Misbehaving ‘baby’ black holes could cause strange brightening of radio galaxies

The radio galaxy Hercules A has an active supermassive black hole at its centre. Here it is pictured emitting high energy particles in jets expanding out into radio lobes.
The radio galaxy Hercules A has an active supermassive black hole at its center. Here it is pictured emitting high energy particles in jets expanding out into radio lobes. NASA/ESA/NRAO

The more we learn about black holes, the more mysterious they seem to be. A new study has looked at “baby” supermassive black holes, which lie at the heart of young galaxies, and found that they may be misbehaving in intriguing ways.

Almost all galaxies have a supermassive black hole at their center, and generally the bigger the galaxy, the bigger the black hole. And one way to measure a distant galaxy is to observe how much light it gives off, and how this light changes over time. Now, researchers from Curtin University have identified a number of galaxies in which the level of light changes much more quickly than expected, and they think that has to do with their supermassive black holes.

“Given their size, you’d expect the amount of light emitted from galaxies would change slowly and steadily, over timescales far beyond a person’s lifetime,” two of the researchers, Kathyrn Ross and Natasha Hurley-Walker of Curtin University, wrote in The Conversation. “But our research, published in the Monthly Notices of the Royal Astronomical Society, found a surprising population of galaxies whose light changes much more quickly, in just a matter of years.”

There are other strange things about these galaxies too. One way that astronomers sort radio galaxies is through the frequencies they give off, classified by “radio color.” When observed in the radio wavelength, younger galaxies appear blue and are brighter at higher radio frequencies. Older galaxies appear red and are brighter at lower radio frequencies. The process of morphing from blue to red should be very, very slow, but some of the galaxies in this sample changed their brightness and their color quickly, which was a puzzle to explain.

The researchers came up with three possibilities to explain the strange brightening and color change: The first and most prosaic explanation is that the light from these galaxies could have been distorted by the dust and gas in our own galaxy, and the galaxies aren’t really changing.

But the other two explanations have to do with the galaxies’ black holes: Either because the black holes happen to be lined up in such a way that they are firing jets of particles called “blazars” directly at us, making the light from the galaxy appear brighter and more variable, or that the black holes are chomping down on extra matter and spewing out a clump of extra particles. As this clump travels toward us, we see it as a change in frequency.

The researchers hope to use upcoming projects like the Square Kilometer Array to understand more about radio galaxies, as they may be far more changeable and dynamic than we previously realized.

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