Posted: 30 Sep 2009 09:04 AM PDT

Galaxies speeding through clusters of their neighbors wereimaged by the Hubble Space Telescope being stripped of their gas.

NGC 4522, the galaxy pictured above, is traveling at 6.2 million milesper hour, astronomers estimate. It's about 60 million light-years away in the Virgo cluster. The bright blue areas to the right and left are star-forming regions.

Unlike our own Milky Way galaxy with its delicate spiral arms, fast-moving galaxies like NGC 4522 get deformed by the strong winds generated by their movement. The process is known as "ram pressure stripping" and it's analogous to what would happen were you to hold a dandelion parachute ball out a car window. The lighter parts get stripped away.

Galaxy NGC 4402, pictured below, shows off the convex gas and dust disc that is characteristic of galaxies undergoing ram pressure stripping. The hot gas between galaxies in a cluster (known as the intra-cluster medium) actually sweeps the gas away, creating the odd shape in the process.

The images were taken by the Advanced Camera for Surveys instrument before it suffered a power failure in 2007. The images were recovered when astronauts restored the unit in May of this year on the last Hubble Servicing Mission. Both are deep enough to show distant background galaxies.

High-res version of the top photo (40 MB): NGC 4522
High-res version of the bottom photo (29 MB): NGC 4402

Images: NASA/ESA

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Weird, Rare Clouds and the Physics Behind Them

Posted: 29 Sep 2009 05:06 PM PDT

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In August, we posted a photograph of some odd, rare clouds known as Morning Glory clouds without providing an explanation for how they form. In response to reader interest, we followed up with meteorologist Roger Smith of the University of Munich, who has studied their formation.

"Over the years we've developed a good understanding of them," Smith said. "It's no longer a mystery, but still very spectacular."

The Morning Glory phenomenon is the result of the particular configuration of the land and sea on the Cape York Peninsula, in a remote part of Australia. The peninsula tapers off from about 350 miles wide to 60 miles as it extends north between the Gulf of Carpentaria to the west and the Coral Sea to the east. The easterly trade winds push the sea breeze across the peninsula during the daytime, which meets the sea breeze from the west coast in the late evening. The collision produces a wave disturbance moving inland to the southwest that is a key part of the cloud formation.

As moist sea air is lifted to the crest of the waves, it cools and condensation forms a cloud. Sometimes there is just one wave, but Smith has seen as many as 10 together in a series.

"If you look at the clouds, it looks as if they are rolling backwards," Smith said. "But in fact the clouds are continuously formed at the leading edge and continuously eroded at the trailing edge. That gives a rolling appearance."

These clouds do occur elsewhere, including Munich, where they form about once in a decade. Cape York is unique because they happen regularly in the fall above the small town of Burketown. And they can also be particularly impressive there as well, growing up to 600 miles long. Pilots fly into the area every year, hoping to see the intriguing clouds.

Not many scientists study them, or really any weird clouds, because their very rarity makes them relatively unimportant for studying precipitation or climate. So, oftentimes, their formation is poorly understood.

"It's hard to get funding to study something that's neat looking," said cloud physicist Patrick Chuang of the University of California, Santa Cruz.

On the following pages, we've gathered photos of some of the strangest, most beautiful cloud types and asked scientists to help us understand how they form.

Images: Above: Ulliver/Wikimedia Commons. Below: Image: Mick Petroff/APOD

T. rex Bite Marks Actually Festering Infections

Posted: 29 Sep 2009 01:27 PM PDT

rex-sores

The biggest problem for Tyrannosaurus rex could have been a single-celled parasite in a paleolithic turn on the tiny-fells-mighty, War of the Worlds story.

A relative of lowly Trichomonas, a microbe commonly found today in pigeons, may have killed off Sue, the famous T. rex skeleton at the American Museum of Natural History, and many other tyrannosaurids, too.

Paleontologist Ewan Wolff of the University of Wisconsin-Madison and colleagues use evidence from modern predatory bird species to argue that the protozoan parasite could have formed lesions along the tyrannosaur mandible, eroding the bone away.

In some cases, the illness might have been bad enough to prevent the animals from feeding, leading to their death from starvation, and creating telltale holes in the jaws of the great beasts.

"I think it's very tempting when you see a hole in a bone to say it's bite marks, but there are innumerable disease you could list that cause holes in bones that have nothing to do with bite wounds," said Ewan Wolff, lead author of a new paper describing the work published Sept. 29 in PLoS One.

There are three pieces of evidence against the bite-wound hypothesis, Wolff said. First, the holes are generally nicely circular or ovoid, not obviously tooth-shaped. Second, dinosaurs obviously had many teeth, not just one, and the holes don't seem to come in groups. Third, there are no smaller marks or scrapes on the bone surrounding the holes.

"We're definitely familiar with what predation traces look like from tooth marks in tyrannosaurus," Wolff said. "And this is not it."

Furthermore, they found evidence in predatory birds that trichomonosis can cause holes in the mandible that look quite similar to those found in nine tyrannosaurid specimens.

The disease could have been passed to dinosaurs from their prey or from tyrannosaur to tyrannosaur during combat, mating, or cannibalism.

"Head or face-biting behavior relating to intraspecific territoriality, social dominance, courtship, feeding or some other unknown aspect of tyrannosaurid behavior would have provided the ideal mechanism for the transmission of this trichomonosis-like disease," Wolff and colleagues wrote.

Other researchers, notably dinosaur anatomy expert Elizabeth Rega of Western University of Health Sciences in Pomona, California, have argued the holes were disease-induced. But Rega's culprit was the bacteria, actinomycosis.

Wolff, however, argued that much of the damage that actinomycosis causes in humans is due to puss. Birds don't actually create puss, and the same was likely true of dinosaurs. It's not part of the avian immune response. Instead, birds coat invading organisms with a fibrous protein.

"So instead of getting puss, you get this very cheesy inflammatory area," Wolff said. "You get chronic infection that just doesn't go away."

He said that the actinomycosis-trichomonosis debate showed how his team's approach differed from other paleopathologists.

"In the past, a lot of comparisons of paleopathological specimens have been made to humans or domesticated animals," Wolff said, instead of with dinosaurs' closer relatives, birds and reptiles. "It makes a lot of sense to look at animals that share a close evolutionary relationship to each other and the diseases that they have. The approach in this article is to do just that and that's relatively novel in the paleopathological community."

Still, not everyone is sold on the new idea about Sue's holey jaw.

"My suspicion is that it's not valid," said Bruce Rothschild, a medical doctor and paleopathologist at the University of Kansas.

Rothschild said the holes in Sue's jaw look a lot like the marks made by trepanation — the practice of puncturing of the skull — in ancient humans.

"I'm not saying it was trepanation," Rothschild said. "I'm saying it was a bite."

And he disputed that the modern examples of presumed trichomonas jaw holes matched up "exactly" with the tyrannosaur fossil holes.

"To me, it's not exactly the same," he said.

Rothschild said that the tyrannosaur fossils showed just "a hole with simple infilling," without the more complex features associated with lesions.

What might be next is looking at dinosaur specimens for other signs of starvation, suggested Thomas Holtz, a tyrannosaur specialist at the University of Maryland.

"I don't know if you'd see maybe bone loss as a sign of starvation," Holtz said.

Wolff said his team could follow up with exactly that type of observation. Another line of inquiry could run in the molecular direction, trying to determine if parasites like trichomonas did in fact exist 65 millions of years ago.

"I don't know to what degree anyone can confirm that this particular group of organisms were around during the Cretaceous," Holtz said. "We don't have a fossil record for them."

Image: Pex Rex. Chris Glen/University of Queensland

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