Mysterious quasar sequence explained

A growing black hole, called a Quasar, can be seen at the center of a faraway galaxy in this artist's concept. 

Credit: NASA/JPL-Caltech

Quasars are supermassive black holes that live at the center of distant massive galaxies.

They shine as the most luminous beacons in the sky across the entire electromagnetic spectrum by rapidly accreting matter into their gravitationally inescapable centers.

Yue Shen

New work from Carnegie's Hubble Fellow Yue Shen and Luis Ho of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University solves a quasar mystery that astronomers have been puzzling over for 20 years.

Their work, published in the September 11 issue of Nature, shows that most observed quasar phenomena can be unified with two simple quantities: one that describes how efficiently the hole is being fed, and the other that reflects the viewing orientation of the astronomer.

Luis Ho

Quasars display a broad range of outward appearances when viewed by astronomers, reflecting the diversity in the conditions of the regions close to their centers, but despite this variety, quasars have a surprising amount of regularity in their quantifiable physical properties, which follow well-defined trends (referred to as the "main sequence" of quasars) discovered more than 20 years ago.

Shen and Ho solved a two-decade puzzle in quasar research: What unifies these properties into this main sequence?

Using the largest and most-homogeneous sample to date of over 20,000 quasars from the Sloan Digital Sky Survey (SDSS), combined with several novel statistical tests, Shen and Ho were able to demonstrate that one particular property related to the accretion of the hole, called the Eddington ratio, is the driving force behind the so-called main sequence.

The Eddington Ratio

The Eddington ratio describes the efficiency of matter fueling the black hole, the competition between the gravitational force pulling matter inward and the luminosity driving radiation outward.

This push and pull between gravity and luminosity has long been suspected to be the primary driver behind the so-called main sequence, and their work at long last confirms this hypothesis.

Of additional importance, they found that the orientation of an astronomer's line-of-sight when looking down into the black hole's inner region plays a significant role in the observation of the fast-moving gas innermost to the hole, which produces the broad emission lines in quasar spectra.

This changes scientists' understanding of the geometry of the line-emitting region closest to the black hole, a place called the broad-line region: the gas is distributed in a flattened, pancake-like configuration.

Going forward, this will help astronomers improve their measurements of black hole masses for quasars.

New work solves a quasar mystery that astronomers have been puzzling over for 20 years. 

It shows that most observed quasar phenomena can be unified with two simple quantities: one that describes how efficiently the hole is being fed, and the other that reflects the viewing orientation of the astronomer.

This graph shows the distribution of about 20,000 luminous Sloan Digital Sky Survey (SDSS) quasars in the two-dimensional space of broad line width versus FeII strength, colour-coded by the strength of the narrow [OIII] line emission.

The strong horizontal trend is the main sequence of quasars driven by the efficiency of the black hole accretion, while the vertical spread of broad line width is largely due to our viewing angle to the inner region of the quasar. 

Credit: Yue Shen

"Our findings have profound implications for quasar research. This simple unification scheme presents a pathway to better understand how supermassive black holes accrete matter and interplay with their environments," Shen said.

"And better black hole mass measurements will benefit a variety of applications in understanding the cosmic growth of supermassive black holes and their place in galaxy formation," Ho added.

More information: The diversity of quasars unified by accretion and orientation, Nature, dx.doi.org/10.1038/nature13712

Astronomers find stream of gas, 2.6 million light years long

The bridge of gas (shown in green) stretches from the large galaxy at the bottom left to the group of galaxies at the top. 

A third nearby galaxy to the right also has a shorter stream of gas attached to it. 

The three insets show expanded views of the different galaxies and the green circle indicates the Arecibo telescope beam.

Credit: Rhys Taylor /Arecibo Galaxy Environment Survey /The Sloan Digital Sky Survey Collaboration

Atronomers and students have found a bridge of atomic hydrogen gas 2.6 million light years long between galaxies 500 million light years away.

They detected the gas using the William E. Gordon Telescope at the Arecibo Observatory, a radio astronomy facility of the US National Science Foundation sited in Puerto Rico.

The results are published today in a paper in Monthly Notices of the Royal Astronomical Society.

The stream of atomic hydrogen gas is the largest known, a million light years longer than a gas tail found in the Virgo Cluster by another Arecibo project a few years ago.

Dr Rhys Taylor, a researcher at the Czech Academy of Sciences and lead author of the paper, said "This was totally unexpected.

We frequently see gas streams in galaxy clusters, where there are lots of galaxies close together, but to find something this long and not in a cluster is unprecedented."

It is not just the length of the stream that is surprising but also the amount of gas found in it. Roberto Rodriguez, a 2014 graduate from the University of Puerto Rico in Humacao who worked on the project as an undergraduate, explained "We normally find gas inside galaxies, but here half of the gas, 15 billion times the mass of the Sun, is in the bridge. That's far more than in the Milky Way and Andromeda galaxies combined!"

The team is still investigating the origin of the stream. One notion surmises that the large galaxy at one end of the stream passed close to the group of smaller galaxies at the other end in the past, and that the gas bridge was drawn out as they moved apart, while a second notion presumes that the large galaxy plowed straight through the middle of the group, pushing gas out of it.

The team plan to use computer simulations to find out which of these ideas can best match the shape of the bridge that is seen with the Arecibo Telescope.

The project involved three undergraduate researchers: Roberto Rodriguez and Clarissa Vazquez from UPR Humacao, and Hanna Herbst, now a graduate student at the University of Florida. Dr Robert Minchin, a staff astronomer at Arecibo Observatory and the principal investigator on the project, said "Student involvement is very important to us."

"We are proud to be inspiring the next generation of astronomers, and particularly proud of the involvement of Puerto Rican students."

The bridge was found in data taken between 2008 and 2011 for the Arecibo Galaxy Environment Survey (AGES), which is using the power of the Arecibo Telescope to survey a large area of sky with a high level of sensitivity.

Journal Reference: R. Taylor, R. F. Minchin, H. Herbst, J. I. Davies, R. Rodriguez And C. Vazquez. The Arecibo Galaxy Environment Survey – VII. A dense filament with extremely long H i streams. Monthly Notices of the Royal Astronomical Society, 2014 DOI: 10.1093/mnras/stu1305

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

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.'