Category Archives: astrophysics

Separating Darks from Lights

Galaxy Cluster Clash Points out Dark Matter

This image just released today may look like a galaxy cluster with some Photoshop airbrushing on it, but it’s really proof of an effect that had been observed once before but not completely proved. The Hubble Space Telescope and Chandra X-ray Observatory both studied the same area of the sky, looking at a collision of galaxy clusters into a giant supercluster, called MACS J0025.4-1222.  The combined observations provide another clue to the existence and distribution of dark matter. This time the mechanism was provided by the cluster collision. When such objects collide, they pack a heck of a punch, and the energy of that punch separates dark matter from ordinary matter.

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.

Optical images from Hubble were used to infer the distribution of the total mass — dark and ordinary matter — using a technique known as gravitational lensing (the blue area shows where light is “bent” as it passes by clumps and regions of dark matter and is influenced by the dark matter’s gravitational pull). Chandra data enabled astronomers to accurately map the position of the ordinary matter, mostly in the form of hot gas, which glows brightly in X-rays (the pink regions.) The separation between the material shown in pink and blue provides direct evidence for dark matter. The fact that it could be separated from baryonic matter in such a powerful collision is another clue to the nature of dark matter.

Understanding this unseen material, particularly how much of it there is in the universe, is key to our understanding of so many other things about the universe. The expansion of the universe, the ordering of large-scale structure (into clusters of galaxies, superclusters of galaxies), and even such aspects of galaxies as their rotation and merger rates are all affected by this dark matter. It permeates the universe, yet it is extraordinarily difficult to detect using conventional observational techniques. It’s “easier” to infer its existence by observing its affect on light, for example. Which is what gravitational lensing does, and what makes it such a useful tool for astronomy.  While we still don’t know the entire story of dark matter, discoveries like these are helping “fill in the puzzle pieces.”

Hubble’s Magnetic Monster

The Power of Magnetic Fields in Space

Yesterday this amazing picture came rumbling through the Intarwebs from the folks at Hubble Space Telescope. It’s from the Advanced Camera for Surveys and shows the galaxy NGC 1275 seemingly surrounded by what looks like a spiderweb of stuff.

Now, there’s a lot going on here, so let’s break it down. First, the galaxy is in the center of the Perseus Cluster of galaxies, and the whole region is permeated with hot gas — REALLY hot — around 100 million degrees. The red stringy things are filaments of cooler hydrogen gas and they’re threaded on magnetic field lines that extend throughout the region.

Magnetic field lines?  Yes, this is indeed true. The actions of a supermassive black hole and its associated jet at the core of the galaxy are the source of very strong magnetic fields that extend quite far out from the core. Gas near the center of the galaxy gets superheated by all the hoopla surrounding the black hole and jet, and that blows bubbles of material out into surrounding space. Those expanding bubbles plow into cooler regions of hydrogen gas, and the expansion carries some of that gas out along with it. The red filaments are the hydrogen gas that looks like it’s draped on the magnetic field lines. Those filaments are actually a very important clue to astronomers. They are the biggest visible-light evidence for some “invisible” (i.e. not visible to our eyes) interactions between NGC 1275’s central black hole and the hot gas that permeates the surrounding interstellar and intergalactic spaces.

Now, the filaments look really delicate, and you’d think that the huffing and puffing of material blasting out from the center of the galaxy via the jet would destroy the hydrogen gas threads. What’s saving them are the magnetic fields. Those lines of force hold the gas in place and help it resist the outward blasts from the core of the galaxy. They also keep the gas from clumping up to form newborn stars, making them a disruptive as well as a unifying force.

For anybody who thought that intergalactic space might be empty and boring, this image and another one taken with multiple instruments (left), reveals just how frenetic the environment in galaxy clusters can be.

While we may not be able to see those magnetic fields, images like these show us the effects that such fields have on the ordinary matter they thread through.

For more information, surf on over to the Space Telescope Science Institute’s HubbleSite page.