About a Third of Supermassive Black Holes are Hiding

Supermassive black holes can have trillions of times more mass than the Sun, only exist in specific locations, and could number in the trillions. How can objects like that be hiding? They’re shielded from our view by thick columns of gas and dust.

However, astronomers are developing a way to find them: by looking for donuts that glow in the infrared.

It seems almost certain that large galaxies like our own Milky Way host supermassive black holes (SMBHs) in their centers. They grow through mergers with other SMBHs and through accretion. When they’re actively accreting material, they’re called Active Galactic Nuclei (AGN) and become so bright they can outshine all of the stars in their entire galaxy. The most luminous AGN are called quasars.

SMBHs, like all black holes, emit no light themselves. Instead, the light comes from the torus of swirling gas and dust that forms an accretion ring around the SMBH. The gas and dust become superheated and emit electromagnetic radiation. So far, scientists have only imaged two SMBHs, both with the Event Horizon Telescope (EHT). (To be clear, the EHT doesn’t actually “see” the SMBH. Instead, it sees the light coming from the accretion disk and the shadow the SMBH casts on the disk.)

Even without seeing them, astronomers are pretty certain that most large galaxies host an SMBH. How? Stars near the center of galaxies move in unusual ways as if they’re under the influence of an extremely massive object. The intense radiation from AGN is also strong evidence of an SMBH. Galaxy formation and evolution models and gravitational lensing provide additional evidence.

However, astronomers still want to find more of them so they can confirm their models or adapt them to suit observational results. The problem is that many of them are hidden from view by gas and dust. If that gas and dust are thick and dense enough, they act as a veil, blocking even low-energy X-ray light. That means our view of the galaxy centre is obscured, even if it is an AGN.

Whether or not we can see the centre of a galaxy like this depends on our viewing. From a “side” view, the torus blocks it out, while from a “top” or “bottom” view, it doesn’t.

Astronomers want to understand how many SMBHs there are in the Universe, but obviously, there’s no way to find them and count them all. What they hope to do is determine the ratio between hidden and unhidden SMBHs. To do that, they need a large enough sample to extrapolate from. That way, they can get a more accurate idea of how many SMBHs there are.

A new survey using data from multiple NASA telescopes has advanced our understanding of SMBHs. The survey and its results are detailed in a paper titled “The NuSTAR Local AGN NH Distribution Survey (NuLANDS). I. Toward a Truly Representative Column Density Distribution in the Local Universe.” It’s published in The Astrophysical Journal, and the lead author is Peter G. Boorman, an astrophysicist from the Cahill Center for Astrophysics at the California Institute of Technology.

The NuLANDS aims to find the thick dust and gas that obscures AGN. Previous efforts to detect AGN have been hampered by relying on hard X-rays, the highest-energy portion of the X-ray spectrum, often defined as X-rays with energies greater than 10 kiloelectronvolts (keV). Accretion disks around SMBHs can be heated to extremely high temperatures and emit hard X-rays.

However, thick enough gas and dust can block even hard X-rays. If the column density of the gas is too high, no hard X-rays can get through. “Hard X-ray-selected samples of active galactic nuclei (AGN) provide one of the cleanest views of supermassive black hole accretion but are biased against objects obscured by Compton-thick gas column densities of NH > 1024 cm-2,” the authors write in their paper. Compton-thick means thick enough to obscure an AGN.

The thick gas and dust that block hard X-rays absorb them and then re-emit them as lower-energy infrared light. This creates a glowing torus, or donut, of gas and dust. This is where IRAS comes in.

IRAS was the Infrared Astronomical Satellite, launched in January 1983 and operated for 10 months. It performed an infrared survey of the entire sky, and it spotted the infrared emissions from the toruses around SMBHs. Critically, it spotted these toruses whether they were face-on or edge-on.

However, IRAS didn’t discriminate against infrared sources. It also spotted galaxies undergoing rapid star formation, which emit similar infrared light as AGN. In this new research, the authors used ground-based telescopes to differentiate between the two.

At that stage, the researchers had a sample of toruses around SMBHs emitting infrared light. However, they didn’t know if they were seeing them face-on or edge-on. Remember, their goal was to determine how many SMBHs are hidden and how many aren’t. With a large enough sample containing good data, they could extrapolate how many SMBHs there are and whether all large galaxies have one.

This is where another NASA satellite comes in. NuSTAR is an X-ray space telescope that was launched in June 2012 and is still operating. One of its primary goals was to detect SMBHs one billion times more massive than the Sun.

NuSTAR can detect high-energy X-rays that pass through thick dust and gas, so it can detect edge-on SMBHs. However, it can use hours of observation time to detect these X-rays, so for it to be effective, it has to know where to look first. That’s what IRAS helped with.

“It amazes me how useful IRAS and NuSTAR were for this project, especially despite IRAS being operational over 40 years ago,” said lead author Boorman. “I think it shows the legacy value of telescope archives and the benefit of using multiple instruments and wavelengths of light together.”

In their NuLANDS survey, the researchers looked at 122 nearby AGN chosen for their warm infrared colours. “To tackle this issue, we present the NuSTAR Local AGN NH Distribution Survey (NuLANDS)—a legacy sample of 122 nearby (z < 0.044) AGN primarily selected to have warm infrared colors from IRAS between 25 and 60 ?m,” the authors write.

Their sample of galaxies is also biased towards those whose AGN is obscured by something close to them rather than by some large-scale feature of the galaxy itself. “By construction, our sample will miss sources affected by severe narrow-line reddening, and thus segregates sources dominated by small-scale nuclear obscuration from large-scale host-galaxy obscuration,” the authors explain.

The researchers found that 35% ± 9% of galaxies have Compton-thick dust, meaning their AGN and SMBH are obscured. So, about one-third of the Universe’s SMBHs are obscured. However, these are only the first results from NuLANDS, and while 122 AGN is a sizeable survey, there’s more to come.

These results support some of the thinking around SMBHs, their masses, and their numbers. SMBHs must consume an enormous amount of material to reach their enormous sizes. That means many of them should be obscured by the very dust they’ll eventually consume. Boorman and his co-authors say their results support this idea.

“If we didn’t have black holes, galaxies would be much larger,” said study co-author Poshak Gandhi, a professor of astrophysics at the University of Southampton in the UK. That’s for two reasons. First, they consume material that would otherwise form more stars. Second, sometimes too much material falls toward the black hole, and they belch up the excess. That ejected material can disperse the clouds of gas where stars form, slowing the galaxy’s star formation.

“So if we didn’t have a supermassive black hole in our Milky Way galaxy, there might be many more stars in the sky. That’s just one example of how black holes can influence a galaxy’s evolution,” said Gandhi.