Ultra-Fast Outflows Help Monster Black Holes Shape Their Galaxies

A curious correlation between the mass of a galaxy's central black hole and the velocity of stars in a vast, roughly spherical structure known as its bulge has puzzled astronomers for years. An international team led by Francesco Tombesi at NASA's Goddard Space Flight Center in Greenbelt, Md., now has identified a new type of black-hole-driven outflow that appears to be both powerful enough and common enough to explain this link.

Most big galaxies contain a central black hole weighing millions of times the sun's mass, but galaxies hosting more massive black holes also possess bulges that contain, on average, faster-moving stars. This link suggested some sort of feedback mechanism between a galaxy's black hole and its star-formation processes. Yet there was no adequate explanation for how a monster black hole's activity, which strongly affects a region several times larger than our solar system, could influence a galaxy's bulge, which encompasses regions roughly a million times larger.

"This was a real conundrum. Everything was pointing to supermassive black holes as somehow driving this connection, but only now are we beginning to understand how they do it," Tombesi said.

Active black holes acquire their power by gradually accreting -- or "feeding" on -- million-degree gas stored in a vast surrounding disk. This hot disk lies within a corona of energetic particles, and while both are strong X-ray sources, this emission cannot account for galaxy-wide properties. Near the inner edge of the disk, a fraction of the matter orbiting a black hole often is redirected into an outward particle jet. Although these jets can hurl matter at half the speed of light, computer simulations show that they remain narrow and deposit most of their energy far beyond the galaxy's star-forming regions.

Astronomers suspected they were missing something. Over the last decade, evidence for a new type of black-hole-driven outflow has emerged. At the centers of some active galaxies, X-ray observations at wavelengths corresponding to those of fluorescent iron show that this radiation is being absorbed. This means that clouds of cooler gas must lie in front of the X-ray source. What's more, these absorbed spectral lines are displaced from their normal positions to shorter wavelengths -- that is, blueshifted, which indicates that the clouds are moving toward us.

In two previously published studies, Tombesi and his colleagues showed that these clouds represented a distinct type of outflow. In the latest study, which appears in the Feb. 27 issue of Monthly Notices of the Royal Astronomical Society, the researchers targeted 42 nearby active galaxies using the European Space Agency's XMM-Newton satellite to hone in on the location and properties of these so-called "ultra-fast outflows" -- or UFOs, for short. The galaxies, which were selected from the All-Sky Slew Survey Catalog produced by NASA's Rossi X-ray Timing Explorer satellite, were all located less than 1.3 billion light-years away.

The outflows turned up in 40 percent of the sample, which suggests that they're common features of black-hole-powered galaxies. On average, the distance between the clouds and the central black hole is less than one-tenth of a light-year. Their average velocity is about 14 percent the speed of light, or about 94 million mph, and the team estimates that the amount of matter required to sustain the outflow is close to one solar mass per year -- comparable to the accretion rate of these black holes.

"Although slower than particle jets, UFOs possess much faster speeds than other types of galactic outflows, which makes them much more powerful," Tombesi explained.

"They have the potential to play a major role in transmitting feedback effects from a black hole into the galaxy at large."

By removing mass that would otherwise fall into a supermassive black hole, ultra-fast outflows may put the brakes on its growth. At the same time, UFOs may strip gas from star-forming regions in the galaxy's bulge, slowing or even shutting down star formation there by sweeping away the gas clouds that represent the raw material for new stars. Such a scenario would naturally explain the observed connection between an active galaxy's black hole and its bulge stars.

Tombesi and his team anticipate significant improvement in understanding the role of ultra-fast outflows with the launch of the Japan-led Astro-H X-ray telescope, currently scheduled for 2014. In the meantime, he intends to focus on determining the detailed physical mechanisms that give rise to UFOs, an important element in understanding the bigger picture of how active galaxies form, develop and grow.

Black Hole Came from a Shredded Galaxy

Astronomers using NASA's Hubble Space Telescope have found a cluster of young, blue stars encircling the first intermediate-mass black hole ever discovered. The presence of the star cluster suggests that the black hole was once at the core of a now-disintegrated dwarf galaxy. The discovery of the black hole and the star cluster has important implications for understanding the evolution of supermassive black holes and galaxies.


"For the first time, we have evidence on the environment, and thus the origin, of this middle-weight black hole," said Mathieu Servillat, who worked at the Harvard-Smithsonian Center for Astrophysics when this research was conducted.
Astronomers know how massive stars collapse to form stellar-mass black holes (which weigh about 10 times the mass of our sun), but it's not clear how supermassive black holes (like the four million solar-mass monster at the center of the Milky Way) form in the cores of galaxies. One idea is that supermassive black holes may build up through the merger of smaller, intermediate-mass black holes weighing hundreds to thousands of suns.
Lead author Sean Farrell, of the Sydney Institute for Astronomy in Australia, discovered this unusual black hole in 2009 using the European Space Agency's XMM-Newton X-ray space telescope. Known as HLX-1 (Hyper-Luminous X-ray source 1), the black hole weighs in at 20,000 solar masses and lies towards the edge of the galaxy ESO 243-49, which is 290 million light-years from Earth.
Farrell and his team then observed HLX-1 simultaneously with NASA's Swift observatory in X-ray and Hubble in near-infrared, optical, and ultraviolet wavelengths. The intensity and the color of the light shows a cluster of young stars, 250 light-years across, encircling the black hole. Hubble can't resolve the stars individually because the suspected cluster is too far away. The brightness and color are consistent with other clusters of young stars seen in other galaxies.
Farrell's team detected blue light from hot gas in the accretion disk swirling around the black hole. However, they also detected red light produced by much cooler gas, which would most likely come from stars. Computer models suggested the presence of a young, massive cluster of stars encircling the black hole.
"What we can definitely say with our Hubble data is that we require both emission from an accretion disk and emission from a stellar population to explain the colors we see," said Farrell.
Such young clusters of stars are commonly seen in nearby galaxies, but not outside the flattened starry disk, as found with HLX-1. The best explanation is that the HLX-1 black hole was the central black hole in a dwarf galaxy. The larger host galaxy then captured the dwarf. Most of the dwarf's stars were stripped away through the collision between the galaxies. At the same time, new young stars were formed in the encounter. The interaction that compressed the gas around the black hole also triggered star formation.
Farrell and Servillat found that the star cluster must be less than 200 million years old. This means that the bulk of the stars were formed following the dwarf's collision with the larger galaxy. The age of the stars tells how long ago the two galaxies crashed into each other.
The future of the black hole is uncertain at this stage. It depends on its trajectory, which is currently unknown. It's possible the black hole may spiral in to the center of the big galaxy and eventually merge with the supermassive black hole there. Alternately, the black hole could settle into a stable orbit around the galaxy. Either way, it's likely to fade away in X-rays as it depletes its supply of gas.
"This black hole is unique in that it's the only intermediate-mass black hole we've found so far. Its rarity suggests that these black holes are only visible for a short time," said Servillat.
More observations are planned this year to track the history of the interaction between the two galaxies.
The new findings are being published in the February 15 issue of the Astrophysical Journal Letters.

Milky Way's Black Hole Found Grazing On Asteroids

The giant black hole at the center of the Milky Way may be vaporizing and devouring asteroids, which could explain the frequent flares observed, according to astronomers using data from NASA's Chandra X-ray Observatory.
For several years Chandra has detected X-ray flares about once a day from the supermassive black hole known as Sagittarius A*, or "Sgr A*" for short. The flares last a few hours with brightness ranging from a few times to nearly one hundred times that of the black hole's regular output. The flares also have been seen in infrared data from ESO's Very Large Telescope in Chile.
"People have had doubts about whether asteroids could form at all in the harsh environment near a supermassive black hole," said Kastytis Zubovas of the University of Leicester in the United Kingdom, and lead author of the report appearing in the Monthly Notices of the Royal Astronomical Society. "It's exciting because our study suggests that a huge number of them are needed to produce these flares."
Zubovas and his colleagues suggest there is a cloud around Sgr A* containing trillions of asteroids and comets, stripped from their parent stars. Asteroids passing within about 100 million miles of the black hole, roughly the distance between Earth and the sun, would be torn into pieces by the tidal forces from the black hole.
These fragments then would be vaporized by friction as they pass through the hot, thin gas flowing onto Sgr A*, similar to a meteor heating up and glowing as it falls through Earth's atmosphere. A flare is produced and the remains of the asteroid are swallowed eventually by the black hole.
"An asteroid's orbit can change if it ventures too close to a star or planet near Sgr A*," said co-author Sergei Nayakshin, also of the University of Leicester. "If it's thrown toward the black hole, it's doomed."
The authors estimate that it would take asteroids larger than about six miles in radius to generate the flares observed by Chandra. Meanwhile, Sgr A* also may be consuming smaller asteroids, but these would be difficult to spot because the flares they generate would be fainter.
These results reasonably agree with models estimating of how many asteroids are likely to be in this region, assuming that the number around stars near Earth is similar to the number surrounding stars near the center of the Milky Way.
"As a reality check, we worked out that a few trillion asteroids should have been removed by the black hole over the 10-billion-year lifetime of the galaxy," said co-author Sera Markoff of the University of Amsterdam in the Netherlands. "Only a small fraction of the total would have been consumed, so the supply of asteroids would hardly be depleted."
Planets thrown into orbits too close to Sgr A* also should be disrupted by tidal forces, although this would happen much less frequently than the disruption of asteroids, because planets are not as common. Such a scenario may have been responsible for a previous X-ray brightening of Sgr A* by about a factor of a million about a century ago. While this event happened many decades before X-ray telescopes existed, Chandra and other X-ray missions have seen evidence of an X-ray "light echo" reflecting off nearby clouds, providing a measure of the brightness and timing of the flare.
"This would be a sudden end to the planet's life, a much more dramatic fate than the planets in our solar system ever will experience," Zubovas said.
Very long observations of Sgr A* will be made with Chandra later in 2012 that will give valuable new information about the frequency and brightness of flares and should help to test the model proposed here to explain them. This work could improve understanding about the formation of asteroids and planets in the harsh environment of Sgr A*.

A new study provides a possible explanation of mysterious X-ray flares detected by Chandra over the period of several years. It suggests that there is a cloud around Sgr A* containing trillions of asteroids and comets, stripped from their parent stars. The flares occur when asteroids of six miles or larger in radius are consumed by the black hole. The panel on the left shows a very long Chandra observation of the region around the Sgr A*, while the three panels on the right are artist's impressions of the path that a doomed asteroid would take on its way to the black hole. (Credit: X-ray: NASA/CXC/MIT/F. Baganoff et al.; Illustrations: NASA/CXC/M.Weiss)


The above story is reprinted from materials provided by Chandra X-ray Center.