Several NASA telescopes recently helped scientists study supermassive black holes, which are billions of times heavier than the Sun. The new study is unique as it has made it equally effective in identifying black holes hidden behind thick clouds of gas and dust and those that are open to observation.

Astronomers believe that almost every major galaxy in the Universe has a supermassive black hole at its center. However, it is not easy to confirm this because it is not possible to directly count all existing black holes. Scientists have to extrapolate data from smaller samples. Accurate determination of the ratio of hidden to open black holes helps to estimate their total number.
A study published in The Astrophysical Journal found that about 35% of supermassive black holes are heavily obscured. This means that clouds of gas and dust block even low-energy X-rays. Previously, it was thought that such objects were less than 15%. Models of galaxy growth suggest that the ratio should be closer to 50/50. If this discrepancy remains, scientists will have to rethink their ideas about the role of black holes in the formation of galaxies.
Hidden black holes
Black holes do not inherently emit light, but they may be the brightest objects in the Universe. When gas spins around a supermassive black hole, gravity creates friction that heats the gas to hundreds of thousands of degrees, making it brighter than all the stars in the galaxy. The clouds of gas and dust that surround the disk are torus-shaped. When viewed from the side, the disk is darkened.
Modeling of a thick torus of gas and dust surrounding a supermassive black hole. The torus can obscure the light that is generated by material falling into the black hole. Observations from NASA telescopes have helped scientists identify more of these hidden black holes. Author: NASA/JPL-Caltech
Most telescopes can detect open black holes, but not hidden ones. However, infrared telescopes, such as NASA’s IRAS satellite, can see the radiation that the torus emits in the infrared. IRAS operated in 1983, imaging the entire sky, and discovered hundreds of potential hidden black holes.
To confirm the presence of heavily obscured black holes, the researchers relied on NuSTAR (Nuclear Spectroscopic Telescope Array), NASA JPL’s X-ray observatory. Lower-energy X-rays are absorbed by the surrounding clouds of gas and dust, while the higher-energy X-rays observed by NuSTAR can penetrate and scatter from the clouds. Detecting these X-rays can take a long time, so scientists working with NuSTAR first need a telescope like IRAS to help tell them exactly where to look.
Why is it important?
Determining the ratio of hidden to open black holes allows us to understand how they evolve and grow. If black holes absorb material, then a significant portion of them must be surrounded by thick clouds. The study confirms this hypothesis.
Black holes also affect the galaxies they are in. As they absorb material, they can eject it back, which slows the rate of star formation. “If we didn’t have black holes, galaxies would be much larger,” said Poshak Gandhi, a professor of astrophysics at the University of Southampton in the U.K. and co-author of the study.
For example, there could be many more stars in the Milky Way without a supermassive black hole, illustrating their influence on the evolution of galaxies.
According to phys.org