Australasian Science Magazine

By David Reneke

The European Southern Observatory’s Very Large Telescope Interferometer has gathered the most detailed observations ever of the dust around the huge black hole at the centre of an active galaxy.

Rather than finding all of the glowing dust in a doughnut-shaped torus around the black hole, as expected, the astronomers found that much of it is located above and below the torus.

These observations show that dust is being pushed away from the black hole as a cool wind – a surprising finding that challenges current theories and tells us how supermassive black holes evolve and interact with their surroundings.

Over the past 20 years, astronomers have found that almost all galaxies have a huge black hole at their centre. Some of these black holes are growing by drawing in matter from their surroundings, creating in the process the most energetic objects in the Universe – active galactic nuclei (AGN).

The central regions of these brilliant powerhouses are ringed by doughnuts of cosmic dust dragged from the surrounding space, similar to how water forms a small whirlpool around the plughole of a sink. It was thought that most of the strong infrared radiation coming from AGN originated in these doughnuts, but new observations of the nearby active galaxy NGC 3783 by the Interferometer at ESO’s Paranal Observatory in Chile have given a team of astronomers a surprise.

Although the hot dust, at 700–1000°C, is indeed in a torus as expected, they found huge amounts of cooler dust above and below this main torus forming a cool wind streaming outwards from the black hole. It’s assumed this wind must play an important role in the complex relationship between the black hole and its environment.

The black hole feeds its insatiable appetite from the surrounding material, but the intense radiation this produces also seems to be blowing the material away. It is still unclear how these two processes work.

Future observations must now take into account this newly discovered effect, leading perhaps to a paradigm shift in the understanding of how supermassive black holes grow and evolve.

A Light Echo
The NASA/ESA Hubble Space Telescope has observed the Cepheid variable star RS Puppis over a period of 5 weeks, showing the star growing brighter and dimmer as it pulsates. These pulsations have created a stunning example of a phenomenon known as a light echo, where light appears to reverberate through the murky environment around the star.

For most of its life a star is pretty stable, slowly consuming the fuel at its core to keep it shining brightly. However, once most of the hydrogen that stars use as fuel has been consumed, some stars evolve into very different beasts – pulsating stars. They become unstable, expanding and shrinking over a number of days or weeks and growing brighter and dimmer as they do so.

RS Puppis is unusual as it is shrouded by a nebula — thick, dark clouds of gas and dust. As the star expands and brightens, we see some of the light after it is reflected from progressively more distant shells of dust and gas surrounding the star, creating the illusion of gas moving outwards. This is analogous to sound bouncing off surrounding objects, causing the listener to hear an audible echo.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com