I admit it. I really dig black holes. As I’ve stated in other articles, when I was growing up they were (for many years) considered a curiosity. Nobody had seen one. People thought they might exist, but until the late 20th century, nobody had observed one.

Well, it turns out you don’t observe the black hole itself. You observe its effect on the surrounding space and material. Today, we have many observations of black hole regions, particularly at the hearts of galaxies.

The Smoking Gun

When I was just finishing graduate school, Hubble Space Telescope finally imaged the region around a supermassive black hole at the heart of galaxy NGC 4261.  At the time it was called the “smoking gun” image evidence for a black hole.  Today, we find them all over the place. Black holes are part of the cosmic zoo – and are thought to play a role in a number of processes inside galaxies. For example, they affect star formation rates in some regions of their galaxies. In a number of barred galaxies, the bars feed material from the outer regions of the galaxy into the region of the black hole.  In others, the black hole feeds from material much closer around it. Astronomers continue to look for answers to questions about the symbiosis between galaxies and their black holes.

Closer to Home

So, our own galaxy has a supermassive black hole, called Sagittarius A*. Not surprisingly, it’s at the heart of our galaxy. It’s also under intense study by astronomers using a range of techniques. This includes the team using the Event Horizon Telescope (which has imaged a supermassive black hole at the heart of M87).

An interesting story about Sagittarius A* story caught my eye this week. It’s focused on how this thing eats.  We know our black hole is relatively quiescent most of the time, but it does gobble up material. Yet, it doesn’t seem to have a jet as other, more active supermassive black holes seem to have. Occasionally, it does wake up and munch down some interstellar material. When that happens, the region around the black hole brightens up at various wavelengths.

An Infrared Peek

That appears to be what has been happening and astronomer Andrea Ghez at UCLA and her group found evidence for this meal in infrared observations. They went through and analyzed 13,000 observations made over 133 nights of telescope time using the Keck Telescope in Hawai’i and the Very Large Telescope in Chile. In April and May this year, the region right at the event horizon – that “point of no return” around the black hole – appeared to be quite bright. In fact, it got about twice as bright as seen in previous observations.  Now, the region does go through some variability in brightness, but these brightenings were WAY out of the “norm”.

So, what was going on ? These kinds of brightenings are kind of unusual for Sgr A*.  Clearly something, probably blobs of gas, have been getting caught up in the black hole’s intense gravitational pull. As they get drawn into the black hole, they heat up and start to glow – and that’s what showed up in the infrared observations that Ghez and her group studied.

What Caused The Brightening?

Now, what they don’t know yet is if this is a one-off occurrence at the event horizon. Could it be caused by, say, a nearby star losing mass to the black hole? The best candidate for that is one called S0-2. It may have lost a great deal of gas during a close pass to the event horizon earlier this year. That material went speeding off. Inside the disk, it was superheated, and presumably, eventually lost to Sagittarius A*.

There’s also an object that could be feeding the black hole, called G2. It may be a pair of binary stars, and it periodically makes close approaches to the inner region of Sgr A*. When it does, the gravitational pull could have stripped off material from the stars outer layer.

There’s a third possibility that involves swarms of asteroids that have gotten too close to the event horizon and have been drawn in. All of these may explain the sudden brightening as materials are heated and eventually disappear into the event horizon.

Black Holes and the Big Picture

The bigger story here is understanding how black holes at the hearts of galaxies sustain themselves. That is, how do they eat and grow? We know they formed early in the history of the universe. How they grew, and how fast is another part of their story. So, astronomers continue to study them and the regions around them. Ghez and her team have studied several thousand stars that orbit Sgr A*.  And, those long-term studies tipped them off to this increased brightness. Sagittarius A* hadn’t brightened like that in the 24 years they studied the region.

So, do all black holes eat like this? What happens when they clear their region?  Do they stop growing? What’s the history of Sgr A*? Is it quiet because it has eaten a lot and cleared some of its neighborhood? Or, is there something else going on with it? The answers will come with increased studies made by ever-more-sensitive techniques and equipment. Stay tuned!