Powered by matter-swallowing supermassive black holes at the centers of galaxies, active galactic nuclei are the most powerful, compact sources of stationary energy in the universe. It has long been known that the brightest AGNs far outshine the combined light of billions of stars in their host galaxies.
A new study suggests that scientists have greatly underestimated the energy output of these objects by not recognizing the extent to which their light is dimmed by dust.
“When there are small particles interfering along our line of sight, it makes things behind them look dimmer. We see this at sunset on any clear day when the sun looks dimmer,” said Martin Gaskell, a research associate in astronomy and astrophysics at UCLA. Santa Cruz.
Gaskell is the lead author of a research paper on the new findings published January 16 in Monthly Notices of the Royal Astronomical Society.
He said that although the possibility of dust dimming light from active galactic nuclei had been recognized for a long time, the amount was controversial and widely believed to be negligible.
“We have shown that this is not the case and that the far ultraviolet light of a typical active galactic nucleus is attenuated by a significant factor,” Gaskell said.
The team reached this conclusion by studying the effect of reddish dust on light from one of the most studied active galactic nuclei, known as NGC 5548. Just as Earth’s atmosphere makes the sun appear redder and less dim at sunset, so too does dust in galactic nuclei. Active makes it look redder than it really is. The amount of redness is related to the amount of opacity.
Scientists measure the colors of an object by measuring the ratios of the intensity of its light at different wavelengths. While we know what the color of the non-reddish Sun is, there has been much debate about the non-reddish colors of the different types of emissions from AGNs. This is because although simple theories predict undiluted intrinsic colors, there has been uncertainty about whether these simple theories apply 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 that they all agreed well. Furthermore, the dust-induced darkening of NGC 5548 was found to be significant, more than ten times the dust-induced darkening when we look outside our own galaxy, the Milky Way.
“The good agreement between the different indices of the amount of redness was a pleasant surprise,” Gaskell said. “It strongly supports simple theories of emission from active galactic nuclei. There is no need for exotic interpretations of the colors. This makes life simpler for researchers and accelerates our understanding of what happens when black holes swallow material.”
He said that NGC 5548’s colors are typical of other active galactic nuclei, which have widespread implications. Due to the darkening effects of the dust, the active galactic nuclei are stronger than realized. Gaskell said the results indicate that in the ultraviolet, where most of the energy is radiated, a typical active galactic nucleus puts out more energy than previously thought.
Another implication, he said, is that the active galactic nuclei are very similar, and what until now were thought to be major fundamental differences between them are really just the consequences of different amounts of flushing by dust.
Estimating the redness of the connected region and the outline of active galactic nuclei: NGC 5548’s mean redness and accretion disk size, Monthly Notices of the Royal Astronomical Society (2023).
Provided by University of California – Santa Cruz
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