Ghostly galaxy may be missing dark matter

By Mary HaltonScience reporter, BBC News

A diffuse fuzzy blob with other galaxies visible behind it — Image captionHubble image: The galaxy is so faint that other spiral galaxies can been seen through it

An unusually transparent galaxy about the size of the Milky Way is prompting new questions for astrophysicists.

The object, with the catchy moniker of NGC1052-DF2, appears to contain no dark matter.

If this turns out to be true, it may be the first galaxy of its kind - made up only of ordinary matter. Currently, dark matter is thought to be essential to the fabric of the Universe as we understand it.

The study is published in Nature.

Artist's impression of a spiral galaxy with blue arms of dust and stars. A faint blue glow surrounds it. — Image captionArtwork: A dark matter halo (blue) is thought to extend beyond the visible star population of galaxies

Ghostly glow

The authors of the study weren't initially on the hunt for a dark matter-free galaxy; instead they had set out to take a closer look at large, ultra-diffuse galaxies.

These are similar in size to the spiral galaxies we're more familiar with, but have a fraction of the number of stars.

When Prof Pieter van Dokkum, lead author of the study, first spotted NGC1052-DF2, "I stared a lot at that image and just marvelled at it... It's like this ghostly glow in the sky."

The galaxy has very few stars, but many of them are grouped together in unusually bright clusters. When the team studied the behaviour of these clusters, they found that the stars seemed to account for all of the galaxy's mass.

Leaving no room for dark matter.

An illustration: a web of purple filaments, thin and thick, meeting with several glowing yellow centres — Image captionArtwork: a map of dark matter in the local universe

This is not the case for most galaxies.

"There's about five times more dark matter in a galaxy than regular matter," explained Dr Michelle Collins, a physicist at the University of Surrey who was not involved in the study.

"As you go further out from the galaxy you have fewer stars and more dark matter. The dark matter halo is much more extended than the stars are in a galaxy," she added.

You can't have one without the other

As it has greater mass than normal matter, dark matter is believed to hold the necessary gas together while galaxies are forming.

"So this galaxy would have to form a different way: maybe from interactions within gas that's flowing into or blowing out of a larger galaxy," North Carolina State University astrophysicist Dr Katherine Mack told BBC News.

A spiral galaxy with blue/purple dust in its outer arms and reddish dust in its interior — Image captionAndromeda: most galaxies have five times more dark matter than regular matter

"It's not just galaxies," explains van Dokkum. "The entire fabric of the universe is really the scaffolding of dark matter and everything else is pasted on it."

For Mack, the most exciting aspect of this galaxy is its potential to prove that dark matter - until now widely theorised but not directly observed - is real.

If dark matter were just an unexplained effect of the gravity from regular matter, its effects would be visible in this galaxy. "So it only makes sense if dark matter is a real substance, that can be present or not, separately from the regular matter," Mack added.

The team has another paper forthcoming that will take a closer look at the bright star clusters, and may unravel more of NGC1052-DF2's mystery.

Dark materials

More work remains to be done on this and similar objects before dark matter theory needs to be fundamentally altered, however.

A large assemblage of gold hexagons, in a clean room workshop with suited workers looking on — Image captionOnce launched, the James Webb Space Telescope will offer a better look at distant galaxies

"You always have to be careful to say it's the first of anything," says van Dokkum. "It's certainly the best candidate for a baryonic [ordinary matter] galaxy."

Dr Collins is cautious to conclude that the galaxy has no dark matter halo, based on the current evidence.

She notes that there may be other galaxies with unusual and unexplained features, but technology will need to improve before they can be properly observed.

"We have some galaxies nearby that show some similar properties but they're much fainter. They're much smaller objects," she told BBC News.

Dr Richard Massey, a physicist at Durham University, agrees: "I'm genuinely very impressed with the work, and I'd use the conclusions to say that we should stare at these objects a lot harder for a lot longer - but I wouldn't conclude anything profound about dark matter quite yet," he told BBC News.

'Lost' film predicting rise of Nazism returns to screen

Media captionThe silent film shows Jewish characters being abused in the street and forced from their homes. Credit: Filmarchiv Austria

A silent film from 1924 predicting the rise of Nazism was found in a Paris flea market in 2015 after being lost for decades. Thanks to a huge fundraising campaign, it has now been restored and returned to cinemas, reports the BBC's Bethany Bell in Vienna.

An Orthodox Jew is set upon by three taunting men.

A woman shopping at a market stall becomes outraged at the high prices. She starts pelting a passing Jewish man with fruit.

Later, huge protesting crowds gather outside the chancellor's office. Inside, the leader consults with an adviser. "It is awful to expel the Jews," he says. "But one must satisfy the people."

The incidents portrayed in the Austrian film The City Without Jews (Die Stadt Ohne Juden) are eerily prophetic.

It was made nearly 20 years before the Holocaust, at a time when the Nazi party was banned in Austria, and when Adolf Hitler was in jail in Germany, working on his book Mein Kampf.

'Growing anti-Semitism'

The film tells the story of a city that expels all of its Jews. They are made the scapegoats for rising prices and unemployment.

Still from The City without Jews (Die Stadt ohne Juden) — Image captionThe book and film aimed to combat intolerance

It is based on a dystopian, satirical novel by the Austrian Jewish writer and journalist, Hugo Bettauer.

"At the beginning of the 1920s, just after Austria's First Republic was founded, political anti-Semitism was growing incredibly, much more than during the monarchy," says Nikolaus Wostry, director of collections at the Filmarchiv Austria.

Bettauer was trying to combat rising intolerance and racism, at a time when Jews displaced after World War One were arriving in Vienna.

"Hugo Bettauer was a person who really tried to make society more tolerant - towards Jews, towards homosexuals, toward women taking part in political life," says Mr Wostry.

"That all brought him into big conflicts with the right wing in Austrian society, which was quite dominant at that time."

'Grotesque' murder

Adapted for the screen by Austrian director HK Breslauer, the City Without Jews premiered in Vienna in 1924.

The film was a success, screened in the five biggest cinemas in Vienna, Mr Wostry says.

"But then the political right reacted against it."

Less than a year after the film was made, Hugo Bettauer was killed by a Nazi, called Otto Rothstock, following a hate campaign against him.

"His private address was published and in the newspapers, it was explicitly said that such a person shouldn't be part of society."

Mr Wostry says the court case against Rothstock was "grotesque". The murderer escaped with a very short sentence.

Desperate goodbyes

Watching it now, the film's scenes of the expulsion are some of the most haunting and prescient.

The city's Jews are told to leave by Christmas Day. Some of the poorer people leave on foot, accompanied by soldiers with bayonets. Whole communities walk slowly through the snow, some on crutches, some carrying Torah scrolls from the synagogues.

Others leave by train. Desperate goodbyes take place on the platforms. A Jewish father embraces his little daughter, who is staying behind with her Christian mother.

In reality, some six million Jews in Nazi-occupied Europe were rounded up and killed in the Holocaust. Many others fled abroad.

One of the actors in the film, Hans Moser, one of Austria's most famous film stars at the time, was married to a Jewish woman.

When the Nazis took over in 1938, he refused to divorce her, writing personally to Adolf Hitler, who was one of his biggest fans, asking for clemency.

She eventually emigrated to Hungary and they were reunited after the war.

Meanwhile, the film itself went missing as silent films fell from favour.

"At the end of the silent period, silent films couldn't be distributed any longer and the only way to make a profit was to destroy them and to take the silver, to use the plastic," says Mr Wostry.

"Worldwide 90% of our silent film heritage is lost."

'Growing relevance'

An incomplete and very damaged version was found in the Netherlands in 1991. Then in 2015, the whole film was rediscovered by a collector in a flea-market in Paris.

The Austrian Film Archive organised a crowd-funding campaign to save it. More than 700 people contributed more than €86,000 (£72,000; $107,000) to it. The film has now been digitally restored and re-released.

It is being screened in Vienna before going on a tour around Austria and reaching European cities including London later this year, where audiences will be able to watch it accompanied by a new score performed live.

Nikolaus Wostry says the movie should be considered "one of the strongest anti-Nazi-statements of the silent film period".

In the film, after the Jews leave, the city celebrates with fireworks. But life soon gets worse. International trade and the economy suffer, and the city's beloved cafes get turned into simple beer halls.

The chancellor eventually invites the Jews back home. Lovers and mixed Christian and Jewish families are reunited.

"It is a statement against excluding people on the grounds of their religion or race from society and has in this respect a growing relevance again for our time," says Mr Wostry.

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Wednesday 28 March 2018