Misbehaving Baby Black Holes May Cause Strange Brightening

The radio galaxy Hercules A has an active supermassive black hole in its center. It shows how high-energy particles are emitted in jets, which expand to form radio beams. NASA / ESA / NRAO

The more we learn about black holes, the more mysterious they seem. A new study looked at “baby” supermassive black holes that lie in the heart of young galaxies and found that they can misbehave in fascinating ways.

Almost all galaxies have a supermassive black hole in their center, and in general, the larger the galaxy, the larger the black hole. One way to measure a distant galaxy is to observe how much light it is giving off and how that light changes over time. Now Curtin University researchers have identified a number of galaxies where the level of light is changing much faster than expected, and they believe this has to do with their supermassive black holes.

“Given their size, one would expect the amount of light emitted by galaxies to change slowly and steadily over periods of time far beyond a person’s life,” wrote two of the researchers, Kathyrn Ross and Natasha Hurley-Walker of Curtin University, in The Konversation. “However, our research, published in the Royal Astronomical Society’s monthly bulletins, revealed a surprising population of galaxies whose light changes much faster within a few years.”

There are other strange things about these galaxies too. One way astronomers sort radio galaxies is in the frequencies they emit, classified according to “radio color”. Younger galaxies appear blue when observed at radio wavelengths and are brighter at higher radio frequencies. Older galaxies appear red and are brighter at lower radio frequencies. The process of turning blue to red should be very, very slow, but some of the galaxies in this sample quickly changed their brightness and color, which was a mystery to be explained.

The researchers had three options to explain the strange brightening and color change: The first and most prosaic explanation is that the light from these galaxies may have been distorted by dust and gas in our own galaxy and the galaxies are not really changing.

The other two explanations have to do with the black holes of the galaxies: Either because the black holes are aligned in such a way that they fire particle beams, so-called “blazars”, directly at us and the light from the galaxy appear brighter and more variable, or that the black holes are crushing extra matter and spitting out a clump of extra particles. When this lump comes towards us, we see it as a change in frequency.

The researchers hope to use upcoming projects like the Square Kilometer Array to learn more about radio galaxies, as these may be far more changeable and dynamic than previously thought.

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