The slingshot

Coming right on the heels of yesterday's post about a star so large the astrophysicists are at a loss to explain how it even exists, today we have...

... a supermassive black hole moving so fast it seems to be exceeding the escape velocity of the entire galactic cluster.

The paper about the discovery, by Yale University astronomer Pieter van Dokkum et al., appeared last week in Astrophysical Journal Letters, and its findings are hard to summarize without lapsing into superlatives.  Data from the James Webb Space Telescope identified a large, rapidly-moving object from its bow shock -- the compression waves surrounding a projectile as it moves through a medium (picture the pile-up of water and resulting waves preceding a boat as it moves across the surface of a lake).  But an analysis of this particular bow shock demonstrated something incredible; the object creating it was ten million times the mass of the Sun -- thus, a supermassive black hole -- and it was moving at an estimated three hundred kilometers a second.

For reference, this is over two hundred times faster than the muzzle velocity of a rifle bullet.

A map of the JWST data that led to the discovery [Image credit van Dokkum et al.]

Amongst the many cool things about this discovery is that there is a higher-than-expected number of very young stars in the wake of this thing.  Apparently, the compression caused by the black hole is triggering gas cloud collapse and star formation as it passes.

What could give something this massive that much momentum?  The quick answer is "no one knows for sure," but a good candidate is a galactic merger.  Two colliding galaxies represent a quantity known to astrophysicists as "a shitload of kinetic energy," and the slingshot effect -- where two moving objects pass close enough to each other that there's a transfer of momentum, causing one to slow down and the other to accelerate -- could be responsible.  It might be that this was once the black hole at the center of a galaxy, but the collision caused it to swing around an even more massive black hole from the other galaxy, resulting in its being jettisoned -- not just from the combined mass of the merger, but from the entire galactic cluster.

The question that naturally comes up is "what if it was headed toward us?"  Well, to start with, it's not; just a glance at the map of the bow shock should tell you that.  Second, its light has a red shift of 0.96, putting it at about a billion light years away, so even if it was, it wouldn't be anything you or I would have to fret about in our lifetimes.

On the other hand, what if there was a black hole like this headed our way?  Being black (as advertised), would we see it coming before the Earth was messily devoured?  The answer is "almost certainly;" not only would there be the effects of the compression waves heating up the gas ahead of it, causing it to emit radiation, there'd be the fact that massive black holes cause gravitational lensing -- they bend and distort the light of objects behind them.  If we were looking down the barrel of a black hole headed our way, we'd see this as an optical effect called an Einstein-Chwolson ring:

[Image licensed under the Creative Commons ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, G. Anselmi, T. Li, CC BY-SA IGO 3.0, Close-up of the Einstein ring around galaxy NGC 6505 ESA506346, CC BY-SA 3.0 IGO]

[Nota bene: black holes that are not moving toward us also cause gravitational lensing and Einstein-Chwolson rings; it'd be the combination of the lensing effect and the heating of the gas in front of the black hole that would tell us it was heading in our direction.]

Given astronomical distances, though, we still wouldn't have to worry about anything in our lifetimes.  It might be bad news for our possible descendants a hundred million years from now, but there are way worse problems to concern ourselves with in the interim.  And in any case, even if there was a supermassive black hole headed our way that was due to arrive either a hundred million years from now or a week from next Tuesday, there'd be absolutely nothing we could do about it.  Altering the trajectory of a something with ten million times the mass of the Sun, traveling at three hundred kilometers per second, gives new meaning to the word "unfeasible."  The only option, really, would be to stick your head between your legs and kiss your ass goodbye.

But like I said, the one van Dokkum et al. discovered isn't going to be a problem, even millions of years from now.  It's something we can goggle at from a safe distance.  A massive bullet flying through space, leaving a spangle of new stars in its wake.  Yet another example of how endlessly awe-inspiring the universe is -- and the more we find out about it, the more wonderful it gets.

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The firehose

There's some weird stuff going on with M87.

M87 is a supergiant elliptical galaxy in the constellation of Virgo.  It was discovered and catalogued in 1781 by French astronomer Charles Messier -- the "M" designation in many of the brightest nebulae and galaxies comes from their listing in the Messier catalogue -- but the telescopes of his time weren't good enough to make out much detail.  Even through better telescopes it looks like an uninteresting fuzzy blob, mostly because it's 53 million light years away.

This belies its magnitude.  It contains over a trillion stars, and is orbited by around fifteen thousand globular clusters (compare this to the Milky Way's paltry two hundred or so), and has a ginormous black hole at its center with a mass 2.4 billion times that of the Sun.  It is this black hole that you undoubtedly remember from the famous photographs in March of 2021:

[Image licensed under the Creative Commons Event Horizon Telescope, A view of the M87 supermassive black hole in polarised light, CC BY 4.0]

So this is impressive enough as is.  But then the astronomers and astrophysicists starting noticing that the black hole itself was behaving... oddly.

Three weeks ago, a team led by Yuzhu Cui of Shanghai Jiao Tong University published a paper in Nature showing that the black hole at the center of M87 was not only spinning (which isn't at all unusual; most black holes spin) but was precessing.  If you've ever played with a gyroscope, you've seen precession; get it started spinning, and for a little bit it'll stand upright, but then it starts to wobble, and its spin axis traces out a cone that gets wider and wider as the spin rate goes down because of friction.  The Earth precesses, with a period of about 26,000 years, meaning that Polaris wasn't the North Star a few thousand years ago, nor will it be a few thousand years in the future.  Twelve thousand years ago, the North Star was the bright star Vega in the constellation Lyra, made famous as the home of the benevolent aliens in the brilliant movie Contact.

[Image licensed under the Creative Commons Tauʻolunga, Precession N, CC BY-SA 2.5]

So precession of a spinning body isn't that unusual, either, but considering the angular momentum of a 2.4 billion solar mass object, it's kind of surprising that the M87 black hole is precessing fast enough to be observable from 53 million light years away.  But it is -- and its period of precession is only eleven years!

This means that the fountain of radiation and debris being shot out along its spin axis is flailing around like the jet from a loose firehose.  

Then, a new paper -- still in the preprint stages -- has added another bizarre twist.  A team of astrophysicists led by Michael M. Shara, Curator of Astrophysics for the American Museum of Natural History, has found that wherever that wildly-precessing jet nozzle is aimed, there's a higher rate of stars going nova.  Novae are explosions less violent than supernovae (those actually blow the unfortunate star to smithereens); they seem to occur mostly when white dwarf stars accrete matter from nearby dust clouds or by stealing it from a binary star partner, triggering instability and a sudden flare-up.  Here, though, the mechanism isn't understood.  Whether the jet of debris from the black hole is compressing the stars that get in the way and triggering detonation, or if it's simply that the material itself is getting caught by white dwarfs and causing the novae, isn't known.

But it's quite a mental image, isn't it?  A careening jet from a spinning supermassive black hole blasts away at stars in its path, and makes them blow up.

Leaves me feeling glad we live in the tranquil outer reaches of our own galaxy.  I know the Milky Way has its own massive black hole at the center, but out here in quiet stellar suburbia, we're pretty insulated from all that craziness.

I'm perfectly happy hearing about the wild gyrations of M87 -- from a safe vantage point 53 million light years away.

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The glowing death spiral

One of the things that always blows my mind about astronomy is how good we've gotten at using indirect evidence to figure out what's going on up there.

In a way, of course, it's all indirect, at least in the sense that everything we're seeing is (1) wicked far away, and (2) in the past.  I remember how weirded out I was when I first ran into the latter concept, back when I was maybe twelve years old.  My first inkling of it happened when I was out on a walk with my dad, and down the street there was a guy using a sledgehammer to pound in a fence post.  The strange thing was, I saw the hammer's head strike the post, and then, a second or two later I heard the bang of the strike.  I asked my dad why that was.

"Well," he said, after a moment's thought, "the sound takes a moment to get to your ears.  It's why we always see the lightning before we hear the thunder.  And the farther away it is, the longer the delay.  So as we get closer to the guy, the delay should get smaller."

Which, of course, it did.

After I'd had a minute to process that, I said, "But light takes time to get to your eyes, too.  A very short amount of time, but still, some time.  So does that mean you're not seeing things as they are, but as they were in the past?"

My dad agreed that must be so.

Upon learning some more physics, I found out that the Sun is far enough away from the Earth that it takes a bit over eight minutes for light to travel the distance in between.  So if the Sun suddenly vanished -- an unlikely eventuality, fortunately -- we not only wouldn't know it for eight minutes, there is no possible way to know it.  Einstein showed that information can't travel any faster than the speed of light -- it really is the ultimate speed limit.

The nearest star, Proxima Centauri, is 4.25 light years, so we're seeing it as it was 4.25 years ago, and have no way of seeing what it looks like right now.  Given that it seems to be a fairly stable star, it probably looks much the same; but the fact remains that we can't know what its current appearance is.  The most distant objects we've seen through our most powerful telescopes are some of the quasars, at thirteen billion light years distant (and thus, what they looked like thirteen billion years ago).  So what those quasars look like right now -- where they are, if they even exist any more -- is impossible to know.  We're seeing them as they looked shortly after the universe began; what they are today is anyone's guess.

Impressively far away, but at least still in our own galaxy, is Sagittarius A*, the supermassive black hole at the center of the Milky Way.  It's 26,000 light years distant.  But despite how far away it is -- and the fact that massive dust clouds lie between it and us, obscuring what light it does emit -- we've been able to find out an astonishing amount about it.

Sagittarius A*, as imaged by NASA's Chandra X-Ray Observatory (Image is in the Public Domain]

This, in fact, is why the topic comes up today -- some research out of the Université de Strasbourg that found evidence of a sudden flare-up of Sagittarius A*, around two hundred years ago.  For such a behemoth, it's been relatively quiet since its discovery in 1990.  But astrophysicist Frédéric Marin has found a cosmic glow that resulted from a brief, powerful flare of x-rays, during which Sagittarius A* was radiating a million times brighter than it is now.

The x-rays caused the clouds of dust surrounding the black hole to fluoresce; from the distance of those clouds from the event horizon of the black hole, Marin and his team determined that they must have been hit by a strong blast of x-rays about two hundred years ago.  (Keep in mind that because of the time-lag effect I described earlier, these times are all as seen from Earth; the actual flare-up occurred 26,200 years ago, or thereabouts.)

What caused the burst isn't known, but is surmised to be the sudden swallowing by the black hole of a denser blob of cosmic dust and gas.  As material goes into a death spiral toward the event horizon of a black hole, it speeds up, and electrons are stripped from atoms, leaving a whirling funnel cloud of charged particles.  These particles radiate away some of that energy in the form of x-rays -- the "smoking gun" that allows us to see black holes, which otherwise would be entirely invisible.

If you get a little nervous about such astronomical violence, there's no cause for alarm; neither Sagittarius A* nor any of its radiation blasts pose any sort of danger to us.  We'd only be in trouble if we were a great deal closer to the galactic center.

So we can just sit back and appreciate the amazing capacity the astrophysicists have for sifting through data and painting us a picture of what the universe looks like.  In this case, the last blaze of glory for a dust cloud that got sucked into a supermassive black hole 26,000 light years away.

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