Powered by supermassive black holes swallowing matter in the centres of galaxies, active galactic nuclei are the most powerful compact steady sources of energy in the universe

The brightest active galactic nuclei have long been known to far outshine the combined light of the billions of stars in their host galaxies.

A new study indicates that scientists have substantially underestimated the energy output of these objects by not recognising the extent to which their light is dimmed by dust.

Lead author Martin Gaskell, a research associate in astronomy and astrophysics at UC Santa Cruz, said: “When there are intervening small particles along our line of sight, this makes things behind them look dimmer. We see this at sunset on any clear day when the sun looks fainter.”

Although the possibility of dust dimming the light from active galactic nuclei has been recognised for a long time, the amount was controversial and was widely believed to be negligible.

Gaskell advised: “We have shown that this is not the case and that the far ultraviolet light of a typical active galactic nucleus is dimmed by a large factor.”

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active galactic nuclei

The team reached this conclusion by studying the reddening effect of dust on the light from one of the most well-studied active galactic nuclei, known as NGC 5548.

Just as the Earth’s atmosphere makes the sun appear redder as well as dimmer at sunset, so dust in active galactic nuclei also makes them appear redder than they really are. The amount of reddening is related to the amount of dimming.

Scientists quantify the colours of something by measuring the ratios of the intensity of its light at different wavelengths.

While we know what the unreddened colour of the sun is, there has been much debate over the unreddened colours of the various types of emission from active galactic nuclei.

This is because, although simple theories predict the intrinsic, unreddened colours, there were doubts about whether these simple theories applied to active galactic nuclei.

In the new study of NGC 5548, the UCSC researchers used seven different indicators of the amount of dust and found them all to be in good agreement. Furthermore, the dimming of NGC 5548 due to dust was found to be large, more than ten times the dimming caused by dust as we look out of our own galaxy, the Milky Way.

Gaskell said: “The good agreement between the different indicators of the amount of reddening was a pleasant surprise.

“It strongly supports simple theories of emission from active galactic nuclei. Exotic explanations of colours are not needed.

“This makes life simpler for researchers and is speeding up our understanding of what happens as black holes swallow material.”

wide-ranging implications

The colours of NGC 5548 are typical of other active galactic nuclei, which has wide-ranging implications. Because of the dimming effects of dust, active galactic nuclei are even more powerful than had been realised.

The results imply that in the ultraviolet, where most of the energy is radiated, a typical active galactic nucleus is putting out an order of magnitude more energy than previously thought.

Another implication, Gaskell said, is that active galactic nuclei are very similar, and what had hitherto been thought to be major fundamental differences between them are really just the consequences of different amounts of reddening by dust.

Gaskell’s co-authors – Frances Anderson (now at Harvey Mudd College), Sufia Birmingham (now at Princeton University), and Samhita Ghosh (now at UC Berkeley) – worked on this project as high school seniors participating in the UCSC Science Internship Program.

The research is published in Monthly Notices of the Royal Astronomical Society.

Image: An artist’s impression of what the dust around an active galactic nucleus might look like seen from a light year away. © Peter Z Harrington.