Mysterious dance of dwarfs may force a cosmic rethink

This is an artist's impression of the coherent orbit of dwarf galaxies about a large galaxy. 

Credit: Geraint Lewis

The discovery that many small galaxies throughout the universe do not 'swarm' around larger ones like bees do but 'dance' in orderly disc-shaped orbits is a challenge to our understanding of how the universe formed and evolved.

The finding, by an international team of astronomers, including Professor Geraint Lewis from the University of Sydney's School of Physics, is announced today in Nature.

"Early in 2013 we announced our startling discovery that half of the dwarf galaxies surrounding the Andromeda Galaxy are orbiting it in an immense plane" said Professor Lewis.

"This plane is more than a million light years in diameter, but is very thin, with a width of only 300 000 light years."

The universe contains billions of galaxies. Some, such as the Milky Way, are immense, containing hundreds of billions of stars. Most galaxies, however, are dwarfs, much smaller and with only a few billion stars.

For decades astronomers have used computer models to predict how these dwarf galaxies should orbit large galaxies. They had always found that they should be scattered randomly.

"Our Andromeda discovery did not agree with expectations, and we felt compelled to explore if it was true of other galaxies throughout the universe," said Professor Lewis.

Using the Sloan Digital Sky Survey (SDSS), a remarkable resource of colour images and 3-D maps covering more than a third of the sky, the researchers dissected the properties of thousands of nearby galaxies.

"We were surprised to find that a large proportion of pairs of satellite galaxies have oppositely directed velocities if they are situated on opposite sides of their giant galaxy hosts", said lead author Neil Ibata of the Lycée International in Strasbourg, France.

"Everywhere we looked we saw this strangely coherent coordinated motion of dwarf galaxies. From this we can extrapolate that these circular planes of dancing dwarfs are universal, seen in about 50 percent of galaxies," said Professor Geraint Lewis.

"This is a big problem that contradicts our standard cosmological models. It challenges our understanding of how the universe works including the nature of dark matter."

The researchers believe the answer may be hidden in some currently unknown physical process that governs how gas flows in the universe, although, as yet, there is no obvious mechanism that can guide dwarf galaxies into narrow planes.

Some experts, however, have made more radical suggestions, including bending and twisting the laws of gravity and motion.

"Throwing out seemingly established laws of physics is unpalatable," said Professor Lewis, "but if our observations of nature are pointing us in this direction, we have to keep an open mind. That's what science is all about."

More information: "Velocity anti-correlation of diametrically opposed galaxy satellites in the low-redshift Universe." Neil G. Ibata, et al. Nature (2014) DOI: 10.1038/nature13481. 

Red stars and big bulges: How black holes shape galaxies

Images of a small fraction of the galaxies analysed in the new study. 

The galaxies are ordered by total mass of stars (rising from bottom to top) and by ‘bulge to total stellar mass ratio’ (rising from left to right). 

Galaxies that appear redder have high values for both of these measurements, meaning that the mass of the bulge –and central black hole – determines their colour. 

Credit: A. Bluck.

The universe we can see is made up of billions of galaxies, each containing anywhere from hundreds of thousands to hundreds of billions of stars.

Large numbers of galaxies are elliptical in shape, red and mostly made up of old stars.

Another (more familiar) type is the spiral, where arms wind out in a blue thin disk from a central red bulge. On average stars in spiral galaxies tend to be much younger than those in ellipticals.

Asa Bluck

Now a group of astronomers led by Asa Bluck of the University of Victoria in Canada have found a (relatively) simple relationship between the colour of a galaxy and the size of its bulge – the more massive the bulge the redder the galaxy.

The researchers publish their results in the Oxford University Press journal Monthly Notices of the Royal Astronomical Society.

Asa and his team used data from the Sloan Digital Sky Survey (SDSS) to group together over half a million galaxies of all different colours, shapes, and masses.

They then used pattern recognition software to measure the shape of each one, to see how the proportion of red stars in a galaxy varies with its other properties.

They found that the mass in the central bulge (regardless of how big the disk surrounding it may be) is the key to knowing the colour of the whole galaxy.

Above a given bulge mass, galaxies are red and have no new young stars. Almost all galaxies have supermassive black holes at their centres.

The mass of the bulge is closely related to the mass of the black hole; the more massive the black hole the more energy is released into the surrounding galaxy in the form of powerful jets and X-ray emission.

This can blow away and heat up gas, stopping new stars from forming.

Asa comments: "A relatively simple result, that large galaxy bulges mean red galaxies, has profound consequences. Big bulges mean big black holes and these can put an end to star formation."

Journal Reference: Asa F. L. Bluck, J. Trevor Mendel, Sara L. Ellison, Jorge Moreno, Luc Simard, David R. Patton, Else Starkenburg. Bulge mass is king: The dominant role of the bulge in determining the fraction of passive galaxies in the Sloan Digital Sky Survey. Monthly Notices of the Royal Astronomical Society, 2014

Dark Matter: Cosmologists weigh cosmic filaments and voids

A zoomed-out view of galaxies identified by the Sloan Digital Sky Survey. 

Filaments and voids are visible at this scale.

Cosmologists have established that much of the stuff of the universe is made of dark matter, a mysterious, invisible substance that can't be directly detected but which exerts a gravitational pull on surrounding objects.

Dark matter is thought to exist in a vast network of filaments throughout the universe, pulling luminous galaxies into an interconnected web of clusters, interspersed with seemingly empty voids.

Researchers at the University of Pennsylvania have measured the "weight" of these cosmic voids and filaments for the first time, showing the former are not as empty as they look.

The studies of voids and filaments are currently available on the ArXiv (arXiv:1402.3302) and were conducted by graduate student Joseph Clampitt and professor Bhuvnesh Jain of the Department of Physics and Astronomy in Penn's School of Arts & Sciences.

Gravitational lensing, the tiny distortions of distant galaxy images due to intervening matter, allows scientists to weigh galaxies by measuring how much their light bends.

Voids, on the other hand, are enormous, seemingly empty spaces in the universe with scarcely any galaxies visible — an arrangement that makes measuring their contents through lensing more difficult.

While galaxies and filaments have more mass than the average regions of the universe, voids have less mass than average.

This unbalanced distribution causes matter to rapidly move away from voids and towards the concentrations of mass along the cosmic filaments that lie between them.

A depiction of filaments and voids from The Max Planck Institute for Astrophysics’ Millennium Simulation Project.

"This means that voids act like objects with an effectively negative mass," Clampitt said, "such that even light rays bend away from them. They act roughly like concave lenses, the opposite of big galaxies, which act like convex lenses."

Clampitt and Jain detected the tiny distortions produced by voids on the images of nearly 40 million galaxies in the Sloan Digital Sky Survey.

This breakthrough came just a few months after they, along with Masahiro Takada of Tokyo University's Institute for the Physics and Mathematics of the Universe, detected the lensing signal from the dark matter filaments that connect galaxies.

"The measurements came as a wonderful surprise," Jain said. "Theoretical studies had predicted that we'd have to wait for much bigger surveys well into the future to detect void lensing. Joseph's ingenious analysis techniques extracted a subtle signal no one had seen before."

Their results show that voids are not as empty as they appear. Dark matter and other dim structures permeate all the way to the center of the voids.

"Although the density of this matter is far less than average," Clampitt said, "it is somewhat surprising that the voids are not as empty as the galaxy distribution suggests."

"The density at the center of a typical void," Jain said, "is about half the mean density in the universe, but that still leaves the voids with an enormous deficit in mass, about a thousand trillion times the mass of the sun."

Painted Stones video: Asteroids observed by the Sloan Digital Sky Survey

Alex Parker is an astronomer at UC Berkeley, where he researches minor planets—asteroids, Kuiper Belt Objects (giant iceballs orbiting past Neptune), and more.

He took the asteroids in the solar system observed by the Sloan Digital Sky Survey (over 100,000 of them) and created an animation showing their orbits, their relative sizes, and even their colors in the survey. 

The resulting video, “Painted Stones”, has been called 'simply wondrous.'

Dwarf galaxies provide clues to origin of supermassive black holes

Dwarf galaxy NGC 4395, about 13 million light-years from Earth, known to harbour a black hole some 300,000 times more massive than the Sun. 

It is a prototypical example of a small galaxy once thought to be too small to contain such a black hole. 

Credit: David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration; NRAO/AUI/NSF.

Pouring through data from a large sky survey, astronomers have found more than 100 small, dwarf galaxies with characteristics indicating that they harbor massive black holes feeding on surrounding gas.

The discovery confounds a common assumption that only much larger galaxies hold such monsters and may help resolve the question of how such black holes originated and grew in the early universe.

Amy Reines

"We've shown that even small galaxies can have massive black holes and that they may be more common than previously thought," said Amy Reines, of the National Radio Astronomy Observatory (NRAO).

"This is really exciting because these little galaxies hold the clues to the origin of the first 'seeds' of supermassive black holes in the early universe," she said. Reines and her colleagues presented their findings to the American Astronomical Society's meeting in Washington, DC.

Black holes are concentrations of mass so dense that not even light can escape their gravitational pull.

Nearly all "full-sized" galaxies are known to have supermassive black holes, millions or billions of times more massive than the Sun, at their cores.

Until recently, however, smaller galaxies were thought not to harbor massive black holes.

Marla Geha

Reines, along with Jenny Greene of Princeton University and Marla Geha of Yale University, analyzed data from the Sloan Digital Sky Survey and found more than 100 dwarf galaxies whose patterns of light emission indicated the presence of massive black holes and their feeding process.

"The galaxies are comparable in size to the Magellanic Clouds, dwarf satellite galaxies of the Milky Way," Geha said.

"Previously, such galaxies were thought to be too small to have such massive black holes," she added.

In the nearby universe, astronomers have found a direct relationship between the mass of a galaxy's central black hole and a "bulge" in its center.

This indicates that the black holes and the bulges may have affected each others' growth.