Unexpected discovery of a link between black hole jets and their host galaxies

In our study published today, November 14, 2024, in the journal Nature Astronomy we demonstrate that there is a connection between the region near a black hole and its host galaxy because the jets emitted by the black hole are aligned with the rest of the galaxy.

A new analysis of radio telescope data links the shapes of galaxies to the black holes they harbor. Black holes are not diverse. They are always one color (black) and one shape (spherical). The only quality that can vary from one black hole to another is mass.

In general, the black holes we have detected are either stellar-mass black holes or supermassive black holes. Stellar-mass black holes are close to the mass of our Sun (10³⁰ kg) and the size of a city. Supermassive black holes are much more massive (a million times the mass of the Sun) and about the size of the Solar System. However, as massive as these black holes are, they still have a fairly low mass, often well below 1% of its galaxy’s total stellar mass. They are also much smaller than their host galaxies, on the order of a million times their radius.

An alignment on very different scales

In new results that we publish today November 14 in Nature Astronomywe discovered that there is a connection between the region near the black hole and the host galaxy, because the jets emitted by the black hole are aligned with the rest of the galaxy, despite the very different scales.

Supermassive black holes are quite rare. Our galaxy, the Milky Way, has one (named Sagittarius A* for the constellation in which it is located) at its center. All galaxies also appear to harbor one (or sometimes two) supermassive black holes at their core. The centers, or cores, of these distant galaxies can become active, as dust and gas are drawn toward the core under the gravitational pull of the black hole.

They do not fall immediately, because they rotate greatly, and form a hot disk of material, called an accretion disk. This accretion disk, because of its intense magnetic field, in turn generates a superheated jet of charged particles which are ejected from the core at very high speeds, close to the speed of light. When this happens, it is called a quasar (quasi-stellar source of radiation).

An Earth-sized telescope

A common way to study quasar jets is to use very long baseline interferometry (VLBI). The VLBI allows different radio telescopes to work in tandem, turning them into a single Earth-sized telescope. The spatial resolution is then much higher than that obtained with optical or infrared telescopes.

This “massive eye” is far more effective at resolving fine details than any individual telescope, allowing astronomers to see objects and structures much smaller than those visible with the naked eye, or even with a optical telescope. This is the technique that was used to create the “Black Hole Image” for the halo of light generated by the supermassive black hole hosted by the M87 galaxy.

Thus, thanks to this high-resolution approach, the VLBI allows astronomers to study these jets up to a few light years or less from their origin: the black hole. The direction of the jet at such small scales tells us about the orientation of the accretion disk, and therefore potentially the properties of the black hole itself. And this is the only way to obtain such data at present.

What about the host galaxies themselves? A galaxy is a three-dimensional object, made up of hundreds of billions of stars. But it appears to us (observed optically or in infrared) in projection, either as a 2D ellipse or as a spiral.

We can measure the shape of these galaxies, by tracing the starlight profile, and measure the major axis and minor axis of the two-dimensional shape.

In our paper, recently published in Nature Astronomy, we compared the direction of quasar jets with the direction of the minor axis of the galaxy’s ellipse, and found that they are connected. This is surprising, because the black hole is so small (the jet we measure is only a few light years long), compared to the host galaxy (which can be hundreds of thousands of light years wide ). It is surprising that such a small (comparatively) object can affect, or be affected, by the environment on such large scales. One might expect to see a correlation between the jet and the local environment, but not with the galaxy as a whole.

Does this have anything to say about how galaxies form? Spiral galaxies sometimes collide with other spiral galaxies and form elliptical galaxies, which appear in the sky as ellipses. During the merger process, the phenomenon of quasars is triggered in ways that we do not fully understand. It is for this reason that almost all jets that can be detected using VLBI are hosted in elliptical galaxies.

The exact interpretation of the result remains mysterious, but it is important in the context of the James Webb Space Telescope’s recent discovery of very massive quasars (with supermassive black holes), which formed much earlier in the universe than foreseen. It is clear that our understanding of galaxy formation and the influence of black holes on this phenomenon needs to be updated.