Deep images of the sky reveal that the universe contains billions of galaxies. Some, such as 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 seem to live together, some in clusters of a thousand individual systems, but most in groups of tens or hundreds.
But new research on dwarf galaxies by my colleagues and me–published in the journal Nature–seems to present a major challenge to ideas of how the universe actually works.
After more than a decade of study, we’ve discovered that small galaxies that accompany the nearest spiral galaxy to our own–the Andromeda Galaxy–are dancing together in a vast plane.
To understand why this is significant, we need to begin with what we know about the universe and our own cosmic backyard.
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 dwarf galaxies in the night sky, namely 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.
Where Are All the Galaxies?
The first problem, known as the “missing satellite problem,” has been known for more than a decade. While the almost-100 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–that is, the idea that dominant component of mass in the universe is invisible to us, the scary-sounding dark matter which shepherds atoms into the stars and galaxies that we see.
Others, however, think there is no crisis, suggesting the dwarfs are 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 our result in the Nature paper.
For more than 10 years I have been part of a collaboration that has been trying to map out the extensive stellar halo of Andromeda.
This tenuous distribution of stars, which extends hundreds of thousands of light years away from the Andromeda Galaxy, is made from the remnants of small galaxies that have strayed too close and have been cannibalised by the larger galaxy.
Over the years, our collaboration has used large telescopes and sensitive instruments to map out the immense portion of the sky that encompasses Andromeda’s halo. Since 2008, we have used the 3.6m Canada-France-Hawaii Telescope to undertake the Pan-Andromeda Archaeological Survey (or, more cutely, PAndAS).
Now that the data-taking and processing is 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 cannibalization of other systems and lots of globular clusters – small balls of a million stars living together.
The dwarf galaxies were found to be rotating in a giant plane around Andromeda. Nature
Within PAndAS we found 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.
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