An artist’s impression of what the dust around an active galactic nucleus might look like as viewed from… [+] a light year away.
Peter Z. Harrington
There are few sights more beautiful than the orange-red disk of our star, the Sun, sinking into the ocean. At sunset it looks far from a powerful, hot star that we feel at noon and can’t even look at safely. If we could only see the Sun at sunset, what would we think of it? It would be fair to conclude that he was much weaker than he actually is.
Such may be the case with astronomers’ observations of galactic centers, a new study suggests.
Active galactic nuclei are the most powerful compact persistent energy sources in the cosmos. They are powered by supermassive black holes that gobble up matter and far outshine the combined light of the billions of stars in their host galaxies.
Published today in the Monthly Notices of the Royal Astronomical Society, new research suggests that astronomers may have vastly underestimated the energy of these objects — and the seemingly fundamental differences between them — simply because their light is obscured by varying amounts of dust.
“When small particles along our line of sight are involved, it makes things behind them look darker,” said Martin Gaskell, lead author and research associate in astronomy and astrophysics at UC Santa Cruz. “We see this at sunset on any clear day when the sun appears weaker.”
The work is based on observations of the active galactic nuclei of NGC 5548, a galaxy 250 million light-years away in the constellation Volovar that hosts a supermassive black hole.
When the Sun is setting, the light that reaches the viewer’s eyes has passed through much more atmosphere than when it is high above. Its radiation hits more molecules. Red and orange have the longest wavelengths of light, so they more easily pass through Earth’s atmosphere to reach your eyes. The intensity of the light is also less because it is filtered by the atmosphere, so you can look at the setting sun with the naked eye. It looks paler.
Similarly, dust in active galactic nuclei also makes them appear redder than they actually are, with the amount of reddening being related to the degree of dimming, the study suggests.
The study shows that “the far-ultraviolet light of a typical active galactic nucleus is obscured by a large factor,” according to Gaskell. It was previously thought to be insignificant.
In the new study of NGC 5548, researchers found that its dimming due to dust is more than ten times greater than the dimming caused by dust as we look outside our Milky Way galaxy.
The bottom line is that in the ultraviolet — where most of the energy is emitted — a typical active galactic nucleus gives off an order of magnitude more energy than previously thought, Gaskell said. Not only that, but NGC 5548’s colors are similar to other active galactic nuclei, making active galactic nuclei in general more powerful than previously realized.
It also means that the centers of galaxies may actually be much more similar than previously thought, with what were thought to be major fundamental differences between them actually just the effects of different amounts of reddening from dust.
“This makes life easier for researchers and accelerates our understanding of what happens when black holes swallow material,” Gaskell said.
The paper was co-authored by three graduate students participating in UCSC’s Research Internship Program – Frances Anderson (now at Harvey Mudd College), Sufia Birmingham (now at Princeton University), and Samhita Ghosh (now at UC Berkeley).
Wishing you clear skies and wide-open eyes.
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