Two New Finds about our Galaxy
I was Born Near a Wanderin’ Star
The hardest galaxy to study in the universe is our own. We’re inside it, and just like trying to figure out the color of a building from a room within it, it’s tough for us to see the shape and density of our galactic home while being stuck inside of it. We have to use indirect evidence to infer galaxy characteristics for the Milky Way. Surveys of stars, particularly those done using infrared-sensitive instruments, tell us a lot about the distribution of stars and other mass in our galaxy. That’s because infrared astronomy lets us peer through the dark clouds and dusty regions of our galaxy, and we can study the chemical distribution of stars throughout the galaxy.
And, from those data sets, we can extrapolate to the areas of the galaxy we can’t see in optical wavelengths of light. We can make models using real data, and challenge long-held suppositions about the state of our galaxy and the stars it contains.
One such long-standing scientific theory holds that stars tend to hang out in the roughly the same galactic neighborhood where they were born. Some astrophysicists decided to look into whether that theory is true, and their simulations show that, at least in galaxies similar to our own Milky Way, stars such as the Sun can migrate great distances from their birthplaces.
If the Sun has actually fled the old ‘hood, that tells us that perhaps there are many regions in galaxies where stars that support planets — and life — could exist in reasonable safety (so-called “habitable zones”).
The group of astrophysicists, led by Rok Roskar, a grad student at the University of Washington, used more than 100,000 hours of computer cluster time at UW and the University of Texas supercomputer site (which I had a chance to visit earlier this year), to simulate the formation and evolution of a galaxy disk from material that had swirled together 4 billion years after the Big Bang.
The orbits of some stars in the galaxies might get larger or smaller but tend to remain very circular after hitting a massive spiral density wave that propagates through the arms of a galaxy. The Sun has a nearly circular orbit, so the findings mean that when it formed 4.59 billion years ago (about 50 million years before Earth did), it could have been either nearer to or farther from the center of the galaxy, rather than halfway toward the outer edge where it is now. As it encountered the density waves that accompany the spiral arms, its orbit around the galaxy would have been shaped by the encounters.
Migrating stars also help explain a long-standing problem in understanding the chemical mix of stars in the neighborhood of our solar system. Simply put, our region of the galaxy has long been known to be more mixed and diluted than you might expect if our stellar neighbors had spent their entire lives where they were born. It makes sense when you think about it: if a given star cloud births many new stars, they’ll all have the same (roughly) chemical composition because they all come from the same cloud. Any stars with different chemical abundances would have to be from another star cloud, maybe one quite far away. If stars from different locations have migrated here, then the Sun’s neighborhood has become a more diverse and interesting place over the past few billion years.
The scientists plan to run a range of simulations with varying physical properties to generate different kinds of galactic disks, and then determine whether stars show similar ability to migrate large distances within different types of disk galaxies.
Stars are the “small-scale” matter that inhabits galaxies. The galaxies themselves (the giant mass of stars, dust clouds, and so forth) are shaped by much larger forces than the wanderings of stars over galactic time. The large-scale evolution of a galaxy and its shape (its morphology) appears to be influenced by dark matter. At least, that’s the current thought, and as scientists find more evidence of dark matter, they are starting to get a clearer picture of its distribution and the effect it has on galaxy morphology.
Unlike the stars, gas and dust that make up what scientists call “baryonic” or “normal” matter, dark matter is invisible. We know its there because it has a gravitational influence on its surroundings. Physicists suspect that it makes up 22% of the mass of the universe (compared with the 4% of normal matter and 74% comprising the mysterious ‘dark energy’). But, despite the fact that it seems to be everwhere, no one is sure what dark matter consists of.)
Today, an international team of scientists is predicting that the Milky Way contains a disk of dark matter. In a paper published in Monthly Notices of the Royal Astronomical Society, astronomers Justin Read, George Lake and Oscar Agertz of the University of Zurich, and Victor Debattista of the University of Central Lancashire used the results of a supercomputer simulation to infer the presence of this disk; they haven’t directly imaged it, they are deducing its existence from its effects on nearby matter and then using their work to create a computer simulation of the galaxy and disk. This simulation could allow physicists to directly detect and identify the nature of dark matter for the first time.
How Does this Work?
For a long time, scientists thought that dark matter forms in roughly spherical lumps called “halos” that envelope galaxies. There is one that surrounds the Milky Way. However, this “standard’ theory is based on supercomputer simulations that model the gravitational influence of the dark matter alone. The new simulation takes into account the gravitational influence of the stars and gas that also make up our galaxy.
Stars and gas are thought to have settled into disks very early on in the life of the universe and this affected how smaller dark matter halos formed. The team’s results suggest that most lumps of dark matter in our local galactic neighborhood merged to form a halo around the Milky Way. But the largest lumps were preferentially dragged towards the galactic disk and were then torn apart. This is what likely created the disk of dark matter within the galaxy.
Detecting the two different dark matter components depends on the density of each, according to Justin Read, the lead scientist on this project. “The dark disk only has about half of the density of the dark matter halo, which is why no one has spotted it before,” he says. “However, despite its low density, if the disk exists it has dramatic implications for the detection of dark matter here on Earth.
The Earth and Sun move at some 220 kilometers per second along a nearly circular orbit about the center of the galaxy. The dark matter halo does not rotate. This means that if we could see the dark matter halo as we rotated through the galaxy, it would appear as if we had a “wind” of dark matter flowing towards us at great speed. By contrast, the disk of dark-matter particles rotates along with us, and so the “wind” from the dark disk is much slower.
This abundance of low-speed dark matter particles could be a real boon for researchers because they are more likely to excite a response in dark matter detectors than fast-moving particles. Is there such a detector? Yes–it’s called XENON (located at the Gran Sasso Underground Laboratory in Italy) and it’s built to directly detect dark matter particles. This detector is very sensitive, and if there’s dark matter out there, particularly the low-speed type in the suggested disk, XENON should be able to see it. This new research raises the exciting prospect that the dark disk of our galaxy and its associated dark matter could be directly detected in the very near future. Will it tell us what dark matter is made of? Let’s hope so!
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Here’s how the Chandra folks describe the mechanics of the collision: two galaxy clusters, each a quadrillion times the mass of the Sun, collided to form the system known as MACS J0025.4-1222. When they merged at speeds of millions of miles per hour, the hot gas in each cluster collided and slowed down, but the dark matter did not.