New Survey Unveils Surprising Surge of Hidden Supermassive Black Holes!

Recently, several NASA telescopes assisted researchers in scanning the heavens for supermassive black holes—those weighing billions of times more than the sun. This recent survey is distinct as it was equally inclined to discover massive black holes obscured by dense clouds of gas and dust as those that are not.

Astronomers propose that each significant galaxy in the universe harbors a supermassive black hole at its core. However, evaluating this theory is challenging since researchers cannot feasibly count the billions or even trillions of supermassive black holes believed to exist in the cosmos. Instead, they must infer from smaller groups to gain insights about the wider population. Thus, accurately gauging the proportion of hidden supermassive black holes in a specific sample enables scientists to more accurately estimate the total number of these black holes in the universe.

The recent research published in The Astrophysical Journal discovered that approximately 35% of supermassive black holes are substantially obscured, indicating that the surrounding clouds of gas and dust are so dense they block even low-energy X-ray light.

Previous comparable searches have indicated that less than 15% of supermassive black holes are so heavily obscured. Scientists believe the actual division should be more like 50/50 based on models regarding how galaxies evolve. If observations continue to suggest that significantly fewer than half of supermassive black holes are concealed, scientists will have to revise some fundamental concepts they have regarding these entities and the influence they exert on shaping galaxies.

Concealed riches

Although black holes are fundamentally dark—not even light can evade their gravitational pull—they can also be among the most luminous objects in the cosmos: When gas is drawn into orbit around a supermassive black hole, resembling water swirling down a drain, the extreme gravitational forces generate such intense friction and heat that the gas reaches hundreds of thousands of degrees and radiates so intensely it can outshine every star in the neighboring galaxy.

The clouds of gas and dust surrounding and replenishing the brilliant central disk may generally form a torus shape, or doughnut. If the hole of the doughnut is oriented towards Earth, the radiant central disk within it is visible; if the doughnut is viewed edge-on, the disk becomes obscured.

Most telescopes can relatively easily recognize face-on supermassive black holes, though edge-on ones can be more challenging. However, there is one exception that the authors of this new research were able to exploit: The torus absorbs light from the central source and reemits lower-energy light in the infrared spectrum (wavelengths slightly longer than what human eyes can perceive). Essentially, the doughnuts emit a glow in the infrared.

These wavelengths of light were observed by NASA’s Infrared Astronomical Satellite, or IRAS, which operated for ten months in 1983 and was managed by NASA’s Jet Propulsion Laboratory in Southern California. As a survey telescope that scanned the entire sky, IRAS was able to detect the infrared emissions from the clouds enveloping supermassive black holes. Most importantly, it could identify both edge-on and face-on black holes with equal effectiveness.

IRAS identified hundreds of initial targets. Some were revealed to be not densely obscured black holes but galaxies with high star formation rates emitting a similar infrared luminescence. Consequently, the authors of the recent study utilized terrestrial, visible-light telescopes to pinpoint those galaxies and distinguish them from the hidden black holes.

To validate the presence of edge-on, heavily obscured black holes, the researchers depended on NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array), an X-ray observatory also overseen by JPL. X-rays are emitted by some of the hottest substances surrounding the black hole. Lower-energy X-rays are absorbed by the surrounding clouds of gas and dust, while the higher-energy X-rays detected by NuSTAR can traverse and scatter off the clouds. Identifying these X-rays may require hours of observation, so scientists employing NuSTAR must first utilize a telescope like IRAS to indicate where they should focus their efforts.

“I am astonished by how instrumental IRAS and NuSTAR were for this project, particularly given that IRAS has been active for over 40 years,” remarked lead researcher Peter Boorman, an astrophysicist at Caltech in Pasadena, California. “This illustrates the enduring value of telescope archives and the advantages of employing multiple instruments and wavelengths of light simultaneously.”

Numerical advantage

Assessing the quantity of hidden black holes in contrast to non-hidden ones can aid scientists in comprehending how these black holes attain such massive sizes. If they expand by consuming matter, a considerable number of black holes should be enveloped in thick clouds and possibly concealed. Boorman and his co-authors assert that their study bolsters this hypothesis.

Furthermore, black holes impact the galaxies they inhabit, primarily by influencing how galaxies develop. This transpires because black holes encased in substantial clouds of gas and dust can ingest considerable—but not limitless—quantities of material. If too much material approaches a black hole simultaneously, the black hole starts expelling the excess and launching it back into the galaxy. This phenomenon can scatter gas clouds within the galaxy where star formation is occurring, thereby hindering the rate of star formation there.

“Without black holes, galaxies would likely be much larger,” commented Poshak Gandhi, a professor of astrophysics at the University of Southampton in the United Kingdom and a co-author of the recent study. “Thus, if we lacked a supermassive black hole in our Milky Way galaxy, there might be considerably more stars visible in the sky. That is merely one illustration of how black holes can impact a galaxy’s evolution.”