were initially discovered, they became the targets of the world’s largest telescopes, using the Doppler shift of light to measure the velocities of the individual galaxies. What we found stopped us in our tracks. Those dwarfs north of Andromeda were moving towards us, while those in the south were moving away from us; this vast plane of dwarfs is rotating!

Remember, nothing like this plane is predicted in our cosmological models.

The mystery deepens when we look closer to home where there has been growing evidence that the Milky Way possesses its own plane of dwarf galaxies known as Vast Polar Structure.

Studying our own galaxy’s stellar halo is more challenging than that of Andromeda as we need to image the entire sky, but the evidence is growing stronger that these two galaxies possess unexplained planes of dwarf galaxies. What are they doing there?

Some have suggested that what we see are not normal dwarf galaxies but are actually “tidal dwarfs” – small agglomerations of stars that form from the debris when a large galaxy tears apart a smaller one. However, such dwarfs are transitory and quickly disperse, so such planes should be rare and we would be extremely lucky to see them in both the Milky Way and Andromeda at the same time.

Furthermore, if what we are seeing are truly tidal dwarfs then we are missing even more of the satellite galaxies predicted by our theories for the growth of structure in the universe.

Hence we are faced with a serious cosmological conundrum, a problem that our current theoretical models have to explain if we are to have faith that they are an accurate description of the universe around us. And if they cannot, while we may not have to go right back to the drawing board we will need to question your underlying assumptions on things like the nature of dark matter. Some already think the existence of these planes are telling us that we should be questioning the more fundamental properties of the universe, including the very existence of dark matter and even the action of gravity.

So, what is the solution? Honestly, I don’t know, but it is going to be fun trying to find out.

Geraint Lewis is Professor of Astrophysics at the University of Sydney.