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The disappearing-reappearing black hole

In astronomical terms, human lifetimes pass by in a blink. As a result, many cosmic phenomena are ponderously slow from our perspective. So, when something happens quickly, astronomers take notice. That’s the case where a black hole in a distant galaxy seemed to nearly vanish, dropping in brightness by more than 90% within the span of a few years.

As regular readers know, black holes are far from being invisible: some of them rank as the most persistently bright objects in the Universe. (I say “persistently” because supernovas can outshine whole galaxies, but that intensity doesn’t last.) The black hole in the galaxy NGC 5548 is sufficiently bright that astronomers have been watching it for nearly 50 years. But when its X-ray emissions dropped by a factor of 25, astronomers had to wonder why.

Further observations showed that something was an incredibly fast flow of gas blowing outward from the black hole. We happen to be looking straight into that flow, as though a fan were turned on directly in our face. That’s a lucky break: while astronomers have measured similar outflows in other galaxies, this is the first time anyone has seen one from this angle, which provides a lot more detailed data than other viewpoints.

Between that privileged angle and the fact that researchers have watched the galaxy change over decades, we can see how outflows from black holes have shaped NGC 5548. After all, galaxies aren’t static environments, and studying their history is a way to understand the complex processes that control the birth of stars and planets. [Read more…]

Cookie Monsters and black holes are both messy eaters.

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