Supermassive black holes, the monsters that reside in the centres of galaxies, are complex beasts. While some, like the black hole in our own galaxy, sit there quietly not doing much, others are active objects disrupting the environment around them.
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Some galaxies are called Active Galactic Nuclei - galaxies that are dominated by a very active supermassive black hole. In these galaxies, the supermassive black hole is sucking up huge amounts of gas and dust, the building blocks of stars, as well as the stars themselves. As all of this stuff falls into the black hole, huge amounts of energy and light are released, becoming some of the brightest objects in the Universe.
After observing many of these galaxies, astronomers found distinct types or categories, which depended on how much light, and what kind of light they emitted.
However, an idea was put forth that instead of having different types of these galaxies, there was only one type of these monster black holes and their galaxies. In fact, we see different types dependent on the angle from which we see the galaxy. In space - stars, planets, and galaxies can be viewed or orientated in any direction. There is no up or down in space. Luckily stars and planets are spherical, so for the most part, it does not matter from which angle we see it.
Galaxies on the other hand, are for the most part giant discs. In fact, there are two main types of galaxies - disc galaxies and elliptical galaxies.
On a dark night, if you go outside, you can see our beautiful Milky Way Galaxy stretching across the sky. We see it as a line going across the sky, because we are looking at the edge or side of our disc.
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Imagine looking at a frisbee perfectly from the side - it would look like a thick line. However, if we could see our Milky Way or that frisbee from the top or bottom, it would look completely different. This was the view put forth to explain the different types of these supermassive black hole-dominated galaxies.
Called the unified model of Active Galactic Nuclei, gas and dust would be swirling around the black hole in a giant disc, kind of like a donut. Depending on the angle we saw this supermassive black hole donut, this would change our view of it.
Imagine you only look at someone's face, and now imagine someone else only sees the back of their head. You'd both be seeing the person in a correct way, but they would look different. And if you looked at the top of their head, you'd get a different view again.
The unified model was showing that these active galaxies had profiles or faces, and they were all powered and active in a similar way, but because in space we can see them from any direction, we'd see them differently.
A recent announcement showed that they had imaged and confirmed in a nearby active galaxy this donut of gas and dust - a confirmation of this unified model. There is a lot to learn about supermassive black holes, and while mysterious, they are not two-faced as we previously thought.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU.
