College Researchers Study Gravitational Forces of a Black Hole

Researchers at the College of Charleston and the Lawrence Livermore National Laboratory recently completed a study concerning the fate of a newly discovered gas cloud that soon will be devoured by a supermassive black hole residing at the center of the Milky Way galaxy. The gas cloud, known as G2, is expected to pass within 40 billion kilometers (270 times the distance between the Earth and the Sun) of the black hole named Sgr A* in June of next year. The researchers’ work helps to provide an unprecedented opportunity to study the disruption of a gas cloud by the intense gravitational forces of a black hole.

Simulations such as the ones done by the College of Charleston researchers are particularly useful at predicting the future behavior of astronomical events. The simulations can also be used to test alternate scenarios. In astronomy, where conventional laboratory experiments are generally impractical, such simulations represent a critical tool of the science.

These simulations were carried out on the Palmetto supercomputer at Clemson University. Each of the six simulations used in the study required more than 50,000 computing hours to complete. By simultaneously running the calculations on over 3,000 processors on Palmetto, the researchers were able to get their results in a few days, rather than the years it would have taken on a single processor.

In the case of G2, the simulations predict that the cloud will begin to experience significant disruption starting in 2013. However, the cloud will not fall directly into the black hole, but will be gradually torn apart over the coming decades. This will lead to a steady feeding of Sgr A*, which may increase its brightness.

Astronomers can’t see black holes directly, but they can often observe radiation emitted by gas falling into them. As gas from G2 is pulled toward the black hole, it may be heated to extreme temperatures that allow it to produce enough light for astronomers on Earth to watch this event unfold. The more rapidly the black hole gorges itself, the brighter the gas will glow. This is important to astronomers since, for its size, Sgr A* is the dimmest black hole system known, as it has long been underfed. The relative feast that G2 could provide may, therefore, have significant effect on the brightness of Sgr A*.

The simulations depict the possible evolution of G2 from the time of its original discovery in 1995 through the year 2020. At the start, the cloud is modeled as a simple gas sphere, near the point in its orbit where it was first discovered. As it approaches Sgr A*, a process known as tidal stretching increasingly distorts the cloud: the gravity felt at the front of the cloud is stronger than the gravity felt at the back; the difference causes it to stretch out. By the end of 2012, the cloud will be nearly five times longer than it is wide. Along with tidal stretching, the cloud also experiences ram pressure forces as it tries to plow through the interstellar gas that already fills the space around Sgr A*.  The interactions of G2 with this background gas cause further disruptions to the cloud. Collectively, these effects gradually strip material from the cloud and feed it into Sgr A*.

An article presenting these results will soon appear in the Astrophysical Journal. The authors of the article include Dr. P. Chris Fragile and student Julia Wilson at the College of Charleston, and Drs. Peter Anninos and Stephen Murray of Lawrence Livermore National Laboratory in Livermore, CA.