From our only vantage point in the cosmos, it is really difficult to understand three-dimensional space.
We can easily map stars into constellations in relation to each other, but knowing which are closer and which are further away is much more difficult to measure.
One way to determine the distance to objects in space is to use standard candles, objects of known intrinsic brightness. Astronomers measure the difference between how bright the object actually is and how bright it appears to us light-years away, and use this difference to calculate how far the light travels.
These candles include pulsating stars whose intrinsic brightness is related to the timing of their pulses and supernovas with a limited peak brightness range.
Now, astronomers have proven the feasibility of what appears to be the most unlikely instrument in the Universe for this set: supermassive black holes. Or, at least, their echoes.
“Measuring cosmic distances is a fundamental challenge in astronomy, so the possibility of having an extra ace up your sleeve is very exciting,”
You might feel a bit puzzled here. While it’s true that we know (more or less) how bright black holes are, it doesn’t help – because they’re, well, the opposite of bright.
They do not give off any visible radiation; they are effectively invisible.
There are up to one billion stellar-mass black holes in the Milky Way; we only identified a few handfuls.
Supermassive black holes that reside in the hearts of galaxies, however, are a very different ballgame.
No, we still can’t see them; but if they are active, the material around them really shines very brightly. And it’s how light behaves in this surrounding environment can be used to calculate its intrinsic brightness.
An active supermassive black hole is one that feeds on material, and this material is structured around the black hole in a known architecture. At the center is the supermassive black hole itself, a beast that can be millions to tens of billions of times the mass of the Sun.
Around this vortex is a disc of material, which gravitationally pours into the black hole, a bit like water that whirls and falls into a drain. This is the accretion disk and the intense gravitational and frictional forces in it heat the material and make it shine brightly. But that’s not what astronomers have measured.
Outside the accretion disk is a larger cloud, a donut-shaped ring of dust called a torus. The whole structure is assembled as in the illustration above. It is that external torus that is the key to a technique known as echo mapping or reverb mapping.
Occasionally, the region of the accretion disk closest to an active supermassive black hole glows brightly in predominantly optical and ultraviolet wavelengths – and when it reaches the torus, it “echoes.”
The optical and ultraviolet light is absorbed by the dusty cloud, which heats and emits that thermal energy as mid-infrared light.
Growth discs can be huge; it can take years for the light to reach the torus and be re-emitted. But because we know the speed of light, astronomers can use the time between the flare and the echo to calculate the distance between the inner edge of the accretion disk and the torus.
This is where it gets really smart. We know that the inside edge of the accretion disc is incredibly hot. And we know the disk cools as we move outward from the black hole.
When the temperature drops to around 1,200 degrees Celsius (2,200 degrees Fahrenheit), that’s when dust clouds can form.
Hence, the distance between the torus and the inner edge of the accretion disk is directly proportional to that insanely hot temperature.
If we know the distance, we can calculate the temperature – and once we know the temperature, we can calculate the amount of light that region emits. Boom. Intrinsic brightness. That link is called R-L relationship (by radius and brightness).
Well, obviously it’s not as simple as “boom”. It is necessary to observe a black hole very carefully for long periods of time to detect the optical / ultraviolet flash and mid-infrared echo.
A team of astronomers led by Qian Yang of the University of Illinois at Urbana-Champaign sifted through nearly two decades of data collected from ground-based optical telescopes to look for the optical flash.
Then they studied data collected between 2010 and 2019 by NASA’s Near Earth Object Wide Field Infrared Survey Explorer looking for matching infrared flares.
They identified 587 supermassive black holes with an optical flash and a mid-infrared echo – the largest single survey of its kind.
And although there is still room to refine the data – infrared probes did not cover the full infrared range, meaning there is a fair amount of uncertainty in distance calculations – they confirmed that the RL relationship scales and the echo behaves similarly through supermassive black holes of all sizes in their sample.
Work to refine the measurements will be ongoing.
The team is working to improve their models to better limit the behavior of the dust and the way it emits infrared light. And, of course, ongoing surveys with better technology will continue to deliver higher quality observations.
“The beauty of the echo mapping technique is that these supermassive black holes aren’t going to disappear anytime soon,” Yang said. “So we can measure the dust echoes over and over again for the same system to improve distance measurement.”
The research was published in The Astrophysical Journal.