Category Archives: dark matter

Vera C. Rubin and a Dark Matter Observatory

Vera Rubin
Dr Vera Cooper Rubin, who (along with a team of observers) confirmed the existence of dark matter through continued observations of galaxy motions.

One of the stories coming from the American Astronomical Society meeting involves a topic that we’ve discussed here before: dark matter. Actually, it focuses directly on one of the astronomers closely associated with determining the existence of this mysterious “stuff”: Dr. Vera C. Rubin.

We’ve all heard of dark matter. It’s a weird, “invisible” material that makes up about a quarter of the mass in the universe. Nobody is quite sure what it is, but astronomers are sure it’s out there. The fact that we know even that much is due largely to Dr. Rubin and her efforts at finding it.

The dark matter story begins with a question: why don’t galaxies rotate at the velocity we expect them to? Over many years, Dr. Rubin and her team observed galaxy rotations. They compiled their data into what are called “rotation curves” and noticed that galaxies don’t always rotate the way they were expected to. Why?

Ultimately, the answer was “dark matter”. This is a type of cosmic “stuff” first suggested by Swiss astronomer Dr. Fritz Zwicky as “dunkel materie”. It could constrain the motions of objects in galaxies. It was largely unknown and theoretical then, but its effects are often observed today.

To honor Dr. Rubin’s work, the Large Scale Synoptic Telescope (LSST), currently under construction in Chile, will be renamed the NSF Vera C. Rubin Observatory (VRO). It’s the first national observatory named after a woman. VRO will be heavily involved in the search for dark matter and the even-more-mysterious dark energy.

The Vera C. Rubin Observatory on Cerro Pachón, in Chile, as seen on Dec. 18, 2019. Credit: LSST/Vera Rubin Observatory.
The newly renamed Vera C. Rubin Observatory after sunset in December 2019. Credit: LSST/Vera Rubin Observatory.

Tracking Galaxy Motions Leads to Dark Matter

Vera Cooper Rubin began her astronomy career at Vassar college at a time when women weren’t expected to do science. She went to Cornell University and Georgetown Universities, gaining her Ph.D. in 1954. Her thesis suggested that galaxies clumped together in clusters. Today, that’s accepted observational fact. Back then, galaxy clusters were still a theoretical and not-too-popular idea. Still, throughout her career, Dr. Rubin studied galaxies both individually and in clusters, and charted the motions of their stars.

In the 1960s, Rubin began working at the Carnegie Institution of Washington’s Department of Terrestrial Magnetism. Rubin’s work focused directly on galactic and extragalactic dynamics. Those subjects deal with the motions of galaxies both singularly and in clusters. In particular, Dr. Rubin studied the rotation rates of galaxies and the material in them. As I mentioned above, the team promptly discovered a puzzle: the predicted motion of a galaxy’s rotation didn’t match the observed rotation.

To understand why that might seem strange, it’s important to understand that galaxies do rotate. Astronomers expected all the material in a galaxy to rotate at rates dependent on their distance from the center. However, if they do it fast enough, they could fly apart, IF the combined gravitational effect of all their stars was the only thing holding them together. In her team’s observations, the rotation of some galaxies didn’t perform according to expectations. It implied that the mass of their stars wasn’t enough to keep them “together”. Why why didn’t they come apart? Something else had to be holding them together. The difference between the predicted and observed galaxy rotation rates was dubbed the “galaxy rotation problem”.

Dark Matter as a Vera C. Rubin’s Solution

Rubin and others decided that there was some kind of unseen mass in or around the galaxy. It was holding the galactic pieces and parts together. Based on many observations made by Rubin and her colleague Kent Ford, the mystery began to unravel. It turned out that galaxies must have at least ten times as much “invisible” mass as they do visible mass in their stars and nebulae.

Calculations showed that this invisible mass really existed. And, it might be that “dark matter” that Zwicky first suggested in the 1930s. Zwicky himself was scoffed at when he came up with the idea and later on, Rubin and her colleagues faced a lot of the same skepticism. Yet, it made sense when they invoked dark matter as an explanation for the odd rotation curves they calculated.

Dark Matter Focus from Chile

Dr. Vera C. Rubin (who died in 2016) spent much of her later life working on the dark matter problem. For that reason, the renaming of LSST in her honor is appropriate. The Vera Rubin Observatory will begin official operations in 2022. Its telescope will be mated to a state-of-the-art 3200-megapixel camera. Together with other instruments, astronomers will use it to study the universe in search of dark matter. It will also look for evidence of dark energy, study the bodies of the solar system, explore the transient optical sky, and map our home galaxy, the Milky Way. It’s a perfectly fitting tribute to a woman who persisted on research that others felt wasn’t important; not only WAS it important, it led to a new area of astrophysical research.

We Come From the Stars

This is Our Home Galaxy, and a Couple of Neighbors

As the Milky Way rises over the horizon at the European Southern Observatory, its companion galaxies also come into view. The Large Magellanic Cloud (LMC) at far left lies about 160,000 light-years away, while the Small Magellanic Cloud (SMC, above and to the right of the LMC) lies about 200,000 light-years away. New simulations show that the LMC stole stars from the SMC when the two galaxies collided 300 million years ago. Microlensing events that have been observed are due to LMC stars passing in front of a stream of stars pulled from the SMC.
Credit: ESO/Y. Beletsky

When you look out at the night sky, you’re looking at our ancient home. Yes, Earth is our current home. But, in the grand scheme of things, the galaxy — and all the elements that make it are also our home.  The elements that make up our bodies, our planets, and our star all were either created in the Big Bang (hydrogen, for example), or inside other stars (carbon, oxygen, nitrogen, etc.).  Multiple generations of stars have lived and died in the galaxy, and we are the resulting “star stuff”.

But, there’s more than star stuff out there.  There are mysterious things that may tell astronomers more about types of matter in the cosmos and distribution of that matter throughout the universe.

Astronomers have been studying one of those two irregular-looking clouds of stars that appear just below our galaxy in this image to understand a category of objects called MACHO (Massive Compact Halo Objects). These were thought to be things about the mass of a star that were so faint they couldn’t be easily detected. Surveys of this region of our galactic neighborhood have been underway to see if MACHOs could be part of that mysterious collection of “stuff” called “dark matter” that seems to be an incredibly important part of the universe.

In order for MACHOs to make up dark matter, they must be very faint. To even decide if they’re “there”, astronomers looked for a phenomenon known as microlensing. During a microlensing event, a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten. If a MACHO does this, then they’d know a little bit more about the object.

By studying the LMC, astronomers hoped to see MACHOs within the Milky Way lensing distant LMC stars. The number of microlensing events observed by various teams was smaller than needed to account for dark matter, but much higher than expected from the known population of stars in the Milky Way. This left the origin of the observed events a puzzle and the existence of MACHOs as exotic objects a possibility.

Instead of MACHOs, a trail of stars removed from the SMC could well be responsible for the microlensing events. How do astronomers know this? They’ve done computer simulations showing that the most likely explanation for the observed microlensing events was an unseen population of stars removed by the LMC from its companion, the SMC. Foreground stars in the LMC are gravitationally lensing the trail of removed stars located behind the LMC from our point of view.

Although the evidence for the trail of lensed stars is persuasive, they haven’t been directly observed yet. That will take time, since these could be faint. A number of teams are searching for the signatures of these stars within a bridge of gas that connects the Magellanic Clouds. The computer models used to simulate the trail will point the way for astronomers to find the other “stuff” that makes up the galaxies… and intergalactic space.