Dark Passage

Change-me Charlie’s not giving up easily. “You said that NASA picture did three things, but you only told us two of them — that dark matter’s a thing and that it’s separate from normal matter. What’s the third thing? What exactly is in that picture? Does it tell us what dark matter is?”

The Bullet Cluster ( 1E 0657-56 )

Physicist-in-training Newt’s ready for him. “Not much of a clue about what dark matter is, but a good clue about how it behaves. As to what’s in the picture, we need some background information first.”

“Go ahead, it’s not dinner-time yet.”

“First, this isn’t two stars colliding. It’s not even two galaxies. It’s two clusters of galaxies, about 40 all together. The big one on the left probably has the mass of a couple quintillion Suns, the small one about 10% of that.”

“That’s a lot of stars.”

“Oh, most of it’s definitely not stars. Maybe only 1-2%. Those stars and the galaxies they form are embedded in ginormous clouds of proton-electron plasma that make up 5-20% of the mass. The rest is that dark matter you don’t like.”

“Quadrillions of stars are gonna make a super-super-nova when they collide!”

“Well, no. That doesn’t even happen when two galaxies collide. The average distance between neighboring stars in a galaxy is 200-300 times the diameter of a star so it’s unlikely that any two of them will come even close. Next level up, the average distance between galaxies in a cluster is about 60 galaxy diameters or more, depending. The galaxies will mostly just slide past each other. The real colliders are the spread-out stuff — the plasma clouds and of course the dark matter, whatever that is.”

Astronomer-in-training Jim cuts in. “Anyway, the collision has already happened. The light from this configuration took 3.7 billion years to reach us. The collision itself was longer ago than that because the bullet’s already passed through the big guy. From that scale-bar in the bottom corner I’d say the centers are about 2 parsecs apart. If I recall right, their relative velocity is about 3000 kilometers per second so…” <poking at his smartphone> “…the peak intersection was about 700 million years earlier than that. Call it 4.3 billion years ago.”

“So what’s with the cotton candy?”

Newt looks puzzled. “Cotton… oh, the pink pixels. They’re markers for where NASA’s Chandra telescope saw X-rays coming from.”

“What can make X-rays so far from star radiation that could set them going?”

“The electrons do it themselves. An electron emits radiation every time it collides with another charged particle and changes direction. When two plasma clouds interpenetrate you get twice as many particles per unit volume and four times the collision rate so the radiation intensity quadruples. There’s always some X-radiation in the plasma because the temperature in there is about 8400 K and particle collisions are really violent. The Chandra signal pink shows the excess over background.”

“The blue in the Jim’s picture is supposed to be what, extra gravity?”

“Basically, yeah. It’s not easy to see from the figure, but there are systematic distortions in the images of the background galaxies in the blue areas. Disks and ellipsoids appear to be bent, depending on where they sit relative to the clusters’ centers of mass. The researchers used Einstein’s equations and lots of computer time to work back from the distortions to the lensing mass distributions.”

“So what we’ve got is a mostly-not-from-stars gravity lump to the left, another one to the right, and a big cloud in the middle with high-density hot bits on its two sides. Something in the middle blew up and spread gas around mostly in the direction of those two clusters. What’s that tell us?”

“Sorry, that’s not what happened. If there’d been a central explosion the excess to the right would be arc-shaped, not a cone like you see. No, this really is the record of one galaxy cluster bursting through another one. Particle-particle friction within the plasma clouds held them back while the embedded galaxies and dark matter moved on.”

“OK, the galaxies aren’t close-set enough for them to slow each other down, but wouldn’t friction in the dark matter hold things back, too?”

“Now that’s an interesting question…”

~~ Rich Olcott

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The Fellowship of A Ring

Einstein ring 2018
Hubble photo from NASA’s Web site

Cathleen and I are at a table in Al’s coffee shop, discussing not much, when Vinnie comes barreling in.  “Hey, guys.  Glad I found you together.  I just saw this ‘Einstein ring’ photo.  They say it’s some kind of lensing phenomenon and I’m thinking that a lens floating out in space to do that has to be yuuuge.  What’s it made of, and d’ya think aliens put it there to send us a message?”

Astronomer Cathleen rises to the bait.  I sit back to watch the fun.  “No, Vinnie, I don’t.  We’re not that special, the rings aren’t signals, and the lenses aren’t things, at least not in the way you’re thinking.”

“There’s more than one?”

“Hundreds we know of so far and it’s early days because the technology’s still improving.”

“How come so many?”

“It’s because of what makes the phenomenon happen.  What do you know about gravity and light rays?”

Me and Sy talked about that a while ago.  Light rays think they travel in straight lines past a heavy object, but if you’re watching the beam from somewhere else you think it bends there.”

I chip in.  “Nice summary, good to know you’re storing this stuff away.”Gravitational lens 1

“Hey, Sy, it’s why I ask questions is to catch up.  So go on, Cathleen.”

She swings her laptop around to show us a graphic.  “So think about a star far, far away.  It’s sending out light rays in every direction.  We’re here in Earth and catch only the rays emitted in our direction.  But suppose there’s a black hole exactly in the way of the direct beam.”

“We couldn’t see the star, I get that.”

“Well, actually we could see some of its light, thanks to the massive black hole’s ray-bending trick. Rays that would have missed us are bent inward towards our telescope.  The net effect is similar to having a big magnifying lens out there, focusing the star’s light on us.”

“You said, ‘similar.’  How’s it different?”Refraction lens

“In the pattern of light deflection.  Your standard Sherlock magnifying lens bends light most strongly at the edges so all the light is directed towards a point.  Gravitational lenses bend light most strongly near the center.  Their light pattern is hollow.  If we’re exactly in a straight line with the star and the black hole, we see the image ‘focused’ to a ring.”

“That’d be the Einstein ring, right?”

“Yes, he gets credit because he was the one who first set out the equation for how the rays would converge.  We don’t see the star, but we do see the ring.  His equation says that the angular size of the ring grows as the square root of the deflecting object’s mass.  That’s the basis of a widely-used technique for measuring the masses not only of black holes but of galaxies and even larger structures.”

“The magnification makes the star look brighter?”

“Brighter only in the sense that we’re gathering photons from a wider field then if we had only the direct beam.  The lens doesn’t make additional photons, probably.”

Suddenly I’m interested.  “Probably?”

“Yes, Sy, theoreticians have suggested a couple of possible effects, but to my knowledge there’s no good evidence yet for either of them.  You both know about Hawking radiation?”

“Sure.”

“Yup.”

“Well, there’s the possibility that starlight falling on a black hole’s event horizon could enhance virtual particle production.  That would generate more photons than one would expect from first principles.  On the other hand, we don’t really have a good handle on first principles for black holes.”

“And the other effect?”

“There’s a stack of IFs under this one.  IF dark matter exists and if the lens is a concentration of dark matter, then maybe photons passing through dark matter might have some subtle interaction with it that could generate more photons.  Like I said, no evidence.”

“Hundreds, you say.”

“Pardon?”

“We’ve found hundreds of these lenses.”

“All it takes is for one object to be more-or-less behind some other object that’s heavy enough to bend light towards us.”

“Seein’ the forest by using the trees, I guess.”

“That’s a good way to put, it, Vinnie.”

~~ Rich Olcott