An artist’s impression of what the dust around an active galactic nucleus might look like seen from … [+]
There are few more beautiful sights than the orangey-red disk of our star, the Sun, sinking into the ocean. At sunset it seems a far cry from powerful, hot star we feel at midday and can’t even look at safely. If we were only able to view the Sun at sunset what would we think of it? It would be fair to conclude that it was far weaker than it actually is.
It could be a similar case for astronomers’ observations of the centers of galaxies, suggests a new study.
Active galactic nuclei are the most powerful compact steady sources of energy in the cosmos. They’re powered by supermassive black holes that swallow matter and far outshine the combined light of the billions of stars in their host galaxies.
Published today in Monthly Notices of the Royal Astronomical Society, new research suggests that astronomers may have hugely underestimated the energy output of these objects—and the seemingly fundamental differences between them—simply because their light is dimmed by differing amounts of dust.
“When there are intervening small particles along our line of sight, this makes things behind them look dimmer,” said Martin Gaskell, lead author and a research associate in astronomy and astrophysics at UC Santa Cruz. “We see this at sunset on any clear day when the sun looks fainter.”
The work is based on observations of the active galactic nuclei of NGC 5548, a galaxy 250 million light-years distant in the constellation Bootes that hosts a supermassive black hole.
When the Sun sets the light that reaches the viewer’s eyes has traveled through a lot more atmosphere than when it’s high above. Its radiation is striking more molecules. Red and orange have the longest wavelengths of light, so they more easily travel through Earth’s atmosphere to reach your eyes. The intensity of light is less, too, having been filtered by the atmosphere, so you can glance at a setting Sun with your naked eyes. It looks fainter.
Similarly, dust in active galactic nuclei also makes them appear redder than they really are, with the amount of reddening related to the amount of dimming, suggests the study.
The study shows that “the far ultraviolet light of a typical active galactic nucleus is dimmed by a large factor,” according to Gaskell. It had previously been presumed to be negligible.
In the new study of NGC 5548 the researchers found its dimming due to dust was found to be more than ten times the dimming caused by dust as we look out of our Milky Way galaxy.
The implication is that in ultraviolet light—where most of the energy is radiated—a typical active galactic nucleus is putting out an order of magnitude more energy than previously thought, Gaskell said. Not only that, but the colors of NGC 5548 are similar to other active galactic nuclei, making active galactic nuclei in general more powerful than had been realized.
It also means that the centers of galaxies may, in fact, be much more similar than previously thought, with what was thought to be major fundamental differences between them really just the consequences of different amounts of reddening by dust.
“This makes life simpler for researchers and is speeding up our understanding of what happens as black holes swallow material,” said Gaskell.
The paper was co-authored by three high school seniors participating in the UCSC Science 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 eyes.
