Why Black Holes Barf Scorching Winds of High-Energy X-Rays
Astronomers are only now getting a handle on one of the driving forces behind galactic growth.
As a black hole chews up its surroundings, inhaling the universe itself into a unknowable, infinite point of space-time, the process is hardly one-way. Black holes give back, in a sense. As a black hole might form the gravitational core of a galaxy—a foundation of sorts—it also shapes that galaxy through violent blasts of cosmic wind. In the early microquasar manifestation of a black hole, the intensity of those winds almost matches the energy being sucked away, offering a turbulent near-equilibrium.
As detailed in a study this week in Science, researchers have for the first time measured the strength of outgoing black hole winds, demonstrating that, indeed, they can pack enough force to shape the fates of galaxies. With help from the Nuclear Spectroscopic Telescope Array (NuSTAR), the astronomers were able to examine one especially luminous black hole in particular, known as PDS 456.
What they found at PDS 456, which lives about 2 billion light-years from Earth, were X-ray winds blasting outward at nearly one-third the speed of light, delivering more energy every second that that emitted by one trillion of our Suns. This is more than enough energy needed for a black hole to have a profound influence on the evolution of its surrounding galaxy. Which is where the black hole finds its raw materials in the very first place. It's a fascinating dynamic that's only now being understood.
The process at work is known as accretion. As a black hole or really any very massive cosmic structure draws in gas and other material from its surroundings, the result is an inward spiral or disk. The closer that spiral gets to the gravitational source, the more extreme things become: more motion, more friction, more heat. As this collected material become more and more compressed, energy is released as electromagnetic radiation. The results are jets shooting outwards at right angles to the accretion disk, which you can see in the accompanying illustration. Not much black about it.
This process of matter-into-energy conversion is among the most efficient in the universe, achieving something like 40 percent conversion, compared with .7 percent in nuclear fusion. It's now thought that this radiation is what makes quasars, the superdense, superbright regions found at the cores of young "active" galaxies—usually surrounding a black hole nucleus—so bright. For perspective: The largest known quasar kicks out the energy of more than a thousand Milky Ways, while consuming the equivalent of about 600 Earths per minute.
It's only been in the past few years that astronomers have been able to characterize (or start to, anyway) these jets of X-ray wind. A major study last year found that at least some portion of this wind is able to defy what's known as the Eddington limit, a theorized boundary relating the mass of a black hole to its maximum luminosity. It turns out that these jets are able to surpass this limit several times over, as they're so concentrated and so energetic that they can pierce the bubbles of gas and other material that collects around a black hole like a bullet through paper.
The effect of these winds is hardly trivial. A supermassive black hole might act as the gravitational glue holding a galaxy together, but, at the same time, the resulting winds blast galactic material deep into space. Less material, less growth potential. It's sort of a catch-22: the larger the black hole, the more gravitational attraction, but, at the same time, there's more outward energy acting as a cosmic leafblower.
"A fundamental correlation exists between the mass of the central black holes and the stellar velocity dispersion of galactic bulges," writes Durham University astronomer Martin Ward and his group in the current study. "That is, hundreds of times beyond the gravitational sphere of influence of the black holes themselves."
"PDS 456 shows a flavor of cosmic feedback, believed to have operated at the peak of the quasar epoch about 10 billion years ago," Ward and co. write. "In distant galaxies in a similar activity phase, such powerful winds would have provided the energy and momentum to self-regulate the black hole growth and control the star formation in stellar bulges."