When Quasars Collide (Script) (Patreon)

When two astrophysical objects go up against each other, one usually comes out on top. Red giant stars incinerate planetary systems, neutron stars cannibalize their red giant neighbors, and stellar-mass black holes rip neutron stars to shreds. But supermassive black holes eat all of the above for breakfast. So what happens when two SMBHs tango? We may be about to find out, because astronomers have spotted a pair of them in a close binary orbit for the very first time.

Today on Space Time Journal Club we’re going to dig into a paper that reports the detection of a pair of supermassive black holes orbiting only one light year apart from each other. We’ve never seen such a system this close to each other. This is extremely cool because we knew for a long time that such tight binaries must exist, but it’s taken until now to spot one. Studying the dance of these giants should tell us a ton about how black holes grow. This paper was just published in Nature Astronomy by Preeti Kharb and Dharam Vir Lal from India’s National Center for Radio Astrophysics, and David Merritt from the Rochester Institute of Technology.

Before we get to this new result, let’s talk about supermassive black holes - SMBHs. These things live in the dead centers of pretty much every decent-sized galaxy. We’ve already talked about the black holes that form in the deaths of massive stars; they start with masses of up to 10 or so Suns. The ones in the cores of galaxies contain the mass of a million to a billion Suns. The largest have event horizons that would envelop most of our solar system. We’re still figuring out how supermassive black holes got so big. Did they get most of their mass from eating gas and stars from their surrounding galaxy? Or do they mostly grow when smaller SMBHs find each other and merge during galaxy collisions?

We know a lot more about the first process because we’ve been watching SMBHs munching on their host galaxies for half a century now. This is what causes the quasar phenomenon. On the other hand, this new observation may turn out to be a pair of supermassive black holes as close to merger as we’ve ever witnessed. If so, it’s incredibly important for understanding that whole aspect of black hole growth. Actually, let me say a bit more on quasars, We talk about them in more detail here, but the TL;DR: when gas from the surrounding galaxy falls into and feeds the central SMBH you get an “active galactic nucleus” - AGN. “Quasar” is the term for the most powerful AGNs, and they contain SMBHs with up to billions of Suns in mass. But lower down on the power scale we have Seyfert galaxies, which typically containing a single SMBH weighing in at millions of solar masses. The purported binary black holes in this new study were found in a known Seyfert galaxy. That means they’re feeding on their surrounding galaxy AND approaching merger, so we get to see everything happening at once.

Let’s talk a bit about how these binary black holes were found, because it wasn’t easy. The Seyfert galaxy in question is Markarian 533, which is around 400 million light years away. The black holes are around one light year apart in the center of the galaxy. In order to measure such a small separation at such a large distance we need resolution around 100 times better than the Hubble Space Telescope. Preeti Kharb and collaborators achieve this using a technique called very long baseline interferometry. In short: the target is observed with radio telescopes on opposite sides of the planet, and phase differences in the incoming radio waves are used to find the origin of each wave with incredible accuracy. In fact the spatial resolution is equivalent to what you would get with a telescope equal in size to the separation of the radio antennae. In this case the Very Long Baseline Array – VLBA – was used, and its antennae span Hawaii to the US Virgin Islands and through the continental United States, to give a baseline of over 8000 km.

Here’s the radio map at 15 GHz frequency. Those two hotspots are the locations of the possible black holes. Black holes themselves are invisible, so what we’re actually seeing here is radio emission from jets. Let’s talk about AGN jets for a minute. When a black hole feeds, the vortex of in-falling plasma – the accretion disk – can produce a powerful magnetic field. That field can accelerate narrow streams of high-energy particles away from the black hole. Those jets can blast through the surrounding galaxy and beyond, carrying their magnetic fields with them. The radio light seen here is from electrons spiraling in those magnetic fields – so-called synchrotron radiation.

This map alone doesn’t tell us that there are two black holes. We frequently see separate knots of radio light in AGN jets, which can splutter as their fuel supply changes, or as the jets smash into denser regions of the surrounding galaxy. In fact we see such hotspots in Markarian 533 when we look at a much larger map. Here we can see two bright spots far from the black holes, presumably from a burst of jet activity some time ago. So how do we know that the hotspots in the core are from two unique black holes, instead of a lumpy jet from one black hole? The researchers tested this by looking at multiple frequencies to get a crude radio spectrum. Typically, knots and lumps in a jet have pretty even energy distributions – spiraling electrons produce radio waves at lots of frequencies, all the way down to very low energies.

But right down where the jet begins the matter is so dense that the lowest energy radio waves have trouble escaping the jet. This is a process called synchrotron self-absorption and it causes the base of AGN jets to be much fainter at long wavelengths. That’s exactly what is seen here. Both knots have the classic energy distribution of completely independent jet launch points. The extreme energy densities or “brightness temperatures” are also what you’d expect from the bases of two distinct jets. The only way this is possible is with two separate black holes, each one powering its own mini-quasar.

OK, let’s assume the researchers are right and we’ve spotted supermassive black holes in a tight binary dance. How did this happen? And when will they merge? Like I said earlier, we already knew that this sort of thing must happen. Galaxies grow by merging with other galaxies. The SMBHs of these galaxies must eventually fall towards the new merged galactic core. This happens through a process called dynamical friction – basically, the black holes slingshot stars outwards through gravitational interactions, and each time they do that they loose a bit of orbital energy, causing them to fall deeper into the gravitational well. You can think of it as a sort of gravitational friction dragging the black holes downwards and towards each other. However by the time the black holes are a few light years apart, there shouldn’t be any stars left in between them. That means they stall and fall into a stable binary orbit around each other.

In fact we don’t know how supermassive black holes merge once they’re within around one parsec of each other, or a few light years. This is called “the central parsec problem”. We know they must merge, we just don’t know how. One possibility is that gas can provide the needed friction. The newly discovered binary definitely has a reservoir of gas – that’s how it powers its jets – so perhaps it’ll give us the answer.

A lot of you are probably thinking “what about gravitational waves?” Can’t gravitational radiation cause supermassive black holes to merge, just like it does with regular stellar-mass black holes? And can LIGO see those waves? The answer is no, and no. Oh, this system is definitely producing gravitational waves, but it’s going to take many billions of years to lose enough angular momentum to merge that way. And while those waves may be powerful, they have an incredibly low frequency – something like one-ten-trillionth of a Hertz. LIGO is sensitive to gravitational waves from 10 to 10,000 Hertz. This binary is just too huge and slow to register with LIGO. There may be ways to detect the actual merger of supermassive black holes – with a galaxy-sized gravitational wave observatory called a pulsar timing array. More on that another time.

For Markarian 533 we’re going to have to stick to traditional observing methods. Longer exposure radio observations will pin down the energy distribution to confirm whether these really are jets produced by two black holes. This galaxy is so dusty that it’s hard to peer into the core at other wavelengths of light. However careful observations of the stars in the galaxy can help us figure out the masses of the black holes and look for signs of galaxy mergers. And if this binary SMBH is the real thing, then it’s certainly not the only one. This finding will inspire astronomers to search for more of these dancing giants, leading us closer to understanding the incredible growth of the largest black holes in all of Space Time