Universe Today

An illustration showing a quasar at the center of the galaxy. Image credit: NASA Click to enlarge
Sometimes the supermassive black holes at the hearts of galaxies are quiet, and nearly invisible. Other times they’re actively gobbling up material, blazing as quasars in the X-ray spectrum. NASA’s Chandra X-Ray Observatory has observed one of these transition times, when the heated material around the supermassive black hole is beginning to ignite. It’s likely the galaxy recently collided or merged with another galaxy, and the turbulence caused material to fall into the black hole.

An artist’s illustration depicts a quasar in the center of a galaxy that has turned on and is expelling gas at high speeds in a galactic superwind. Clouds of hot, X-ray producing gas detected by Chandra around the quasars 4C37.43 and 3C249.1, provide strong evidence for such superwinds.

The X-ray features seen at five, six, ten and eleven o’clock in the 4C37.43 image are located tens of thousands of light years from the central supermassive black hole that powers the quasar. They are likely due to shock waves in the superwind.

Mergers of galaxies are a possible cause for the ignition, or turn-on, of quasars. Computer simulations show that a galactic merger drives gas toward the central region where it triggers a burst of star formation and provides fuel for the growth of a central black hole.

The inflow of gas into the black hole releases tremendous energy, and a quasar is born. The power output of the quasar dwarfs that of the surrounding galaxy and pushes gas out of the galaxy in a galactic superwind.

Over a period of about 100 million years, the superwind will drive most of the gas away from the central regions of the galaxy, quenching both star formation and further supermassive black hole growth. The quasar phase will end and the galaxy will settle down to a relatively quiet life.

Original Source: Chandra X-ray Observatory

A hot gas cloud whirling around a miniature ‘cannibal’ star. Image credit: ESA Click to enlarge
ESA’s XMM-Newton space telescope has observed the tiny cores of dead stars wrapped up in a nice warm blanket of superheated gas. These “low-mass X-ray binary” are pulling a steady stream of material from a larger companion star, and then whipping it up into a disk. This observation answers the question of why these dead stars sometimes blink off in the X-ray spectrum. That’s the time when we’re seeing this disk edge-on, and it’s obscuring our view of the star.

ESA’s XMM-Newton has seen vast clouds of superheated gas, whirling around miniature stars and escaping from being devoured by the stars’ enormous gravitational fields – giving a new insight into the eating habits of the galaxy’s ‘cannibal’ stars.

The clouds of gas range in size from a few hundred thousand kilometres to a few million kilometres, ten to one hundred times larger than the Earth. They are composed of iron vapour and other chemicals at temperatures of many millions of degrees.

“This gas is extremely hot, much hotter than the outer atmosphere of the Sun,” said Maria Diaz Trigo of ESA’s European Science and Technology Research Centre (ESTEC), who led the research.

ESA’s XMM-Newton x-ray observatory made the discovery when it observed six so-called ‘low-mass X-ray binary’ stars (LMXBs). The LMXBs are pairs of stars in which one is the tiny core of a dead star.

Measuring just 15?20 kilometres across and comparable in size to an asteroid, each dead star is a tightly packed mass of neutrons containing more than 1.4 times the mass of the Sun.

Its extreme density generates a powerful gravitational field that rips gas from its ‘living’ companion star. The gas spirals around the neutron star, forming a disc, before being sucked down and crushed onto its surface, a process known as ‘accretion’.

The newly discovered clouds sit where the river of matter from the companion star strikes the disc. The extreme temperatures have ripped almost all of the electrons from the iron atoms, leaving them carrying extreme electrical charges. This process is known as ‘ionisation’.

The discovery solves a puzzle that has dogged astronomers for several decades. Certain LMXBs appear to blink on and off at X-ray wavelengths. These are ‘edge-on’ systems, in which the orbit of each gaseous disc lines up with Earth.

In previous attempts to simulate the blinking, clouds of low-temperature gas were postulated to be orbiting the neutron star, periodically blocking the X-rays. However, these models never reproduced the observed behaviour well enough.

XMM-Newton solves this by revealing the ionised iron. “It means that these clouds are much hotter than we anticipated,” said Diaz. With high-temperature clouds, the computer models now simulate much better the dipping behaviour.

Some 100 known LMXBs populate our galaxy, the Milky Way. Each one is a stellar furnace, pumping X-rays into space. They represent a small-scale model of the accretion thought to be taking place in the very heart of some galaxies. One in every ten galaxies shows some kind of intense activity at its centre.

This activity is thought to be coming from a gigantic black hole, pulling stars to pieces and devouring their remains. Being much closer to Earth, the LMXBs are easier to study than the active galaxies.

“Accretion processes are still not well understood. The more we understand about the LMXBs, the more useful they will be as analogues to help us understand the active galactic nuclei,” says Diaz.

Original Source: ESA Portal

An image of the cool brown dwarf orbiting a star near the Sun. Image credit: UA Steward Observatory. Click to enlarge
Astronomers have discovered a brown dwarf in our galactic neighbourhood, only 12.7 light years away – this makes it the second closest brown dwarf ever discovered. The failed star is circling another star that was only recently discovered in the southern constellation Pavo. The primary star is small, with only 1/10th the mass of our Sun, and the brown dwarf orbits at 4.5 times the distance of the Earth to the Sun.

Astronomers have discovered a unique “brown dwarf” right in our solar neighborhood.

If your city were the galaxy, it would be like finding someone you didn’t know about living upstairs in your house, one of the discoverers said.

The rare object is only 12.7 light years from Earth, circling a primary star that itself was discovered only recently in the southern hemisphere constellation Pavo (the Peacock).

Only one other brown dwarf system has been found closer to Earth, and it’s only marginally closer.

The primary star is only one-tenth the mass of our sun. This is the first time astronomers have found a cool brown dwarf companion to such a low-mass star. Until now, none has been found orbiting stars less than half the mass of our sun.

The brown dwarf is 4.5 AU from the star, or four and one-half times farther from its star than Earth is from our sun. Astronomers estimate that the brown dwarf is between nine and 65 times as massive as Jupiter.

Brown dwarfs are neither planets nor stars. They are dozens of times more massive than our solar system’s largest planet, Jupiter, but too small to be self-powered by hydrogen fusion like stars.

Only about 30 similarly cool brown dwarfs have been found anywhere in the sky, and only about 10 have been discovered orbiting stars.

“Besides being extremely close to Earth and in orbit around a very low-mass star, this object is a ‘T dwarf ‘ – a very cool brown dwarf with a temperature of about 750 degrees Celsius (1,382 degrees Fahrenheit),” said Beth Biller, a graduate student at The University of Arizona.

“It is also likely the brightest known object of its temperature because it is so close,” Biller said. “And it’s a rare example of a brown dwarf companion within 10 astronomical units of its primary star.”

Biller, along with Markus Kasper of the European Southern Observatory (ESO) and Laird Close of UA’s Steward Observatory, led the team who discovered the brown dwarf, designated SCR 1845-6357B.

“What’s really exciting about this is that we found the brown dwarf around one of the 25 stellar systems nearest to the sun,” Close said. “Most of these nearby stars have been known for decades, and only just recently a handful of new objects have been found in our local neighborhood.”

Close said, “If you think of the galaxy as being the size of Tucson, it’s kind of like finding someone living in the upstairs of your house that you didn’t know about before.”

Close helped develop the special adaptive optics camera, the NACO Simultaneous Differential Imager(SDI), that the team used to image the brown dwarf. The camera is used on ESO’s Very Large Telescope (VLT) in Chile. Another SDI camera is used at the 6.5-meter MMT Observatory on Mount Hopkins, Ariz.

“This is also a valuable object to the scientific community because its distance is well known,” said ESO’s Markus Kasper. This will allow astronomers to measure the brown dwarf’s luminosity accurately and, eventually, to calculate its orbital motion, Kasper said. “These properties are vital for understanding the nature of brown dwarfs.”

The discovery of this brown dwarf suggests there may be more cool brown dwarfs in binary systems than single brown dwarfs floating free in the solar neighborhood, Close said. A “binary system” is where a brown dwarf revolves around a star or another brown dwarf.

Astronomers now have found five cool brown dwarfs in binary systems but only two single, isolated cool brown dwarfs within 20 light years of the sun, Close noted. They can expect to find more T dwarf companions in some newly found stellar systems within 33 light years of our solar system, he added.

Evidence that T dwarfs in binary systems outnumber single, isolated T dwarfs in the solar neighborhood has ramifications for theories that predict single brown dwarfs will form more often than binary ones, Close said.

The NACO Simultaneous Differential Imager(SDI) uses adaptive optics to remove the blurring effects of Earth’s atmosphere to produce extremely sharp images. The camera enhances the ability of the VLT to detect faint companions that would otherwise be lost in the glare of their primary stars.

Close and Rainer Lenzen of the Max Planck Institute for Astronomy in Heidelberg, Germany, developed the SDI camera to search for methane-rich extrasolar planets. The SDI camera splits light from a single object into four identical images, then passes the beams through three slightly different methane-sensitive filters. When the filtered light beams hit the detector array, astronomers subtract the images so the bright star disappears and its far dimmer, methane-rich companion pops into view.

The team will publish the discovery in the Astrophysical Journal Letters in the article, “Discovery of a Very Nearby Brown Dwarf to the Sun: A Methane Rich Brown Dwarf Companion to the Low Mass Star SCR 1845-6357.” In addition to Biller, Kasper and Close, team members include Wolfgang Brandner of the Max Planck Institute in Heidelberg, Germany, and Stephan Kellner of the W.M. Keck Observatory in Waimea, Hawaii.

Original Source: UA News Release

Inward migration of a group of protoplanets, where they’re represented by white circles. Image credit: QMUL Click to enlarge
Astronomers think they’ve got a handle on many aspects of planetary formation. But two British researchers have discovered a problem with the formation of gas giant planets. Under their model, the cores of these massive planets should be drawn inward by their parent star in only 100,000 years – not nearly enough time to form into a stable orbit. It could be that the first generations of planets never get past the “clump” stage before they’re destroyed. It’s only the later generations that actually survive long enough to become planets.

Two British astronomers, Paul Cresswell and Richard Nelson present new numerical simulations in the framework of the challenging studies of planetary system formation. They find that, in the early stages of planetary formation, giant protoplanets migrate inward in lockstep into the central star. Their results will soon be published in Astronomy & Astrophysics.

In an article to be published in Astronomy & Astrophysics, two British astronomers present new numerical simulations of how planetary systems form. They find that, in the early stages of planetary formation, giant protoplanets migrate inward in lockstep into the central star.

The current picture of how planetary systems form is as follows: i) dust grains coagulate to form planetesimals of up to 1 km in diameter; ii) the runaway growth of planetesimals leads to the formation of ~100 ? 1000 km-sized planetary embryos; iii) these embryos grow in an “oligarchic” manner, where a few large bodies dominate the formation process, and accrete the surrounding and much smaller planetesimals. These “oligarchs” form terrestrial planets near the central star and planetary cores of ten terrestrial masses in the giant planet region beyond 3 astronomical units (AU).

However, these theories fail to describe the formation of gas giant planets in a satisfactory way. Gravitational interaction between the gaseous protoplanetary disc and the massive planetary cores causes them to move rapidly inward over about 100,000 years in what we call the “migration” of the planet in the disc. The prediction of this rapid inward migration of giant protoplanets is a major problem, since this timescale is much shorter than the time needed for gas to accrete onto the forming giant planet. Theories predict that the giant protoplanets will merge into the central star before planets have time to form. This makes it very difficult to understand how they can form at all.

For the first time, Paul Cresswell and Richard Nelson examined what happens to a cluster of forming planets embedded in a gaseous protoplanetary disc. Previous numerical models have included only one or two planets in a disc. But our own solar system, and over 10% of the known extrasolar planetary systems, are multiple-planet systems. The number of such systems is expected to increase as observational techniques of extrasolar systems improve. Cresswell and Nelson’s work is the first time numerical simulations have included such a large number of protoplanets, thus taking into account the gravitational interaction between the protoplanets and the disc, and among the protoplanets themselves.

The primary motivation for their work is to examine the orbits of protoplanets and whether some planets could survive in the disc for extended periods of time. Their simulations show that, in very few cases (about 2%), a lone protoplanet is ejected far from the central star, thus lengthening its lifetime. But in most cases (98%), many of the protoplanets are trapped into a series of orbital resonances and migrate inward in lockstep, sometimes even merging with the central star.

Cresswell and Nelson thus claim that gravitational interactions within a swarm of protoplanets embedded in a disc cannot stop the inward migration of the protoplanets. The “problem” of migration remains and requires more investigation, although the astronomers propose several possible solutions. One may be that several generations of planets form and that only the ones that form as the disc dissipates survive the formation process. This may make it harder to form gas giants, as the disc is depleted of the material from which gas giant planets form. (Gas giant formation may still be possible though, if enough gas lies outside the planets’ orbits, since new material may sweep inward to be accreted by the forming planet). Another solution might be related to the physical properties of the protoplanetary disc. In their simulations, the astronomers assumed that the protoplanetary disc is smooth and non-turbulent, but of course this might not be the case. Large parts of the disc could be more turbulent (as a consequence of instabilities caused by magnetic fields), which may prevent inward migration over long time periods.

This work joins other studies of planetary system formation that are currently being done by a European network of scientists. Our view of how planets form has drastically changed in the last few years as the number of newly discovered planetary systems has increased. Understanding the formation of giant planets is currently one of the major challenges for astronomers.

Original Source: Astronomy & Astrophysics

Spitzer spots galactic clusters bonding back when the universe was about 4.6 billion years old. Image credit: NASA Click to enlarge
NASA’s Spitzer Space Telescope recently turned up clusters of galaxies located 7-9 billion light years from Earth. These galaxies are located at the very limits of Spitzer’s observing ability with its 85-centimeter (33-inch) telescope. Galaxy clusters are some of the largest structures in the Universe, consisting of thousands of galaxies and trillions of stars. This discovery gives astronomers more evidence about what kinds of structures existed in the early Universe.

Astronomers using NASA’s Spitzer Space Telescope have conducted a cosmic safari to seek out a rare galactic species. Their specimens — clusters of galaxies in the very distant universe — are few and far between, and have hardly ever been detected beyond a distance of 7 billion light-years from Earth.

To find the clusters, the team carefully sifted through Spitzer infrared pictures and ground-based catalogues; estimated rough distances based on the cluster galaxies’ colors; and verified suspicions using a spectrograph instrument at the W.M. Keck Observatory in Hawaii.

Ultimately, the expedition resulted in quite a galactic catch — the most distant galaxy cluster ever seen, located 9 billion light-years away. This means the cluster lived in an era when the universe was a mere 4.5 billion years old. The universe is believed to be 13.7 billion years old.

“Detecting a galaxy cluster 9 billion light-years away is very exciting,” said the study’s lead investigator, Dr. Peter Eisenhardt of NASA’s Jet Propulsion Laboratory. “It’s really amazing that Spitzer’s 85-centimeter telescope can see 9 billion years back in time.”

Using the same methods, the astronomers also found three other clusters living between 7 and 9 billion light-years away.

“Spitzer is an excellent instrument for detecting very distant galaxy clusters because they stand out so brightly in the infrared,” said co-investigator Dr. Mark Brodwin, also of JPL. “You can think of these distant galaxy cluster surveys as a game of ‘Where’s Waldo?’ With an optical telescope you can spot ‘Waldo,’ or the distant galaxy clusters, by carefully searching for them amongst a sea of faint galaxies.”

“But in the Spitzer data, it’s as though Waldo is wearing a bright neon hat and can be easily picked out of the crowd,” Brodwin added.

Galaxy clusters are the largest gravitationally bound structures in the universe. A typical cluster can contain thousands of galaxies and trillions of stars. Because of their huge size and mass, they are relatively rare. For example, if Earth were to represent the entire universe, then countries would be the equivalent of galaxies, and continents would be the galaxy clusters.

Galaxy clusters grow like snowballs, picking up new galaxies from gravitational interactions over billions of years. For this reason, team members say these behemoths should be even rarer in the very distant universe.

“The ultimate goal of this research is to find out when the galaxies in this and other distant clusters formed,” said co-investigator Dr. Adam Stanford, of the University of California at Davis. Stanford is the lead author of a paper on the most distant galaxy cluster’s discovery, which was published in the December 2005 issue of Astrophysical Journal Letters.

This is the second time Eisenhardt and Stanford have broken the record for capturing the most distant galaxy cluster. Both say they accidentally broke the record in 1997 when they detected a cluster located 8.7 billion light-years away. The discovery was made by a deep survey of a 0.03-degree patch of sky, or an area significantly smaller than a pea held out at arms length, for 30 nights at the Kitt Peak National Observatory in Arizona.

“We were lucky in 1997 because we weren’t looking for galaxy clusters and found the most distant one ever detected in a very small patch of sky,” said Stanford. “Because galaxy clusters are so massive and rare, you typically need to deeply survey a large area of sky to find them.”

“With Spitzer’s great infrared sensitivity we surveyed more deeply in 90 seconds than we could in hours of exposure in the 1997 observations, and we used this advantage to survey a region 300 times larger,” adds Eisenhardt.

The 9 billion-year-old cluster is just one of 25 the team captured on their Spitzer safari. They are currently preparing for more observations this spring at the W.M. Keck Observatory to confirm the distance of additional galaxy clusters from their sample. According to Eisenhardt, some of the clusters awaiting confirmation may be even more distant than the current record holder.

Original Source: Spitzer Space Telescope