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Simulating our black hole's meal time – in 3D

A 3D movie of the simulated G2 gas cloud approaching Sgr A* from the years 1995 to 2020. The color corresponds to the density of cloud material (red is the highest density, blue the lowest), and the gray sphere represents the point beyond which gas is considered accreted by the black hole. Image courtesy Chris Fragile.

Black holes have received a lot of attention recently. Last month, one of the biggest and most out-of-proportion black holes relative to its host galaxy was discovered in NGC 1277. And in our very own Milky Way galaxy the supermassive black hole, known as Sagittarius A*, got astrophysicists excited when it emitted the brightest flare ever detected, which could have been caused by the swallowing of an asteroid. Now, researchers have developed a unique 3D simulation that sheds light on how Sagittarius A* consumes matter and what's left after it has had its feast. This research was published in The Astrophysical Journal.

The meal simulated is a newly discovered dust and gas cloud called G2. Researchers made a total of six simulations that forecast the cloud's journey towards our galactic center in June 2013. The cloud will pass within 40 billion kilometers or 270 astronomical units (one astronomical unit is about the average distance between the Earth and the Sun) of Sagittarius A*. However, this isn't close enough to fall into the 'point of no return' where light cannot escape. This is known as the Schwarzschild radius and is less than one astronomical unit for Sagittarius A*.

A slow meal

The parallel and multi-dimensional code used to forecast the G2 cloud is called Cosmos++, developed by computational physicist Peter Anninos, of the Lawrence Livermore National Laboratory's AX division within the Weapons and Complex Integration Directorate (WCI), California, US, and Chris Fragile, a computational astrophysicist at the College of Charleston, South Carolina, US.

The Cosmos++ code is one of the most sophisticated tools for studying black hole accretion in the world. In astrophysics accretion disks are of diffuse material in orbit around a central body, such as a black hole. Taking more than 3,000 processors and 50,000 computing hours on the Palmetto supercomputer at Clemson University's Tech Center, South Carolina, US, the code shows how the G2 cloud will undergo heavy disruption due to the black hole's 'gravity well' over several months. Tidal stretching will make it five times as long as it is wide by the end of 2012. At the beginning of next year, G2 will also experience ram pressure as it plows through hot interstellar gas around Sagittarius A*. This black hole's feeding will strip G2 down within a decade, making it so diffuse it may eventually be destroyed in 2020.

If all this stretching and ram pressure goes over your head Chris Fragile says these forces aren't that difficult to comprehend.

"The black hole's gravity is called a 'tidal' force because it's the same force that causes high and low tides in the Earth's oceans. The same sort of stretching is already happening in the G2 cloud. Eventually, the stretching will become much more extreme than the tides on Earth, ripping the cloud apart," says Fragile.

The other force acting on the cloud is hydrodynamic instabilities. An example of this in everyday life is steam rising from a hot cup of coffee which interacts with the cooler gas in a room. "If you watch the steam carefully, you see it rolling, twisting, and mixing with the air around it. This is the same thing that will happen to G2," says Fragile.

This cosmic process will give astronomers unparalleled access to how material accretes onto supermassive black holes and maybe observable by next September. As G2 begins shedding its energy, reaching very high temperatures, it could produce a luminosity 1,000 times that of the Sun.

Meshing in a different way

Artist's impression of dust and gas cloud called G2 and its simulated orbit around the Milky Way's central black hole SgrA*.
This is an artist's conception of G2's journey. The red line in the image indicates the orbital path of the G2 cloud, if it were undisturbed. The red cloud in the image shows an interpretation of the final stage of the cloud. The blue lines show the orbits of stars near the center of our Galaxy. The bright white stars indicate the stars themselves. Click to enlarge. Image courtesy M. Schartmann and L. Calcada / European Southern Observatory and Max-Planck-Institut fur Extraterrestrische Physik.

"Previous simulations were in two-dimensions," says Stephen Murray, a researcher at WCI who was involved with the research. "The third dimension, out of the plane, was not simulated, and various assumptions had to be made about the behavior of the gas in that direction."

This was because regions of space were divided by the code into a mesh. The finer the mesh the better the simulations are generally. But, getting a fine enough mesh for 3D simulations takes hundreds of times longer to run.

Therefore, the researchers applied a 'moving mesh' method that improved their computational-time efficiency by six times. "Only part of the problem immediately around the gas cloud is directly simulated, so that we don't waste resources computing what's going on in irrelevant regions. By not having to simulate the entire region through which the cloud might at some time move, we are able to do high fidelity 3D models," says Murray.

The next steps for the researchers is to create a better resolution model to see what happens to the gas in the last 10 billion kilometers before reaching the black hole itself.

Sera Markoff, an astrophysicist at the Astronomical Institute in the University of Amsterdam, Netherlands, says this research group's forecast for next year is potentially exciting, but emphasizes that their definition of G2 as a gas cloud is a 'claim' for now.

"I do put it like that because there are other equally reputable groups who have alternative explanations, such as that this object is not a gas cloud, but rather a type of star, in which case it would much less likely get dismantled and eaten by the black hole."

Markoff says that the real innovation by the team is simulating the gas cloud using numerical techniques to follow the complex physical-scale and time-scale evolution of how it interacts with the plasma surrounding the black hole and gravitational effects.

"The group concluded that the cloud most likely would not create much of a 'bang' going down the black hole's gullet, but we won't be able to test this until sometime later next year," says Markoff.

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