Weekend Feature: Phantom Black Holes of the Milky Way –Could They Pose a Danger to Our Solar System?



The effect of a primordial black hole hitting the Sun ought to be easily observable, say physicists at New York University and Princeton University, noting what we think might be the obvious. But they go on to suggest that such an event wouldn’t be as catastrophic as it sounds.

The likelihood is that a primordial black hole with mass of an asteroid or comet would pass straight through the Sun, generating a small puff of X-rays in the process. The burst would be less even than the background rate of X-rays, so it would be impossible for astronomers to see. Instead, the collision would generate supersonic turbulence that would set the Sun ringing like a bell — sort of a "solar hiccup," so we might have seen them already.

As we reported previously, the Milky Way may be full of phantom, planet-devouring black holes. A recent computer simulation revealed that there could be literally hundreds of rogue black holes scattered across the Milky Way galaxy. Each one would weigh several thousand times the mass of the sun, so if these bad guys exist — why haven’t we identified them already?

“Rogue black holes like this would be very difficult to spot,” says Vanderbilt astronomer Kelly Holley-Bockelmann. “Unless it's swallowing a lot of gas, about the only way to detect the approach of such a black hole would be to observe the way in which its super-strength gravitational field bends the light that passes nearby. This produces an effect called gravitational lensing that would make background stars appear to shift and brighten momentarily.”

The research modeled "intermediate mass" black holes –the first of which was just discovered  by an international team using the  European Space Agency's XMM-Newton X-ray space telescope  (read below). Astronomers have ample evidence that small black holes less than 100 solar masses are produced when giant stars explode. They also have evidence that “super-massive” black holes weighing the equivalent of millions to billions of solar masses sit at the heart of most galaxies, including the Milky Way.

Theoreticians have predicted that globular clusters –- ancient, gravitationally bound groups of 100,000 to a million stars –- should contain the third class of intermediate mass black holes. But so far there have only been a couple controversial observations of these objects. The existence of intermediate-mass black holes have been proposed as a possible power source for ultra-luminous X-ray sources.

In the past couple years, scientists have succeeded in numerically simulating black hole mergers that incorporate Einstein’s theory of relativity. One of the most intriguing predictions is that when two black holes that are rotating at different speeds or are different sizes combine, the newly merged black hole receives a big kick due to conservation of momentum, pushing it out in random directions at velocities as high as 4,000 kilometers per second.

“This is much higher than anyone predicted. Even the average kick velocity of 200 kilometers per second is extremely high when compared to the escape velocities of typical astronomical objects,” says Holley-Bockelmann. “We realized that basically any black hole merger would kick the new remnant out of a globular cluster, because the escape velocity is less than 100 kilometers per second.”

Using the facilities of Vanderbilt’s Advanced Center for Computation, Research and Education, Holley-Bockelmann’s team ran a number of simulations of the growth of intermediate mass black holes as they combine with a number of stellar-sized black holes, which are plentiful in globular clusters, paying close attention to the kick they received after each merger.

“We used different assumptions for the initial black hole mass, for the range of stellar black hole masses within a globular cluster, and assumed that the spins and spin orientations were distributed randomly. With our most conservative assumptions, we found that, even if every globular cluster started out with an intermediate-sized black hole, only about 30 percent retain them through the merger epoch. With our least conservative assumptions, less than two percent of the globular clusters should contain intermediate mass black holes today,” she says.

There are about 200 globular clusters in the Milky Way that may have already spawned intermediate-sized black holes, which means that hundreds of them would be wandering invisibly around the Milky Way. These could be engulfing the nebulae, stars and planets that are unfortunate enough to cross their paths, but apparently this poses no imminent danger to Earth— or at least not as far as anyone knows at this point in time.

“These rogue black holes are extremely unlikely to do any damage to us in the lifetime of the Universe,” Holley-Bockelmann stresses. “Their danger zone, the Schwarzschild radius, is really tiny, only a few hundred kilometers. There are far more dangerous things in our neighborhood!”

Astronomers using NASA's Hubble Space Telescope have found a cluster of young, blue stars in the spectacular edge-on galaxy (ESO 243-49 above), 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.

Until now, identified black holes have been either super-massive (several million to several billion times the mass of the Sun) in the center of galaxies, or about the size of a typical star (between three and 20 Solar masses).

The 2009 team led by astrophysicists at the Centre d'Etude Spatiale des Rayonnements in France, detected HLX-1, approximately 290 million light years from Earth with the European Space Agency's XMM-Newton X-ray space telescope.

It had been long believed by astrophysicists that there might be a third, intermediate class of black holes, with masses between a hundred and several hundred thousand times that of the Sun.  However, such black holes had not been reliably detected until then.

"While it is widely accepted that stellar mass black holes are created during the death throes of massive stars, it is still unknown how super-massive black holes are formed," says Farrell, who was a member of the 2009 effort.

"One theory is that super-massive black holes may be formed by the merger of a number of intermediate mass black holes. To ratify such a theory, however,  you must first prove the existence of intermediate black holes.

"The identification of HLX-1 is therefore an important step towards a better understanding of the formation of the super-massive black holes that exist at the centre of the Milky Way and other galaxies."

HLX-1 (Hyper-Luminous X-ray source 1), which lies towards the edge of ESO 243-49, is ultra-luminous in X-rays, with a maximum X-ray brightness of approximately 260 million times that of the Sun.

The Daily Galaxy via science.unsw.edu.au and eurekalert.org and Transient solar oscillations driven by primordial black holes, arXiv.org, May 31, 2011 [arXiv:1106.0011v1]

Image credit: www.arizona.edu/

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