If black holes are so bad at growing, how do we get supermassive black holes?

If supermassive black holes are so bad at feeding from the stars around them, how did they get so big?
This illustration shows a glowing stream of material from a star, disrupted as it was being devoured by a supermassive black hole. The feeding black hole is surrounded by a ring of dust.   Image credit:  NASA/JPL-Caltech

This illustration shows a glowing stream of material from a star, disrupted as it was being devoured by a supermassive black hole. The feeding black hole is surrounded by a ring of dust. Image credit: NASA/JPL-Caltech

This is such a good question that the answer is not particularly settled yet - we have some ideas for how some supermassive black holes may have gotten so large, but it’s not clear that the explanations we’ve come up with so far hold true for all supermassive black holes.

So a few definitions before we launch into our ideas - by and large, all massive galaxies have a gigantic black hole at their centers. To distinguish these central black holes from the type of black hole created when a single, massive star ends its life, we named the big ones “supermassive”. The black holes created from single stars are less entertainingly named “stellar-mass black holes”.

This artist concept of the local galaxy Arp 220, captured by the Hubble Space Telescope,  shows the bright core of the galaxy, paired with an overlaid artist's impression of jets emanating from it, to indicate that the central black hole's activity is intensifying.  Image Credit:  NASA/JPL-Caltech

This artist concept of the local galaxy Arp 220, captured by the Hubble Space Telescope,  shows the bright core of the galaxy, paired with an overlaid artist's impression of jets emanating from it, to indicate that the central black hole's activity is intensifying. Image Credit: NASA/JPL-Caltech

Black holes of all sizes are extremely inefficient at gathering new material to themselves. Rather than absorbing nearby material in one fell swoop, the material is more often than not pulled into a tight disk of extremely hot material, or flung away from the black hole entirely, sometimes at relativistic speeds. For all black holes have a reputation of being cosmic garbage disposals, if you had a garbage disposal this terrible in your own kitchen, after the first time it blasted superheated onion bits on your ceiling you’d call a professional to have it removed ASAP.

So it is an eminently reasonable question - how do you make a black hole that’s thousands of times larger than the ones that are made by stars, when they’re exceptionally bad at growth? This question is made more complicated because right now, getting a star to explode in a supernova, and the remnant collapsing down into a black hole is the most robust theoretical model we have to make a black hole, so you have to somehow bridge the gap between something that contains a few times the mass of the Sun and something that contains thousands of times the mass of the Sun, while that thing steadfastly refuses to grow rapidly by accreting new material onto itself.

What can we do? Well, we can either 1) start larger, or 2) grow differently. If you start larger, then you have the benefit of not having to grow by a factor of several thousand but only by a couple. This method means you have to start with “seed black holes” very early in the universe. You can do this in one of two ways. First is the direct collapse model - the thinking goes that it might have been possible, very early in the universe for enough gas to collect together that its gravity would just collapse all the way down to a black hole, skipping the star phase entirely. The second method is effectively to go through a star first, but to go through a very, very large star - something much larger than the Universe makes nowadays, which would burn through its hydrogen much faster, and would make a larger black hole as a remnant.

Artist's illustration of supermassive black hole pair.Image credit:  NASA/CXC/A.Hobart

Artist's illustration of supermassive black hole pair.Image credit: NASA/CXC/A.Hobart

What about growing differently? It’s possible that instead of growing by slurping tiny fractions of gas and dust from its surroundings, the black holes grow by absorbing other black holes. This raises a whole string of other questions, like “are there even enough black holes around for that to work?”. We know that there should be times when galaxies gain a second supermassive black hole - during galaxy collisions. The remnant of the two galaxies should have two black holes which sink together, immediately doubling their mass. But it’s unclear if you can guarantee that enough galaxies will smash together enough times for this to work for all the galaxies we see, especially since not all galaxies are expected to merge with another galaxy the same number of times.

There’s no reason the answer will wind up being one or the other - some combination is likely to be in play. If you can start with a larger seed in the early Universe, you can grow more easily through a combination of colliding with the supermassive black holes in other galaxies, and by gathering gas inefficiently to themselves.


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What's in a Jet from a Black Hole?

Originally posted at Forbes!

As far as we can tell, it’s mostly electrons, neutrons, and protons getting flung out into these jets. In the end, it turns out that the electrons are the key factor in making the jets so visible to our telescopes.

The jets themselves are interesting objects. Not every galaxy’s black hole produces a jet, even though all sizable galaxies have black holes. It seems that in order to produce this jet, the supermassive black hole in the very center of the galaxy has to be actively trying to gather new material into itself. Supermassive black holes are hilariously inefficient at growing larger, even when there’s material around for it to work with. Our Milky Way’s supermassive black hole isn’t growing at the moment because there’s no material nearby, but even if there were a lot more gas and dust very near the black hole, the black hole wouldn’t be able to grow very fast.

Part of the reason a black hole doesn’t grow very fast is that the material orbiting a black hole has to continue to lose energy to keep falling into the black hole, and that process of energy loss is driven by inefficient things like friction and heat. On top of all of this, there are probably crazy things happening with the magnetic fields within the rapidly rotating material around the black hole. Magnetic fields are a bit of a bugbear for studies of galaxies – we know that there are magnetic fields around, but we’re not quite sure how much of an effect they have on the galaxy, and they’re stupendously difficult to model correctly.

False-colour X-ray image of the giant elliptical active galaxy Centaurus A (NGC 5128) taken with the orbiting Chandra X-ray Observatory, featuring its 30,000 light-years long jet. Credit: NASA/SAO/R.Kraft et al.

False-colour X-ray image of the giant elliptical active galaxy Centaurus A (NGC 5128) taken with the orbiting Chandra X-ray Observatory, featuring its 30,000 light-years long jet. Credit: NASA/SAO/R.Kraft et al.

In the case of jets, we know that there must be strong magnetic fields, because we observe a type of glow that only happens if you have both very rapidly moving electrons and a magnetic field. It’s called synchrotron radiation, and its happens when you get a relativistic electron (which means that it’s moving at a significant fraction of the speed of light) caught in a twisting orbit around a magnetic field line. The electron moves in a helix around the magnetic field line, and emits light that we can observe with a wide range of different telescopes.

This VLA radio composite image shows the active galaxy 3C 348, also known as Hercules A. The VLA data, which record frequencies from 4-9 GHz, were taken in 2010-2011. Image Credit: R. Perley and W. Cotton (NRAO/AUI/NSF)

This VLA radio composite image shows the active galaxy 3C 348, also known as Hercules A. The VLA data, which record frequencies from 4-9 GHz, were taken in 2010-2011. Image Credit: R. Perley and W. Cotton (NRAO/AUI/NSF)

It’s usually this synchrotron radiation that we see in the images of jets coming from a supermassive black hole. If you’re looking at a radio image or an optical image, what you’re looking at is the glowing byproducts of nearly speed-of-light electrons bending under the influence of a magnetic field.

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Can there be more than one black hole in a galaxy?

This artist's conception illustrates one of the most primitive supermassive black holes known (central black dot) at the core of a young, star-rich galaxy. Astronomers using NASA's Spitzer Space Telescope have uncovered two of these early objects, dating back to about 13 billion years ago.  This illustration also shows how supermassive black holes can distort space and light around them (see warped stars behind black hole). Stars from the galaxy can be seen sprinkled throughout, and distant mergers between other galaxies are illustrated in the background.  Image credit: NASA/JPL-Caltech

This artist's conception illustrates one of the most primitive supermassive black holes known (central black dot) at the core of a young, star-rich galaxy. Astronomers using NASA's Spitzer Space Telescope have uncovered two of these early objects, dating back to about 13 billion years ago.  This illustration also shows how supermassive black holes can distort space and light around them (see warped stars behind black hole). Stars from the galaxy can be seen sprinkled throughout, and distant mergers between other galaxies are illustrated in the background. Image credit: NASA/JPL-Caltech

Sure!  But it depends on what type of black hole.  There are two main flavors of black hole: supermassive black holes, and stellar mass black holes.  As their names indicate, stellar mass black holes are about the mass of a single star, and supermassive black holes are super massive.

By supermassive, we mean that they’re at least a million suns worth of mass crammed into a very tiny space. (For this kind of thing, astronomers have defined the handy unit of the solar mass: this is the mass of our sun.) They can go up to solar masses of several billion (several billion times the mass of the sun). These are usually the types of black holes people think about when they’re talking about galaxies, and the center of each galaxy should have exactly one of these. If we find a galaxy with more than one supermassive black hole in its center, we have found ourself a very unusual galaxy, which has very likely just devoured another galaxy about as large as itself. The two black holes will eventually fall together and combine to make one, larger, black hole, as the rest of the galaxy recovers from the consumption of another galaxy. We think that supermassive black holes got to be supermassive by gradually going through this process of munching on smaller galaxies and absorbing their black holes over the course of the universe’s lifetime.

Stellar mass black holes, on the other hand, are about as common as dirt. You get a stellar mass black hole any time a large star (usually more than about 8 solar masses) reaches the end of its life and dies in a spectacular supernova fashion. The mass of the star plus the energy of the supernova compresses the remaining star matter down past the density you would need to make a white dwarf or neutron star, and you’ve got yourself a new stellar mass black hole. There are tons of these black holes in the galaxy, but since they’re only as massive as the star that they formed from, they don’t have nearly as big an effect on their surroundings as the supermassive ones do. This makes them much harder to find, so we’re still getting a handle on exactly how many there should be in the galaxy, but the number can be safely rounded to “a lot.”

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