A Second Supermassive Black Hole May Hide at the Center of the Milky Way

Could a second supermassive black hole be orbiting the one we already know about near the mysterious core of the Milky Way galaxy? New studies suggest our black hole may have a dark, mysterious companion.

Nearly every galaxy astronomers have ever seen contains a supermassive black hole near its center, and our own Milky Way is no exception. However, new research suggests that this invisible behemoth may also have a dark, mysterious partner, orbiting it from afar.

Near the center of the Milky Way sits Sagittarius A* (pronounced “Sag A star”), our local supermassive black hole. Despite its mass, four million times greater than the Sun, this is a sleeping giant. The object is relatively quiet (as far as galactic black holes go), although some signs of increased activity have recently been seen.

A primitive supermassive black hole at the center of a young star-rich galaxy. Image credit: NASA/JPL-Caltech

Scientists have long speculated about the nature of black holes, and four types of the objects are currently known. The largest of these, supermassive black holes (SMBH’s), have masses of more than a million suns and are usually found near the center of galaxies. There, apart from brief periods of fasting, they consume everything around them for eons on end, growing to enormous proportions.

Sgr A* compresses the mass of more than four million suns into a region no larger than the distance between the Earth and Sun.

The region surrounding this galactic black hole is densely packed with stars. By studying the motions of these stars, Smadar Naoz, an astrophysicist at UCLA, and her team became convinced Sgr A* is accompanied by another unseen companion.

“For more than 20 years, scientists have been monitoring the orbits of these stars around the supermassive black hole. Based on what we’ve seen, my colleagues and I show that if there is a friend there, it might be a second black hole nearby that is at least 100,000 times the mass of the Sun,” Naoz writes.

A Long Time Ago in a Galaxy…

Just 100 million years after the Big Bang, the first tiny galaxies began to coalesce. A typical galaxy of this era was just one percent of one percent as large as our modern day Milky Way. Within these structures, enormous stars hundreds or thousands of times larger than the Sun were born, lived out their short lives, and quickly exploded, leaving behind massive black holes. These fell together toward the center of their home galaxies, and merged into newly-born galactic black holes.

Supermassive black holes, and the regions surrounding them, are home to some of the most extreme physics in the Universe. Theories of the formation of galaxies suggest that pairs of supermassive black holes may be common throughout the Universe.

The aftermath of the collisions of galaxies. This luminescent debris will, one day, form into a new, elliptical galaxy, comprised of material from each of its parents. Image credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt

Over time, galaxies collide, and often merge with one another — over the course of the next few billion years, the Milky Way will impact both the Large Magellanic cloud and the Andromeda galaxy. If a second supermassive black hole is discovered near the core of the Milky Way, it would lend strong evidence to the idea that our galaxy experienced at least such collision in fairly recent history.

Bend Me, Shape Me

If there is a second SMBH orbiting with Sgr A*, the pair are caught in a complex dance, accompanied by countless other stars near then, drawn by the irresistible call of gravity.

As stars in the region race around these tremendous black holes, gravitational forces warp its orbit, changing the path over time. Studying how actual orbits compare to predictions can reveal information about gravity from nearby objects — even invisible ones like black holes.

A supermassive black holes warps space and time, as seen in this simulation, provided by the European Southern Observatory.

One of the stars near the stealthy pair, named SO-2, orbits Sgr A* once every sixteen years. Study of its orbit allowed Naoz and her team to better understand the size and orbit of a theoretical second SMBH near the galactic core.

The star at the center of this study, SO-2, also allowed other researchers to test a prediction made by Albert Einstein — that light racing away from a powerful gravitational field would be stretched by the curvature of space.

“The general theory of relativity predicts that a star passing close to a supermassive black hole should exhibit a relativistic redshift. In this study, we used observations of the Galactic Center star S0–2 to test this prediction,” researchers announced in August 2019.

Mists of red with a bright egg shaped region near the bottom.
Mists of red with a bright egg shaped region near the bottom.
The center of the Milky Way galaxy, as seen in radio waves. Image credit: Farhad Zadeh, VLA, NRAO, APOD

Naoz suggests a second SMBH near the core of the Milky Way would affect material in its local area, potentially resulting in waves of radiation which could be detected and measured by astronomers.

“[I]f there is a disk of accreting matter surrounding the massive member of the pair, the passage of the secondary black hole through the disk could induce variability in the output of electromagnetic radiation from the vicinity of Sgr A*,” researchers report.

Astronomers are on the cusp of an exciting new age of astronomy, as they learn how to hear — and map — gravitational waves. Just as we once looked at the stars in visible light, then radio waves, ultraviolet, and infrared frequencies, we are now learning how to see ripples in spacetime, formed in the wake of the cosmic dance of the most massive objects in the Universe. New instruments like the Laser Interferometer Space Antenna (LISA) may soon record the dance of a pair of supermassive black holes at the center of our own galaxy.

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Writing about space since I was 10, still not Carl Sagan. Mailing List/Podcast: https://thecosmiccompanion.substack.com

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

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