Egg-shaped galaxies may be aligned to the black holes at their hearts, astronomers find

Published in Nature Astronomy

Black holes are famously elusive. They have few identifying features—one color (black) and one shape (spherical)—but their most notable distinction is mass. Some black holes weigh roughly the same as a star like our Sun, while others weigh millions of times more. Stellar-mass black holes are scattered throughout galaxies, while supermassive black holes, which are much larger, are typically found at the centers of galaxies.

Even though supermassive black holes are colossal, they are still tiny in cosmic terms. Typically, they make up only around 1 percent of their galaxy’s mass, and their size spans just a millionth of the galaxy's width. However, our recent discovery has revealed an unexpected link between activity near these black holes and the shape of the galaxies they reside in. Our findings were published in Nature Astronomy.

When black holes light up

Supermassive black holes are rare. Our Milky Way hosts one at its center—Sagittarius A*—and many other galaxies have similar giants at their cores. Under certain conditions, gas and dust falling into these black holes can form a hot, spinning disk of material, known as an “accretion disk.” This disk generates a superheated jet of charged particles that shoot out at nearly the speed of light. When a supermassive black hole emits this jet, it’s known as a quasar.

Watching Quasars in action

To study the jets of quasars, astronomers often use radio telescopes. Sometimes, they combine data from multiple radio telescopes located around the globe. Using a technique called Very Long Baseline Interferometry (VLBI), astronomers create a virtual telescope the size of the Earth. This allows us to see incredibly fine details, even finer than what the James Webb Space Telescope can detect.

In fact, VLBI was instrumental in capturing the first-ever “image” of a black hole in 2019, showing the halo of light around the supermassive black hole at the center of the galaxy M87. Quasar jets, which can stretch millions of light-years, are almost always found in elliptical galaxies. With VLBI, we can track these jets all the way down to just a few light-years from their source—providing key insights into the black hole's properties.

The surprising connection to galaxy shape

Galaxies are three-dimensional objects, containing hundreds of billions of stars. However, when viewed from Earth, they appear as two-dimensional shapes—usually elliptical or spiral. By measuring the long and short axes of these shapes, astronomers can determine the galaxy’s orientation.

In our study, we compared the direction of quasar jets with the short axis of their host galaxy’s elliptical shape. We found a surprising pattern: the jets tend to align with the galaxy's shorter axis, more so than would be expected if the alignment were random. This result is striking, considering the vast size difference between the black hole (just a few light-years across) and its host galaxy (which can span hundreds of thousands, or even millions, of light-years).

A puzzle for galaxy formation

Why does this alignment occur? While the black hole is small relative to its galaxy, it appears to have a profound influence on the shape of the galaxy. This is unexpected, as we might expect to see a correlation between the quasar jet and its local environment—but not on such a large, galactic scale.

So, what does this mean for how galaxies form? Most galaxies we see today are spiral-shaped, but these can merge to form elliptical galaxies. This merger process is known to trigger quasar activity, although the details remain unclear. Almost all of the quasars detectable via VLBI reside in elliptical galaxies.

The connection between black holes, galaxy shape, and quasar jets raises new questions about galaxy formation. Recent discoveries, like the detection of highly massive quasars by the James Webb Space Telescope, suggest that massive black holes formed much earlier in the universe than previously thought. Clearly, our understanding of both black holes and galaxy formation is in need of revision.