Messy galactic mergers lead to delicate spiral shapes

Most of the spiral galaxies that decorate our universe have emerged from surprisingly violent pasts, says a new study. They grew their delicate spiral arms after being mashed into a pulp by vast collisions.

This conclusion comes from comparing two snapshots of the universe. A group led by Francois Hammer of the Paris Observatory first took a sample of relatively nearby galaxies observed by the Sloan Digital Sky Survey in New Mexico. It shows that most large galaxies today are disc-shaped, with bright new stars forming along spiral arms – similar to our galaxy and its neighbour Andromeda.

The picture was very different when the universe was about half its present age. To look back to that time, Hammer's team used the GOODS survey, a set of data from Hubble and other space-based observatories. It includes very distant galaxies whose light has been stretched and reddened by the expansion of the universe. The galaxies in Hammer's sample are seen as they were 6 billion years ago.

Among the large galaxies in this era, Hammer's group finds that there are far fewer tidy spirals, and far more with peculiar unclassifiable shapes. Those anomalous galaxies must evolve into spirals to explain the galactic distribution we see today.

Even at first glance, those anomalous galaxies look as though something has smashed them up, and indeed Hammer's simulations show that their shapes and internal motions can be created by two smaller galaxies colliding.

Butterfly from pupa

This is a surprise because spirals need a good supply of cool gas to form new stars, and a big collision should disperse and heat up the gas. Astronomers thought that major collisions would produce relatively dead elliptical galaxies with little new star formation.

The solution in Hammer's simulations is to load the progenitors with gas. If there is more gas than stars, then enough cool gas can be salvaged from the collision to form a disc and then spiral arms, like a butterfly being formed out of a pulpy pupa. "It takes 2 to 3 billion years," says Hammer.

From the high proportion of galaxies that were going through this painful change 6 billion years ago, he calculates that it must have happened to almost all of today's spirals.

So was our own our Milky Way a smashed-up mess 6 billion years ago? It turns out not. We are one of the few exceptions: a spiral that has rotated serenely, undisturbed except by the impact of a few small dwarf galaxies, for 11 billion years – most of the age of the universe.

Journal reference: arxiv.org/abs/0903.3962

Biggest black holes grow inside 'quasistars'

The biggest black holes in the universe might have grown within the bellies of giant stars, a new study suggests. If these hole-bearing "quasistars" exist, then they might be bright enough to see from across the universe.

Quasistars are one attempt to explain the existence of supermassive black holes, which astronomers have detected at the hearts of most large galaxies, and whose origin is still unknown.

Smaller black holes are easier to account for - a massive star's core can sometimes collapse into a black hole with around 10 times the mass of the Sun. But their big brothers can be a billion times as massive.

It is possible that the smaller siblings can grow that big by eating stars and gas or by colliding with each other and merging. But they would have to grow up very quickly in cosmic terms, because some supermassive black holes were already around just a few hundred million years after the big bang.

Mitchell Begelman and colleagues at the University of Colorado in Boulder, US, have worked out how the big holes might have gotten a head start in life.

Large clouds of hydrogen and helium were common in the early universe. Begelman says that if such a cloud collapsed into a massive star, a dense knot of the gas could pile up so rapidly in its core that it would collapse into a small black hole.

When that happens in stars just a few times as massive as the Sun, the enormous energy released is enough to blast away the surrounding layers of gas, revealing a brilliant supernova explosion.

Great bulk

But as long as a quasistar is at least 1000 times the mass of the Sun, its great bulk could have absorbed all that energy, containing the supernova with no more than a shudder, becoming a black-hole sun.

The black hole embryo could then grow fast, nourished by the dense body of the quasistar. Gas falling onto the hole would heat up and release an immense amount of light, so much that its pressure would hold up the layers of the star above it.

That could lead to a potentially unstable situation, with dense gas sitting on a lighter layer. Begelman suspects that the pressure would be released as some of the light would escape in "photon bubbles", large blobs of radiation that would burst from the surface of the star. "My guess is it would have to be bubbly," Begelman told New Scientist.

Gestation would last about a million years, at which point the hole could reach at least 10,000 solar masses - not yet an adult supermassive black hole, but a pretty big baby. With such a head start, it would be relatively easy to reach a billion solar masses on a diet of stars and other black holes.

Bright beacons

Astronomers may be able to test the idea by searching for the objects. A quasistar would be a little cooler than our Sun, Begelman calculates, but at more than 10 billion kilometres across, it would produce about as much light as a small galaxy.

Detecting them will be difficult, however. They are most likely to have existed in the early universe, when stars are thought to have been much more massive than today. The expansion of space since then would have stretched their light into a band of the infrared spectrum that is absorbed by Earth's atmosphere.

Loner galaxy is seed of giant black hole

NGC 4178 enjoyed the single life. Even though the flat, disc-shaped galaxy was getting on a bit, it had a svelte spiral figure to be proud of. Its central black hole was perfect: not too small, not too large. It had never been involved in a major merger with another galaxy, and wanted to keep it that way. None of the unsightly bulges and warps associated with too much socialising for NGC 4178.

But other, more gregarious, galaxies were getting together all around it. They merged into grand spiral galaxies in a firework display of star formation which left them with impressive bulging bellies. They pooled their central black holes until they were billions of times larger than the sun. NGC 4178 watched it all from the sidelines, glad to maintain its trim appearance, although it couldn't help wondering if it wasn't missing out on something.

Cosmic hermit

Unsociable galaxies are unusual. Astronomers think that galaxies grow from scraggly clusters of stars to elegant spirals like the Milky Way by merging and pooling their resources. Loners like NGC 4178, which has spent most of the lifetime of the universe avoiding the company of other galaxies, are useful tools for disentangling how this happens. They are rare snapshots of a simpler time.

"They are more representative of the initial stuff, from when structure started to form in the universe," says Nathan Secrest, a graduate student at George Mason University in Fairfax, Virginia. Galaxies like NGC 4178 are about "as pristine as you can get".

One of the puzzles they can help solve is the origin of supermassive black holes. Most large galaxies seem to have a giant black hole, millions or billions of times larger than the sun, at their centres. How these black holes got so big is still a mystery: did they grow gradually from mergers of smaller black holes, coalescing when their host galaxies merged? Or did they form when gas clouds collapsed in the early universe?

If these giants did grow by devouring their more diminutive counterparts, then the universe should also be riddled with middleweight black holes, tens of times the size of the sun. But only a few of these have ever been spotted.

If galaxies that have never been through a merger, like NGC 4178 – detectable by their lack of stellar bulges – have their own central black holes, their properties could help tell the story.

"We've been looking around and trying to find these galaxies with no bulges," Secrest says. "We've been slowly determining that many of them do in fact have black holes."

Missing middleweights found?

And some of them are in just the right mass range. Secrest and colleagues compiled observations of NGC 4178 in a range of wavelengths of light to narrow down the mass of its central black hole. It came in at between 10,000 and a million solar masses. If it turns out to be on the lower end of that range, it could be the smallest black hole ever seen in a pristine disc galaxy, and one that is still growing by sucking in material.

The fact that several such pristine galaxies turn out to have a small, still-expanding black hole at their core suggests that black holes can grow to intermediate size without mergers, but then need to pool their resources to get much bigger. This supports the idea that supermassive black holes form as a result of galactic unions, Secrest says. In other words, the black hole at NGC 4178's core could be an example of a seed from which larger black holes could grow.

But NGC 4178 will not be a singleton forever. The galaxy sits at the edge of the Virgo cluster of galaxies, and will enter the main part of the cluster in the next 100 million years or so.

"It will definitely at some point go through a merger," Secrest says. When it does it will probably develop a central bulge and a supermassive black hole, just like its more social neighbours. "In some ways, its pristineness will be lost," Secrest says – but at least it will have made some friends.

Journal reference: Astrophysical Journal, DOI: 10.1088/0004-637X/753/1/38

Shapely galaxy is ahead of the curve

Object: BX442, a sophisticated spiral at the dawn of time

Characteristics: three arms, inner turmoil and a hidden companion

Behold, the cosmic Stone Age. In this brutish era 11 billion years ago, galaxies are forming – primitive objects that appear as incoherent jumbles or featureless blobs. All except one. Among the rough-hewn masses sits a single sophisticated spiral with three shapely arms. It seems as finely sculpted as the most glamorous galaxies of today.

"When we saw it we were astounded," says David Law at the University of Toronto. "Common wisdom was that these things don't exist."

Law and his colleagues spotted the cosmic anachronism, dubbed BX442, in infrared images from the Hubble Space Telescope. In their observations, the team counted 306 galaxies whose light has been so stretched and reddened by the expansion of the universe – a phenomenon known as redshift – that it must have been emitted a mere three billion years after the big bang. Of the lot, only BX442 looks like a spiral. Other surveys of such ancient times have turned up no spiral galaxies at all.

Galaxies of this era tend to be turbulent places, where gas and stars churn violently. Such dynamism should prevent any delicate spiral structures from forming. For example, some modern spirals are thought to be carved as gravity gathers their stars and gas into relatively crowded density waves, which ripple through the galactic disk. But the rapid random motions within primordial galaxies such as BX442 should overwhelm that gentle process.

Could the stunning spiral somehow be a closer, more mature galaxy just pretending to be young and far away? Law and his colleagues checked the galaxy's redshift to make sure. "We have 16 independent measurements of redshift," says Law, and they all agree.

Law then wondered whether the spiral shape might be a mere fluke, a chance arrangement of colliding lumps that mimics a more mature galaxy. Using theKeck telescope in Hawaii, the team sampled the spectrum of light at different spots across BX442, which revealed that the whole thing is rotating as a unified disk of gas and stars. It is a true spiral galaxy.

Better still, the galactic beauty is a "grand design" spiral. That means the object has clearly defined arms stretching all the way from core to outskirts, unlike most spiral galaxies, including the Milky Way, which are broken-up "flocculent" spirals.

A smudge in the Hubble images may be a vital clue to the galaxy's comely shape. The group's observations show a small galaxy in the process of merging with BX442. Today's grand-design spirals often have merging companions too, says Law, and astronomers have suggested that the right kinds of mergers could stir up spiral patterns.

When two great arrays of stars collide, you might expect the result to be a big mess. But the group simulated this process for BX442 and found that if the merging companion isn't too heavy and it approaches along the right track, its gravity could create spiral ripples in the larger object's disk.

If so, the galaxy's rare glamour won't last. The merger will eventually be complete, and the turbulent motions within BX442 should wipe out its spiral pattern within 100 million years, a twinkle in cosmic time. The galaxy will then resume the same plain appearance as its contemporaries. Until, perhaps, billions of years later, when a more mature BX442 could stretch out new spiral arms.

Journal reference: Nature, DOI: 10.1038/nature11256