Supermassive black holes have a complicated lifecycle. Sometimes they’re “on”, blasting out tremendous amounts of energy, and sometimes they’re “off’, where they sleep like dragons in their caves. By comparing the proportion of high-energy to low-energy waves emitted by quasars, astronomers are beginning to pin down how many black holes are sleeping, and when they’re likely to wake back up.
Here’s how it works. As far as we can tell, every galaxy has a supermassive black hole in its center. When material falls onto this black hole, it compresses and heats up (because the extreme gravity of the black hole is trying to drag a whole bunch of material into a relatively small volume). All that friction drives the release of tons of high-energy radiation, something astronomers call a quasar.
Along with the hard stuff comes radio waves, and we can use radio telescopes like LOFAR (the LOw Frequency ARray) and the WSRT (Westerbork Synthesis Radio Telescope) to detect them.
But the intense radiation blasts material away from the black hole, and it can no longer feed, so the black hole goes to sleep and the quasar shuts off, along with the radio emission.
Astronomers are trying to understand the overall lifecycle, which can take hundreds of millions of years to play out. They want to know how often quasars light up, how long they burn, and when they’re likely to switch on again.
And using a combination of high-frequency and low-frequency radio waves, they’ve got a new tool.
“High frequency radio waves quickly lose their energy…while those in the lower frequency do so much more slowly,” according to Prof. Dr. Raffaella Morganti, first author of the paper The best of both worlds: Combining LOFAR and Apertif to derive resolved radio spectral index images.
By using different radio telescopes to observe different frequencies of radio waves, and using the combined data to measure the ratio of high-frequency to low-frequency waves, astronomers can tell how recently a quasar shut off: the less of the high-frequency stuff, the more time has passed since the last feeding event.
From there, astronomers can build up a survey of active quasars, silent ones, and all the rest in between.
While powerful, the technique will require new radio surveys to observe as many galaxies as possible, to build up a proper population census of the black holes in our universe.