The study shows that active galactic nuclei are even more powerful than previously thought

The study shows that active galactic nuclei are even more powerful than previously thought

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

Powered by supermassive black holes gobbling up matter from the centers of galaxies, active galactic nuclei are the most powerful compact sources of stable energy in the universe. It has long been known that the brightest active galactic nuclei 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 recognizing the extent to which their light is dimmed by dust.

“When small particles come across our line of sight, it makes things behind them appear fainter. We see this at sunset, on any clear day, when the sun appears fainter,” said Martin Gaskell, associate of research in astronomy and astrophysics at UC. Santa Cruz.

Gaskell is the lead author of a paper on the new findings published on January 16 in Monthly Notices of the Royal Astronomical Society.

Although the possibility of dust dimming light from active galactic nuclei has long been recognized, the amount has been controversial and thought to be negligible, he said.

“We have shown that this is not the case and that the far-ultraviolet light of an active galactic nucleus is dimmed by a large factor,” Gaskell said.

The team came to this conclusion by studying the reddening effect of dust on light from one of the best-studied active galactic nuclei known as NGC 5548. Just as Earth’s atmosphere makes the sun appear redder and dimmer at sunset, likewise, dust in active galactic nuclei makes them appear redder than they really are. The amount of reddening is related to the amount of fading.

Scientists quantify the colors of a thing by measuring the ratios of its light intensity at different wavelengths. Although we know what the sun’s unreddened color is, there has been much debate about the unreddened colors of the various types of emission from active galactic nuclei. This is because, although simple theories predict the intrinsic, unreddened colors, there were doubts about whether these simple theories applied to active galactic nuclei.

In the new study of NGC 5548, UCSC researchers used seven different indicators of the amount of dust and found that they all agree. In addition, the attenuation of NGC 5548’s light due to dust was found to be large, more than ten times greater than the attenuation caused by dust as viewed from our own galaxy, the Milky Way.

“The good agreement between the different indicators of the amount of reddening was a pleasant surprise,” Gaskell said. “It strongly supports simple theories of emission from active galactic nuclei. No exotic color explanations needed. This makes life simpler for researchers and speeds up our understanding of what happens when black holes swallow material.”

The colors of NGC 5548 are typical of other active galactic nuclei, he said, which has broad implications. Because of the dimming effects of dust, active galactic nuclei are even more powerful than previously thought. The results suggest that in the ultraviolet, where most of the energy is radiated, a typical active galactic nucleus releases an order of magnitude more energy than previously thought, Gaskell said.

Another implication, he said, is that active galactic nuclei are very similar, and what were previously thought to be major fundamental differences between them are actually just the consequences of different amounts of dust reddening.

More information:
Estimating the reddening of the continuum and the broad-line region of active galactic nuclei: the mean reddening of NGC 5548 and the size of the accretion disc, Monthly Notices of the Royal Astronomical Society (2023).

Provided by University of California – Santa Cruz

Citation: Study Shows Active Galactic Nuclei Even More Powerful Than Thought (2023, January 16) Retrieved January 16, 2023, from

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