What's That Strange Disk Around That Black Hole?
One of the first questions that someone asks me upon learning that I am an astronomer is, “So what’s up with black holes?”
We are fascinated by these bizarre objects that can trap anything that comes to close, even light. Since astronomers can’t directly see a black hole, we can infer its presence by what is going on around it.
Recently, using the Hubble Space Telescope, the light from the accretion disk around a black hole has been measured for the first time.
WATCH VIDEO: Did you know there’s a black hole in the center of our galaxy?
My first reaction to the image above was, “Oh, pretty!” My second reaction, however, was, “Wait, that could be misleading.”
The study talks about how astronomers have detected a disk around a black hole, and this image shows what looks to be two bright objects surrounded by disks. Those are NOT the black hole disk in question, but rather artifacts of the optics of the telescope itself, along with the spots and rays around them.
These artifacts fundamentally limit the resolution of your telescope, so you actually want to be able to see the “Airy disk” when you zoom in so you know that you’ve removed any other imaging artifacts. (Or so I’ve been told. I’ll admit that it’s been a while since I’ve used anything other than radio telescope.)
No optical system is perfect, and all telescopes with a circular aperture — or opening — will impose a pattern called an “Airy disk.” Any support structure that impedes on that circular aperture will add features like “rays.” Hubble has a secondary mirror with several support struts that cause this effect. With these imperfections you are actually getting a glimpse of how the Hubble Telescope itself is set up! As an instrumentation nerd, I find that very cool.
So, this is a very high resolution of the quasar that appears twice due to gravitational lensing. A quasar is a type of active galaxy where the central supermassive black hole is pulling material on to it, causing material around it to shine brightly.
It may look like a star in the image, but a spectrum of the light reveals its true identity. Gravitational lensing is a process by which some massive object in the universe causes the light from a further object to bend and be distorted. If you look really closely, you’ll notice the faint smudge of light that is the lensing galaxy.
To pick out the light of the black hole’s disk, the group led by Jose Muñoz, measured the color of the light received from the quasar. You may be surprised to find that color is a quantifiable measure to astronomers, but it really just means the ratio of brightness of the light as measured by two different color filters.
This definition is not limited to visible light and can be measured in ultraviolet and infrared as well. Gravitational lenses bend different colors of light slightly differently, and provides information on the structure of the quasar source, in this case, estimated to be 11 light-days, or about 160 billion miles (260 billion kilometers). That’s right, light-DAYS, not light years. Larger than the solar system, but much smaller than the distance between the sun and the next nearest star.
So, if you are looking for a direct image of a black hole disk, this isn’t it. However, such a thing is not out of the question for the future, at least not with radio telescope arrays that span the globe.
In fact, some astronomers are already working on that, and they released their first measurements a few years ago. It’s not an image yet, but with the addition of more radio telescopes with millimeter-wave capabilities in the coming years, I may be able to post such an image here one day.
Images: Top – Image of the gravitationally lensed quasar and its lensing foreground galaxy. Credit - NASA, ESA and J.A. Muñoz (University of Valencia); Center – Cutaway diagram of Hubble. Credit – NASA; Bottom – Diagram of a gravitational lens and its effect on the color of the light from the quasar. Credit – NASA/ESA