An Answer To Why Our Galaxy’s Black Hole Is A Finicky Eater

Thursday, August 29, 2013
by Jennifer Chu, MIT News Office

For years, scientists have observed that the black hole at the center of our galaxy has a surprisingly small appetite.

While the black hole, named Sagittarius A* (pronounced Sagittarius A-star), is 4 million times as massive as the sun, it is unusually inactive for its size, devouring very little of the surrounding gas and other galactic material.

Now a team including researchers from MIT and the University of Massachusetts at Amherst has analyzed what little activity exists around the black hole. By studying 3 million seconds of observations taken by the Chandra X-Ray Observatory, the team found that much of the nearby radiation comes from material that is ejected before reaching the black hole, with very little energy arising from the black hole itself.

“The black hole doesn’t have a chance to do its meat-grinder thing and turn that matter into energy,” says Joey Neilsen, who contributed to the research as a postdoc at MIT’s Kavli Institute for Astrophysics and Space Research. “All of that stuff basically escapes before the black hole can destroy it.”

The researchers publish their results in this week’s issue of Science.

In a related paper, published in The Astrophysical Journal, many of the same researchers took a closer look at the small amount of energy originating from the black hole, finding that what little activity there is comes from low-level, continuous flares close to the event horizon — the very outermost edge of the black hole.

“It’s this sort of constant burn — this little sizzle of flares that is always happening,” Neilsen explains. “It’s doing all this flaring and popping, and all sorts of little activity on a fairly faint scale.”

The new results provide the most detailed look yet at the activity surrounding the center of the Milky Way.

A finicky eater

Over the years, the most powerful telescopes have only been able to detect a faint trace of activity, in the form of X-ray emissions, from the center of the galaxy. Theories abound as to why a supermassive black hole such as Sagittarius A* would be so lackluster: Some scientists have suggested that material escapes the region before the black hole has a chance to devour it, while others say the black hole is simply ineffective at producing radiation. Still others theorize that the observed X-ray emissions arise not from the black hole, but from a cluster of nearby stars.

For each theory, scientists have developed models to simulate the radiation around the black hole, with each model consisting of a characteristic X-ray spectrum. To see which theory is most likely to explain the black hole’s inactivity, the team first looked at X-ray emissions surrounding Sagittarius A*.

The researchers analyzed 3 million seconds of X-ray data from the Chandra observatory, NASA’s orbiting X-ray telescope. The group dissected the X-ray data and focused on the observed emission lines, signifying the characteristic light given off by individual atoms. The researchers then zeroed in on the emissions from atoms of iron, a relatively abundant element in the galaxy.

Iron can be found on the surface of stars, as well as in the gas surrounding a black hole — but stars tend to be many times cooler than a black hole’s circulating gas. To determine where the majority of X-rays were coming from, the researchers calculated the temperature of the iron, given its X-ray emission lines. They observed a large amount of very hot iron atoms, suggesting much of the X-ray activity arose not from a cluster of stars, but from gas surrounding the black hole.

While the group’s observations also appear to suggest that there is plenty of hot, gaseous material available for the black hole to consume, the minimal activity of the black hole itself suggests that that is not the case. The likeliest explanation for such finicky eating, the team concluded, was that the gaseous material ejects itself in the form of hot wind, escaping before the black hole can devour it.

“We think most of the energy, or a lot of it, is going toward pushing the gas away from the black hole and not letting it fall in,” says Mike Nowak, a research scientist at MIT Kavli. “Now we have a better understanding of what parts are active, and what aren’t.”

A buzz of activity

As for what little energy is given off by the black hole itself, Neilsen, Nowak and others looked at some of the very faintest X-ray signals from the same Chandra dataset — signals that indicated flares, or small peaks in activity, from close to the black hole’s event horizon.

Within the data, the group was able to detect large and medium-sized flares occurring about once every few days. They wondered, however, whether smaller flares might be present, but too faint to see.

The researchers calculated the frequency of observed flares at various luminosities, and based on that distribution, estimated the frequency of even smaller flares that were undetectable by the satellite. The result was a near constant “buzz” of low-grade flares that exactly matched the group’s previous estimates of X-ray activity at the galactic center.

“Rarely do we have a chance to answer these questions in detail,” Neilsen says. “This is a really great chance to actually dive in and say, ‘How do we understand what normal galaxies are doing, as opposed to gigantic luminous quasars and active galaxies?’ This is the first time we could really work on answering a question like this with high-quality data.”

Reinhard Genzel, a professor at the Max Planck Institute for Extraterrestrial Physics, says the activity — or lack thereof — observed near Sagittarius A* may be typical of other nearby black holes, most of which are relatively dormant.

“However, the degree of inactivity — a factor of a million or so — is remarkable, in part because we have so much better and higher-resolution data in this case,” notes Genzel, who was not part of the research team. “The beautiful high-resolution Chandra spectroscopy presented in this paper will, for the foreseeable future, never be available in other galactic nuclei.”

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