Posted on by Jason Major

A Galaxy Grows Fat on Nearby Gas

An artist’s impression showing a galaxy in the process of pulling in cool gas from its surroundings. (ESO/L. Calçada/ESA/AOES Medialab)

If you live in the U.S. you may be enjoying a sultry summer day off in honor of Independence Day, or at least have plans to get together with friends and family at some point to partake in some barbecued goodies and a favorite beverage (or three). And as you saunter around the picnic table scooping up platefuls of potato salad, cole slaw, and deviled eggs, you can also draw a correlation between your own steady accumulation of mayonnaise-marinated mass and a distant hungry galaxy located over 11 billion light-years away.

Astronomers have always suspected that galaxies grow by pulling in material from their surroundings, but this process has proved very difficult to observe directly. Now, ESO’s Very Large Telescope has been used to study a very rare alignment between a distant galaxy and an even more distant quasar — the extremely bright center of a galaxy powered by a supermassive black hole. The light from the quasar passes through the material around the foreground galaxy before reaching Earth, making it possible to explore in detail the properties of the in-falling gas and giving the best view so far of a galaxy in the act of feeding.

“This kind of alignment is very rare and it has allowed us to make unique observations,” said Nicolas Bouché of the Research Institute in Astrophysics and Planetology (IRAP) in Toulouse, France, lead author of the new paper. “We were able to use ESO’s Very Large Telescope to peer at both the galaxy itself and its surrounding gas. This meant we could attack an important problem in galaxy formation: how do galaxies grow and feed star formation?”

A beam from the Laser Star Guide on one of the VLT's four Unit Telescopes helps to correct the blurring effect of Earth's atmosphere before making observations (ESO/Y. Beletsky)
A beam from the Laser Star Guide on one of the VLT’s four Unit Telescopes helps to correct the blurring effect of Earth’s atmosphere before making observations (ESO/Y. Beletsky)

Galaxies quickly deplete their reservoirs of gas as they create new stars and so must somehow be continuously replenished with fresh gas to keep going. Astronomers suspected that the answer to this problem lay in the collection of cool gas from the surroundings by the gravitational pull of the galaxy. In this scenario, a galaxy drags gas inwards which then circles around it, rotating with it before falling in.

Although some evidence of such accretion had been observed in galaxies before, the motion of the gas and its other properties had not been fully explored up to now.

Astronomers have already found evidence of material around galaxies in the early Universe, but this is the first time that they have been able to show clearly that the material is moving inwards rather than outwards, and also to determine the composition of this fresh fuel for future generations of stars. And in this particular instance, without the quasar’s light to act as a probe the surrounding gas would be undetectable.

“In this case we were lucky that the quasar happened to be in just the right place for its light to pass through the infalling gas. The next generation of extremely large telescopes will enable studies with multiple sightlines per galaxy and provide a much more complete view,” concluded co-author Crystal Martin of the University of California Santa Barbara.

This research was presented in a paper entitled “Signatures of Cool Gas Fueling a Star-Forming Galaxy at Redshift 2.3”, to appear in the July 5, 2013 issue of the journal Science.

Source: ESO news release

Posted on by Tammy Plotner

Hubble Telescope Directly Observes Quasar Accretion Disc Surrounding Black Hole

A team of scientists has used the NASA/ESA Hubble Space Telescope to observe a quasar accretion disc — a brightly glowing disc of matter that is slowly being sucked into its galaxy’s central black hole. Their study makes use of a novel technique that uses gravitational lensing to give an immense boost to the power of the telescope. The incredible precision of the method has allowed astronomers to directly measure the disc’s size and plot the temperature across different parts of the disc. Image credit: NASA, ESA, J.A. Munoz (University of Valencia)

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Thanks to the magic of the NASA/ESA Hubble Space Telescope, a team of international astronomers have made an incredible observation – a quasar accretion disc surrounding a black hole. By employing a technique known as gravitation lensing, the researchers have been able to get an accurate size measurement and spectral data. While you might not think this exciting at first, know that this type of observation is akin to spotting individual grains of sand on the Moon!

Of course, we know we can’t see a black hole – but we’ve learned a lot about them with time. One of their properties is a bright, visible phenomenon called a quasar. These glowing discs of matter are engaged in orbit around the black hole, much like a coil on an electric stove. As energy is applied, the “coil” heats up and unleashes bright radiation.

“A quasar accretion disc has a typical size of a few light-days, or around 100 billion kilometres across, but they lie billions of light-years away. This means their apparent size when viewed from Earth is so small that we will probably never have a telescope powerful enough to see their structure directly,” explains Jose Munoz, the lead scientist in this study.

Because of the diminutive size of the quasar, most of our understanding of how they work has been based on theory… but great minds have found a way to directly observe their effects. By employing the gravity of stars in an intervening galaxy like a scanning microscope, astronomers have been able to observe the quasar’s light as the stars move. While most of these types of features would be too small to see, the gravitation lensing effect ramps up the strength of the quasar’s light and allows study of the spectra as it cruises across the accretion disc.

This diagram shows how Hubble is able to observe a quasar, a glowing disc of matter around a distant black hole, even though the black hole would ordinarily be too far away to see clearly. Credit: NASA and ESA

By observing a group of gravitationally lensed quasars, the team was able to paint a vivid color portrait of the activity. They were able to pick out small changes between single images and spectral shifts over a period of time. What causes these kaleidoscopic variances? For the most part, it’s the different properties in the gases and dust of the lensing galaxies. Because they travel at different angles to the quasar’s light, scientists were even able to distinguish extinction laws at work.

But there was something special about one of the quasars. Says the Hubble Team, “There were clear signs that stars in the intervening galaxy were passing through the path of the light from the quasar. Just as the gravitational effect due to the whole intervening galaxy can bend and amplify the quasar’s light, so can that of the stars within the intervening galaxy subtly bend and amplify the light from different parts of the accretion disc as they pass through the path of the quasar’s light.”

By documenting these color changes, the team could build a profile of the accretion disc. Unlike our Earthly electric stove coil which glows red as it heats up, the accretion disc of a black hole turns blue as it gets closer to the event horizon. By measuring the blue hue, the team was able to measure the disc diameter and the various tints gave them an indicator of distances from its center. In this case, they found that the disc is between four and eleven light-days across (approximately 100 to 300 billion kilometres). Of course, these are only rough estimates, but considering just how far away we’re looking at such a small object gives these types of observations great potential for future studies… and even improvements on accuracy.

“This result is very relevant because it implies we are now able to obtain observational data on the structure of these systems, rather than relying on theory alone,” says Munoz. “Quasars’ physical properties are not yet well understood. This new ability to obtain observational measurements is therefore opening a new window to help understand the nature of these objects.”

Original Story Source: ESA/Hubble News Release. For Further Reading: A Study of Gravitational Lens Chromaticity With the Hubble Space Telescope.

Posted on by Nancy Atkinson

Huge Reservoir of Water Discovered in Space 30 Billion Trillion Miles Away

This artist's concept illustrates a quasar, or feeding black hole, similar to APM 08279+5255, where astronomers discovered huge amounts of water vapor. Gas and dust likely form a torus around the central black hole, with clouds of charged gas above and below. Image credit: NASA/ESA

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From a Caltech Press Release:

Water really is everywhere. Two teams of astronomers, each led by scientists at the California Institute of Technology (Caltech), have discovered the largest and farthest reservoir of water ever detected in the universe. Looking from a distance of 30 billion trillion miles away into a quasar—one of the brightest and most violent objects in the cosmos—the researchers have found a mass of water vapor that’s at least 140 trillion times that of all the water in the world’s oceans combined, and 100,000 times more massive than the sun.

Because the quasar is so far away, its light has taken 12 billion years to reach Earth. The observations therefore reveal a time when the universe was just 1.6 billion years old. “The environment around this quasar is unique in that it’s producing this huge mass of water,” says Matt Bradford, a scientist at NASA’s Jet Propulsion Laboratory (JPL), and a visiting associate at Caltech. “It’s another demonstration that water is pervasive throughout the universe, even at the very earliest times.” Bradford leads one of two international teams of astronomers that have described their quasar findings in separate papers that have been accepted for publication in the Astrophysical Journal Letters.

Read Bradford & team’s paper here.

A quasar is powered by an enormous black hole that is steadily consuming a surrounding disk of gas and dust; as it eats, the quasar spews out huge amounts of energy. Both groups of astronomers studied a particular quasar called APM 08279+5255, which harbors a black hole 20 billion times more massive than the sun and produces as much energy as a thousand trillion suns.

Since astronomers expected water vapor to be present even in the early universe, the discovery of water is not itself a surprise, Bradford says. There’s water vapor in the Milky Way, although the total amount is 4,000 times less massive than in the quasar, as most of the Milky Way’s water is frozen in the form of ice.

Nevertheless, water vapor is an important trace gas that reveals the nature of the quasar. In this particular quasar, the water vapor is distributed around the black hole in a gaseous region spanning hundreds of light-years (a light-year is about six trillion miles), and its presence indicates that the gas is unusually warm and dense by astronomical standards. Although the gas is a chilly –53 degrees Celsius (–63 degrees Fahrenheit) and is 300 trillion times less dense than Earth’s atmosphere, it’s still five times hotter and 10 to 100 times denser than what’s typical in galaxies like the Milky Way.

The water vapor is just one of many kinds of gas that surround the quasar, and its presence indicates that the quasar is bathing the gas in both X-rays and infrared radiation. The interaction between the radiation and water vapor reveals properties of the gas and how the quasar influences it. For example, analyzing the water vapor shows how the radiation heats the rest of the gas. Furthermore, measurements of the water vapor and of other molecules, such as carbon monoxide, suggest that there is enough gas to feed the black hole until it grows to about six times its size. Whether this will happen is not clear, the astronomers say, since some of the gas may end up condensing into stars or may be ejected from the quasar.

Bradford’s team made their observations starting in 2008, using an instrument called Z-Spec at the Caltech Submillimeter Observatory (CSO), a 10-meter telescope near the summit of Mauna Kea in Hawaii. Z-Spec is an extremely sensitive spectrograph, requiring temperatures cooled to within 0.06 degrees Celsius above absolute zero. The instrument measures light in a region of the electromagnetic spectrum called the millimeter band, which lies between infrared and microwave wavelengths. The researchers’ discovery of water was possible only because Z-Spec’s spectral coverage is 10 times larger than that of previous spectrometers operating at these wavelengths. The astronomers made follow-up observations with the Combined Array for Research in Millimeter-Wave Astronomy (CARMA), an array of radio dishes in the Inyo Mountains of Southern California.

This discovery highlights the benefits of observing in the millimeter and submillimeter wavelengths, the astronomers say. The field has developed rapidly over the last two to three decades, and to reach the full potential of this line of research, the astronomers—including the study authors—are now designing CCAT, a 25-meter telescope to be built in the Atacama Desert in Chile. CCAT will allow astronomers to discover some of the earliest galaxies in the universe. By measuring the presence of water and other important trace gases, astronomers can study the composition of these primordial galaxies.

The second group, led by Dariusz Lis, senior research associate in physics at Caltech and deputy director of the CSO, used the Plateau de Bure Interferometer in the French Alps to find water. In 2010, Lis’s team was looking for traces of hydrogen fluoride in the spectrum of APM 08279+5255, but serendipitously detected a signal in the quasar’s spectrum that indicated the presence of water. The signal was at a frequency corresponding to radiation that is emitted when water transitions from a higher energy state to a lower one. While Lis’s team found just one signal at a single frequency, the wide bandwidth of Z-Spec enabled Bradford and his colleagues to discover water emission at many frequencies. These multiple water transitions allowed Bradford’s team to determine the physical characteristics of the quasar’s gas and the water’s mass.

Read Lis & team’s paper here.