By Françoise Combes - Astrophysicist at the Paris Observatory
A new analysis of radiotelescope data links the shapes of galaxies to the black holes they host. Black holes are not diverse. They are always a single color (black) and a single shape (spherical). The only quality that can vary from one black hole to another is mass.
The galaxy M87 with its center hosting the black hole M87*. Black holes are thousands of times less massive than galaxies, yet they produce jets that point in the same direction.
- Top right: GMVA/Lu et. al. 2023 (Nature)
- Bottom right: Event Horizon Telescope Collaboration
Credit: M87: HST/NASA
In general, the black holes we have detected are either stellar-mass black holes or supermassive black holes. Stellar-mass black holes have a mass close to that of our Sun (10
30 kg) and are about the size of a city.
Supermassive black holes are far more massive (a million times the mass of the Sun) and are approximately the size of the solar system. However, as massive as these black holes may be, they still have a relatively low mass, often well below 1% of the total stellar mass of their galaxy. They are also much smaller than their host galaxies, by a factor of about a million in radius.
An alignment on vastly different scales
In
new results we are publishing today, November 14, in Nature Astronomy, we have discovered that there is a connection between the region near the black hole and the host galaxy, as the jets emitted by the black hole are aligned with the rest of the galaxy, despite the vastly different scales involved.
Supermassive black holes are quite rare. Our galaxy, the Milky Way, hosts one (named Sagittarius A* for the constellation in which it is located) at its center. All galaxies also appear to host one (and sometimes two) supermassive black holes at their core. The centers, or nuclei, of these distant galaxies can become active as dust and gas are drawn toward the nucleus under the black hole's gravitational pull.
They don't fall in immediately because they are rotating at high speeds and form a hot disk of material called an accretion disk. This accretion disk, due to its intense magnetic field, in turn generates an overheated jet of charged particles ejected from the nucleus at very high speeds, close to the speed of light. When this phenomenon occurs, we refer to it as a quasar (quasi-stellar radiation source).
A telescope the size of the Earth
A common way of studying quasar jets is using very long baseline interferometry (VLBI). VLBI enables different radio telescopes to operate in tandem, essentially turning them into one telescope the size of the Earth. The resulting spatial resolution is far superior to what is obtained with optical or infrared telescopes.
This "giant eye" is much more effective in resolving fine details than any single telescope, which allows astronomers to observe objects and structures far smaller than those visible to the naked eye or even with an optical telescope. This is the technique used to produce
the black hole image capturing the light halo generated by the supermassive black hole hosted by the galaxy M87.
Thus, through this high-resolution approach, VLBI allows astronomers to study these jets at distances of just a few light-years or even less from their origin: the black hole. The direction of the jet on such small scales provides information about the orientation of the accretion disk and therefore potentially about the properties of the black hole itself. At present, this is the only way to obtain such data.
What about the host galaxies themselves? A galaxy is a three-dimensional object formed by hundreds of billions of stars. However, when observed (optically or in infrared), it appears to us as a two-dimensional shape, either as an ellipse or a spiral.
We can measure the shapes of these galaxies by mapping the light profile of their stars and measuring both the major and minor axes of their two-dimensional shape.
In our paper, recently published in Nature Astronomy, we compared the directions of quasar jets with the orientation of the minor axis of the galaxy's elliptical shape 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 span hundreds of thousands of light-years in width). It is astonishing that such a relatively tiny object can affect, or be affected by, its environment on such vast scales. One might expect to see a correlation between the jet and the local environment, but not with the entire galaxy.
Does this tell us something about how galaxies form? Spiral galaxies sometimes collide with other spiral galaxies, forming elliptical galaxies, which appear in the sky as ellipses. During the merger process, the quasar phenomenon is triggered in ways we do not fully understand. This is why almost all detectable jets via VLBI are hosted by elliptical galaxies.
The exact interpretation of this finding remains mysterious, but it is significant in the context of the recent discovery by the James Webb Space Telescope of very massive quasars (hosting supermassive black holes) that formed much earlier in the universe than previously expected. Clearly, our understanding of galaxy formation and the influence of black holes on this process needs updating.