Radio signals reveal secrets of hidden supermassive black holes

Astronomers have found a striking correlation between the amount of dust surrounding a supermassive black hole and the strength of radio radiation produced in extremely bright galaxies.

An international team of astronomers led by the University of Newcastle and Durham University in the UK used new data from the Dark Energy Spectroscopic Instrument (DESI), which is conducting a five-year survey of the large-scale structure of the universe covering approx. 3 million quasars; ultra-bright galaxies powered by supermassive black holes.

They found that quasars that contained more dust and therefore appeared redder were more likely to have higher radio emissions than quasars that had very little dust and appeared very blue. The results have been published Monthly Notices of the Royal Astronomical Society.

Almost every known galaxy contains a supermassive black hole, which are black holes with millions to billions of the mass of our sun at their center, including our own Milky Way. At the center of some galaxies, there is a lot of material that feeds and grows this supermassive black hole, making it very energetic and “active”.

The most powerful type of these active galaxies are called “quasars,” which are among the brightest objects in the universe. Most quasars appear very blue due to the bright disk of matter orbiting and feeding the central supermassive black hole, which is very bright at optical and ultraviolet wavelengths.

However, astronomers have discovered that a significant number of these quasars are very red, although the nature of these objects is still not well understood.

Understanding the physics of these red quasars requires “spectroscopic” measurements that can analyze quasar light at different wavelengths. The shape of the quasar spectrum can indicate the amount of dust surrounding the central region. Observing radio emission from quasars can also tell you about the energetics of the central supermassive black hole; either it sets off powerful “winds” or “jets” that can shape the surrounding galaxy.

Understanding the mysteries of red quasars

This new study, led by Dr Victoria Fawcett of Newcastle University and formerly of Durham University, uses DESI spectroscopic observations to measure the amount of dust (reddening) in a sample of ~35,000 quasars and relate this to the observed radio emission. The researchers find that DESI is able to observe much more extreme red (dusty) quasars compared to similar/earlier spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS). They also find that redder quasars are much more likely to have strong radio emission compared to typical blue ones.

Dr Fawcett said: “It was really exciting to see the amazing quality of the DESI data and to discover thousands of these previously rare red quasars. I feel that this study puts many pieces of the puzzle together in our understanding of red quasars and definitively links the dust in the quasar to its radio emission. I think this is the strongest evidence yet that red quasars are a key element in galaxy evolution.”

This reddening radio connection is likely the result of a powerful outflow of gas from the supermassive black hole that slams into the surrounding dust, causing shocks and radio emissions. These outflows eventually blow away all the dust and gas in the galactic center, exposing the blue quasar and resulting in weaker radio emission.

This is consistent with the emerging picture that red quasars are a younger, “blow-out” phase of galaxy evolution. Therefore, red quasars can be extremely important for understanding the evolution of galaxies over time.

Dr. Fawcett continues: “There are still many unanswered questions surrounding red quasars, such as whether black hole winds or radio jets are ultimately responsible for this enhanced radio emission. But as DESI’s sample of red quasars continues to grow over the next few years of study, I am confident that we will on the verge of a complete understanding of nature.”