Energy output of active galactic nuclei underestimated by scientists

A new study has found that the energy output of active galactic nuclei has been underestimated by not recognising the effect of dust dimming.

Active galactic nuclei, powered by supermassive black holes swallowing matter in the centres of galaxies, are the most powerful, compact sources of energy in the Universe. Scientists have long known that the brightest of these objects far outshine the combined light of billions of stars in their host galaxies.

However, a recent study has revealed that the energy output of these objects has been significantly underestimated due to scientists not considering the extent to which their light is dimmed by dust.

The paper, ‘Estimating reddening of the continuum and broad-line region of active galactic nuclei: the mean reddening of NGC 5548 and the size of the accretion disc,’ is published in Monthly Notices of the Royal Astronomical Society.

The amount of dust dimming was previously believed to be insignificant

“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,” said Martin Gaskell, a research associate in astronomy and astrophysics at UC Santa Cruz and lead author of the paper.

Although the possibility of dust dimming the light from active galactic nuclei has previously been acknowledged, it was widely believed that the amount of this was negligible.

“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,” Gaskell said.

The team studied the reddening effect of dust

This conclusion was reached through the team’s analysis of the reddening effect of dust on the light from one of the most well-studied active galactic nuclei, known as NGC 5548. Similar to the Earth’s atmosphere making the Sun appear redder and dimmer at sunset, dust in active galactic nuclei makes them appear redder than they are. The amount of reddening correlates to the amount of dimming.

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The colours were quantified by scientists by measuring the intensity of object’s light at different wavelengths. There has been great debate around the colours of the various types of emission from active galactic nuclei, despite simple theories predicting their unreddened colours. This is because there were doubts about whether these simple theories could be applied to these objects.

In the new study, the researchers used seven different indicators of the amount of dust and found them all to agree. The team found that the dimming of NGC 5548 was large, more than ten times the dimming caused by dust as we look out of the Milky Way.

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

The results of the study have huge implications for active galactic nuclei

The colours of NGC 5548 are typical of others, which has wide-ranging implications. The dimming effects of the dust have revealed that these objects are even more powerful than had been thought. A typical active galactic nucleus is therefore putting out an order of magnitude with more energy than initially realised.

Gaskell believes that the results also suggest that active galactic nuclei are very similar, and what had previously been thought to be major fundamental differences between them are simply the consequences of different amounts of reddening by dust.

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