Category Archives: cosmology

cosmology, galaxy distributions

Void Riders

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The distribution of galaxies in the region around the Milky Way (in galactic coordinates). Each little dot represents a galaxy of typically 100 billion stars. The colors indicate the relative motions of galaxies with accurately measured distances, with shades of gree and blue indicating motions toward us, and shades of yellow to red indicating motions away from us. (For more information, see the IFA press release.
The distribution of galaxies in the region around the Milky Way (in galactic coordinates). Each little dot represents a galaxy of typically 100 billion stars. The colors indicate the relative motions of galaxies with accurately measured distances, with shades of gree and blue indicating motions toward us, and shades of yellow to red indicating motions away from us. (For more information, see the IFA press release.

We are riding on the edge of a huge void in space, hurtling along at 600,000 miles per hour. Yes, it’s true. We’re moving along with the expansion of the universe AND with the coalescence of galaxies along filaments, in clusters at places where the filaments intersect. Yet, as we go about our daily lives, we’re largely (if not completely) unaware of the ride we’re taking through space and time as part of the Milky Way Galaxy. Yet, our motion tracks with the continuing evolution of the universe.

Did that get your attention? It’s a compelling story, and one that astronomer Brent Tully is telling in his latest research into the motions of galaxies in our neighborhood of the universe. In the past, our distance measurements to other galaxies could give us some very broad information about our galaxy’s motion through space. In addition, other measurements of the Cosmic Microwave Background Radiation tell about some aspects of our galaxy’s motion. But, there’s always been a part of our motion that was unexplained—until now. Tully and a team of colleagues have done extremely precise measurements of distances to galaxies around us. Those measurements tell a great story of motion and action of galaxies through time and helped them finger a largely unexplored component of space as the culprit: the Local Void. This is a gap in space that is 50 megaparsecs across. (A megaparsec is about 3,260,000 light-years, and a light-year is the distance that light travels in a year.)

Concentrations of matter (like the filaments and clusters) are aggregates of matter that is pulled together. Concentrations of matter have gravity; they PULL on things. The Local Void, on the other hand, is empty. It seems to PUSH on things, including our galaxy. Tully explains it as the absence of a pull. If an object is surrounded by matter in all directions, except for one empty sector, then the absence of a pull from that sector is actually the same as a push. And, it can have a large effect on our region of space, in effect turning us into void riders moving along the edge of the Local Void.

It’s quite fascinating that we (all of us in the Milky Way Galaxy) should live on the edge of such a huge gap of nothingness; and, we ride along not really feeling our motion across the depths of space and time.

cosmology, dark matter, galaxy clusters

I’m Going Down an Invisible Ring of Dark Matter…

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With Apologies to the Late Johnny Cash

The galaxy cluster CL 0024+17 (ZwCl 0024+1652) as seen by Hubble Space Telescopes Advanced Camera for Surveys.
The galaxy cluster CL 0024+17 (ZwCl 0024+1652) as seen by Hubble Space Telescope's Advanced Camera for Surveys.
A ghostly ring of dark matter in the same galaxy cluster, as inferred by observations of the cluster made by Hubble Space Telescope.
A ghostly ring of dark matter in the same galaxy cluster, as inferred by observations of the cluster made by Hubble Space Telescope.

It’s out there. Dark matter. You’ve probably read about it in the papers, or heard scientists talk about it. Nobody’s quite sure exactly what it is, yet. But, we do know it’s out there, distributed in clumps throughout the galaxies in the universe.

Tracking down dark matter, which is one of the great “Holy Grails” of astronomy and astrophysics these days, is no easy matter (so to speak). Dark matter doesn’t give off light, it doesn’t reflect light. It just sits there, making its presence known by the gravitational effect its mass has on light. To find it, astronomers study how the light from more distant galaxies (beyond or “behind” the dark matter clumps) is distorted and smeared into arcs and streaks by the gravity of the dark matter. The smears are produced by a phenomenon called “gravitational lensing.”

One Ring to Rule Them All… and in the Dark Matter Bind Them

Today a team of astronomers announced that they found a RING of dark matter at the heart of a galaxy cluster. An ordinary “visible light” image of the cluster shows galaxies and some smears of light that are distorted images of galaxies behind the cluster. The smears indicate that there’s something in the cluster causing the light from the distant galaxies to bend or distort. That “something” turns out to be a ring of dark matter. Now, if much of the dark matter we’ve “seen” so far in the universe is clumpy, how does a ring of it end up at the heart of a galaxy cluster?

Good question.

According to one of the astronomers, Dr. M. James Jee of Johns Hopkins University in Baltimore, MD, the collision between two galaxy clusters created a much larger cluster, but it also formed a ripple of dark matter separate from the gas and dust components of the cluster that left distinct footprints in the shapes of the background galaxies. “It’s like looking at the pebbles on the bottom of a pond with ripples on the surface,” he said. “The pebbles’ shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring.”

This is the first time such a distinct ring of dark matter has been seen, and at first Jee did not want to believe the ring-like structure was anything but a statistical fluke in the data analysis. But, like any good scientist does, he systematically went through the data, and eventually was convinced it was real.

“I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data reduction,” Jee explained. “I couldn’t believe my result. But the more I tried to remove the ring, the more it showed up. It took more than a year to convince myself that the ring was real. I’ve looked at a number of clusters and I haven’t seen anything like this.”

It’s really quite a cosmic detective story. Dark matter is sort of like the “last frontier” of matter studies in some ways. We know it’s ubiquitous in the universe, and we’re now starting to trace out its distribution, its “shapes” (if you will). There’s more dark matter than “regular” (so-called “baryonic” matter in the universe) and it serves as a sort of gravitational glue to bind galaxies and clusters together.

If you want to read more about this story, and see some very cool animations, go to the Space Telescope Institute website and visit their news center. And stay tuned. I think there’s going to be more news about this “binding force” of dark matter as scientists do more cosmic mapping!