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Galactic Dinosaurs Aren’t Extinct

By David Reneke

Astronomers have found that compact massive galaxies that roamed the early universe have been hiding in plain sight.

One of the biggest mysteries in galaxy evolution is the fate of the compact massive galaxies that roamed the early universe. Astronomers at Swinburne University of Technology believe they have discovered the answer.

“When our universe was young, there were lots of compact, elliptical-shaped galaxies containing trillions of stars,” Prof Alister Graham said. “Due to the time taken for light to cross the vastness of space, we see these distant galaxies as they were in our young universe. However, in the present-day universe, very few such spheroidal stellar systems have been observed.”

The most popular theory had been that, over time, galaxy mergers might have led to their destruction and transformation into larger elliptical galaxies. However, there have not been enough galactic collisions to account for the reduction in the number of these compact spheroids.

The Swinburne astronomers have eliminated the need for this problematic theory because they have now located the missing galaxies. “They were hiding in plain sight,” said co-author

Dr Bililign Dullo. “The spheroids are cloaked by discs of stars that were likely built from the accumulation of hydrogen gas and smaller galaxies over the intervening eons.” Furthermore, the number of such hidden systems roughly matches the number of compact massive galaxies in the early universe.

“Unlike the massive dinosaurs that existed when the Earth was much younger, the galactic dinosaurs of our universe are not extinct,” Graham said. “They are simply embedded in large, relatively thin discs of stars.”

Due to the enormity of modern galaxy surveys, it had become common practice to treat individual galaxies as single entities. However, by carefully disentangling each galaxy’s components, namely their inner spheroid and outer disc, the researchers uncovered the missing population.

“While the inner component is compact and massive, the full galaxy sizes are not compact,” said PhD student Giulia Savorgnan. “This explains why they had been missed. We simply needed to better dissect the galaxies rather than consider them as single objects.”

Closer to home, the central spheroid of our own Milky Way seems to have, in part, also existed when our universe was young. We know some stars are 12 billion years old, not much younger than the age of our universe. The uncertain question is what fraction of our galaxy’s bulge may have subsequently been built via other processes.

Astronomers Find Impossibly Large Black Hole

An international team of astronomers has found a huge and ancient black hole that was powering the brightest object early in the universe. The black hole’s mass is 12 billion times that of the Sun, and was at the centre of a quasar that pumped out a million billion times its energy.

Team member Dr Fuyan Bian of the Australian National University said the discovery challenges theories about how black holes form and grow in the early universe. “Forming such a large black hole so quickly is hard to interpret with current theories,” he said.

A quasar is an extremely bright cloud of material in the process of being sucked into a black hole. As the material accelerates towards the black hole it heats up, emitting an extraordinary amount of light that actually pushes away material falling behind it.

“This process, known as radiation pressure, is thought to limit the growth rate of black holes,” Bian said. “However, this black hole at the centre of the quasar gained enormous mass in a short period of time.”

The team, led by Xue-Bing Wu at Peking University, selected the quasar from the Sloan Digital Sky Survey of more than 500 million objects in the northern skies, because of its distinctive red colour. They then followed up with three other telescopes to study the object in detail.

We’re fast becoming a leader in many of these fields here in Australia. Bian expects more surprising objects will be discovered during the ANU’s Skymapper survey of the southern skies.

“Skymapper will find more of these exciting objects. Because they are so luminous, we can see further back in time and can use them to explore the early universe,” Bian said.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio.