Not all black holes are black — and researchers found more than 75k of the brightest
Black holes may be the “darkest” objects in the cosmos, but some can shine trillions of times brighter than the Sun
The brightest black holes
SomeAGN violently spew out matter via a jet, which travels millions of kilometers through space and can be seen by radio telescopes. Others produce “winds” at the center of the galaxy, capable of pushing any gas (the fuel needed for stars to form) out of the galaxy.
With such destructive forces in the middle of a galaxy, astronomers are certain this must have a big impact on the galaxy itself. We know most galaxies are slowlyturning off their star formation processes, and AGN might be one of the culprits.
AGN can therefore not only help us to better understand elusive black holes but studying them also teaches us about galaxies themselves.
Finding bright black holes
Depending on how much a black hole is “eating”, what galaxy it’s in, and the angle from which we can see it, AGN can look very different from one another. Even when looking at the same galaxy, one astronomer with an X-ray telescope may see it glow and discover an AGN, whereas another astronomer using a radio telescope might see nothing, if the AGN doesn’t happen to produce jets that are visible in the radio spectrum.
Because of this, it was thought they were all different objects, but by looking at the same objects with different telescopes astronomers discovered they had many similarities and realized the benefits of using more of the electromagnetic spectrum to find them.
The relative brightness of a galaxy across different parts of the electromagnetic spectrum is called its “spectral energy distribution”. This can be used to measure how many stars are in a galaxy, how old they are, what they’re made of, and how much dust is blocking the light.
In our research,published todayin Monthly Notices of the Royal Astronomical Society, we show that this technique can also be used to spot AGN. This means we can now measure not just the properties and histories of the stars in the galaxy, but also the brightness of its central black hole.
It’s not a simple thing to do. The difference between starlight and the light from an AGN is incredibly subtle, so it’s possible to confuse young stars for a bright black hole, and vice versa.
Here in Australia, astronomers have beenusing Australian telescopesto make 3D maps of galaxies in specific patches of the sky. These maps let us scour hundreds of thousands of galaxies, spanning 11 billion years of history, for possible AGN.
By applying our new method to 700,000 galaxies we identified and quantified more than 75,000 AGN to begin understanding how their number has evolved over time and how they have impacted their host galaxies. Astronomers think the number of AGN in the Universe is linked to the amount of star formation, which we know was almost ten times higher roughly 10 billion years ago. But until we can be certain we’ve identified all the AGN across cosmic time in our galaxy samples, we won’t know for sure.
Right now, the astronomical community is still passionately debating the nature of active black holes. While we haven’t yet answered the questions needed to soothe the debate, we’re now one step closer to reliably being able to spot these fascinating objects within galaxies. And that’s an important step towards shedding more light on the mystery of black holes.
Article byJessica Thorne, Astrophysics PhD Candidate,The University of Western AustraliaandSabine Bellstedt, Research Associate in Astronomy,The University of Western Australia
This article is republished fromThe Conversationunder a Creative Commons license. Read theoriginal article.
Story byThe Conversation
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