A new analysis of radio telescope data links the shapes of galaxies to the black holes they house. 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, however massive these black holes may be, they still have a fairly low mass, often well below 1% of the total stellar mass of its galaxy. They are also much smaller than their host galaxies, on the order of a million times their radius.
Alignment on very different scales
In new results published 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 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 lies) at its center. All galaxies also seem to have 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 towards the core by the black hole’s gravitational attraction.
They don’t fall out immediately, however, as they rotate rapidly, forming a hot disk of matter known as an accretion disk. This accretion disk, due to its intense magnetic field, in turn generates a superheated jet of charged particles that are ejected from the core at very high speeds, close to the speed of light. When this happens, we speak of a quasar (a source of quasi-stellar radiation).
A telescope the size of the Earth
A common way to study quasar jets is to use Very Long Baseline Interferometry (VLBI). VLBI allows different radio telescopes to operate in tandem, transforming them into a single Earth-sized telescope. Spatial resolution is far superior to that achieved with optical or infrared telescopes.
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This “massive eye” is far more effective at resolving fine detail than any individual telescope, enabling astronomers to see objects and structures far smaller than those visible to the naked eye, or even with an optical telescope. This is the technique used to create the “Black Hole Image” for the halo of light generated by the supermassive black hole hosted by galaxy M87.
Thanks to this high-resolution approach, the VLBI enables astronomers to study these jets to within a few light-years of their origin : the black hole. The direction of the jet at such small scales informs us about the orientation of the accretion disk, and therefore potentially about 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 profile of starlight, and measure the major and minor axes of the two-dimensional shape.
In our paper, recently published in Nature Astronomy, we compared the direction of the 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’re measuring is only a few light-years long) compared to the host galaxy (which can be hundreds of thousands of light-years across). It’s surprising that such a small object (by comparison) can affect, or be affected by, the environment on such large scales. We 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 to form elliptical galaxies, which appear in the sky as ellipses. During the merging process, the phenomenon of quasars is triggered in a way we don’t fully understand. This is why almost all the jets that can be detected with the VLBI are hosted in elliptical galaxies.
The exact interpretation of the result remains a mystery, but is important in the context of the recent discovery by the James Webb Space Telescope of very massive quasars (with supermassive black holes), which formed much earlier in the universe than previously thought. Clearly, our understanding of galaxy formation and the influence of black holes on this phenomenon needs updating.
This article is republished from The Conversation under a Creative Commons license. Read the original article.