The Missing ‘Ghost’ in the Machine –Why Massive Black Holes are Not Found in Observatory Data

Black Holes in Milky Way Plane 


Black holes have been described as “the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time” by Nobel-Prize laureate Subrahmanyan Chandrasekhar.

Massive Stars are Born in the Dusty, Gaseous Plane of Our Milky Way

These elusive objects have been missing from data captured by the world’s astronomical observatories according to new research by a team of astronomers led by Peter Jonker (Radboud University/SRON). They realized that telescope observations are biased against detecting massive black holes in our Milky Way Galaxy. Such massive black holes can, in principle, be observed if they eat mass from a companion star. However, the circumstances for those observations have been too difficult in practice, explaining the lack of detections of massive black holes through telescope observations. 

Massive stars are born in the dusty, gaseous plane of our Milky Way in dense star-forming environments. “Massive stars don’t live very long so they stay close to their natal environment,” says astrophysicist and editor, Jackie Faherty. “In our Galaxy that means they will be found where the majority of gas and dust are located and that’s the plane of the Milky Way,” she explains. “It’s where the majority of star formation is happening.  Since they live for such a short period of time, they don’t get very far from their birth locations, hence you find them within the spiral arms in the disk of the Galaxy.”

Fast Facts About the Milky Way Galaxy

After a few million years, a star with 20-30 times the mass of the sun will explode as a supernova and leave behind a low-mass black hole remnant with 3 – 10 solar masses. The supernova explosion provides a modest kick so that the low-mass black hole (and its binary companion) recoil out of the plane of the Milky Way. Meanwhile, a star with 30 – 100 times the mass of the sun is expected to implode and directly collapse into a massive black hole with 10 – 70 solar masses. Most of the binary black hole mergers detected by LIGO have pre-merger black hole masses in this range (see image  below). However, the implosions of the most massive stars do not kick the massive black hole remnants, and so the most massive black holes remain shrouded in dust and gas in the plane of our Milky Way galaxy.

“Invisible Monsters” –Supermassive Black Holes Roam the Milky Way 

Massive Black Holes Will Rarely Reveal Their Existence

Aggravating this bias, as realized by Jonker and colleagues, is that any companion star of a massive black hole must orbit at a relatively large distance, making it rarer for a companion star to be devoured in an observable frenzy. Such episodes are what gives away the existence and location of black holes. Thus, the more massive black holes will more rarely give away their location.

Fast Facts About Old Stars of the Universe

The imminent launch of the James Webb space telescope (JWST) on December 22 will allow astronomers to test these ideas. JWST will for the first time allow the measurement of the mass of several systems of candidate black holes in the plane of the Milky Way. JWST will be sensitive to infrared light, and such light is much less affected by dust and gas than is the optical light typically used by ground-based telescopes. Furthermore, the large size of JWST, and its advantageous position in space, allows JWST to pick out the right star to study among the millions of stars in the plane of the Milky Way. Finally, being above the Earth’s atmosphere, JWST will not be hindered by the infrared light emitted by the atmosphere.


Stellar Graveyard

Measurements using electromagnetic (EM) radiation only revealed stellar black holes less massive than about 20 solar masses (purple circles). These black holes all have a companion star that is losing mass to the black hole. This gas stream reveals the existence of the black hole and detailed study of the motion of the companion allows for the mass of the black hole to be measured.

LIGO/Virgo measurements of gravitational wave radiation emitted when two black holes merge have allowed the masses of several tens of black holes to be measured since 2015 (blue circles). These black holes are generally more massive than those found through EM radiation. We know now that the lack of massive black holes studied through EM techniques can be caused by a bias against finding and studying the massive black holes. Incidentally, the LIGO/Virgo measurements favor the detection of massive black holes because the signal of their mergers is louder and thus can be detected from systems further out in the Universe compared with the signal of merging lower mass black holes. 

Nevertheless, LIGO/Virgo is also detecting lower-mass merging black holes. In the near future, the JWST telescope will remove the EM bias. Due to its sensitivity, astronomers will be able to measure the mass of black hole candidate systems located at places where the most massive black holes are thought to reside.

Image Credit: Aaron M. Geller, Northwestern University and Frank Elavsky, LIGO-Virgo

Reference: “The Observed Mass Distribution of Galactic Black Hole LMXBs Is Biased against Massive Black Holes” by Peter G. Jonker, Karamveer Kaur, Nicholas Stone and Manuel A. P. Torres, 9 November 2021, The Astrophysical Journal.

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Jackie Faherty and Radboud University

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