Seeing the Invisible

Overlay of the profile of the Bullet Cluster measured
using three different techniques. The light orange, round
galaxies that make up the cluster are seen clearly
in the image taken from optical telescopes. Overlaid is the
distribution of gas measured from X-ray observations in red
and the distribution of dark matter in blue. Composite Credit:
X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map:
NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.
Optical: NASA/STScI;Magellan/U.Arizona/D.Clowe et al.;

Dark matter continues to be a hot topic at my dinner table. I am continually amazed by how my non-physicist friends are interested in the research on this elusive material that makes up 25 percent of the universe. Just last night I was bombarded with questions like "How do we know it’s out there?" and "Can we see it?"

In my last post, I tried to explain what makes observations of dark matter so difficult. We see ordinary matter through the very strong and efficient electromagnetic force. The problem is that this force does not appear to affect dark matter.

That leaves gravity as the most obvious force for tracking dark matter, but the effects of gravity are really difficult to observe without enormous masses. This difficulty is the main reason that the existence of dark matter wasn’t generally accepted until the 1970’s.

To study dark matter in detail, we have to learn how to pit the forces of gravity against the biggest objects in the universe. One such trick was used in a project at Stanford University, at the Kavli Institute for Particle Astrophysics and Cosmology. In a press release from August of last year, Marusa Bradac describes some pretty convincing observations of dark matter in a massive cluster of galaxies known as the Bullet Cluster.

Measurements of the normal matter in this cluster were pretty easy. The electrons and protons were studied with observations of the hot cluster gas from an X-ray telescope and galaxies from an optical telescope. These can be seen in the figure at the top of the page. Notice the way the X-ray gas (in the hot pink) appears in two vertical bands?

Observations of the dark matter were much more difficult. These researchers measured the effects of dark matter indirectly by weak gravitational lensing. Lensing is a complicated topic that will have to wait for another time, but for the impatient, a couple more good descriptions can be found here and here.

For now, I will just say that we study this hidden matter by its effect on the appearance of the galaxies behind the cluster. The gravity is so strong that it acts like a lens and distorts the image behind it. The blue fog in the figure represents the distribution of dark matter inferred from the distortion of those galaxies.

So why is this interesting? From the image above, we see that the enormous cloud of gas and the enormous cloud of dark matter have completely different experiences in this cluster. The flattened structure of the two bands of gas provides strong evidence in support of a recent collision between two large masses. In the collision, the dark matter passed through relatively unfazed, but the gas in the clusters swirled around like the wind between two fronts in a storm system. In the process, the gas slowed down and is now lagging behind the main material from the two colliding masses. This merger event is demonstrated by a very nice computer animation.

In re-reading this post, I realize that there’s a lot of abstract material. I guess this is why the topic keeps coming up over dinner. Next time I should just apologize to my guests and try to shift the conversation to something less complicated, maybe politics?

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and
development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.