Bad Astronomy

Hubble catches a jet collision

The Milky Way – our home galaxy – is pretty sleepy, as galaxies go. While bigger than most, it’s a good neighbor, generally behaving itself and keeping the noise down.

M87, on the other hand is a galactic frat house.

Deep in the heart of every big galaxy is a supermassive black hole. The Milky Way is no exception, but our black hole is quiet. The one in the core of M87, though is actively feeding. As material swirls around the Point of No Return, magnetic forces align to channel out twin beams or jets of energy and matter that scream out from just above and below the black hole at nearly the speed of light.

Bad neighbor indeed. At 50+ million light years distant it’s no danger to us, though it does keep astronomers up at night: our view of M87’s jet is pretty clear, and when it hiccups, that can be a mighty eructation! Behold:

What you are seeing are ultraviolet images from Hubble of the heart of M87. The bright spot on the left is the core of the galaxy where the black hole sits, and you can see it stays about the same brightness over time. The jet is blasting out to the upper right. The jet isn’t smooth; it’s clumpy, presumably due to past eruptions from the black hole as more matter tries to choke down the drain.

Optical image of the jet in M87
Optical long view of M87’s jet. The black hole
and the HST-1 blob seen in the Hubble image
above are seen here as a single elongated
blob on the left.
The inner clump or knot is called HST-1. This time sequence shows it changing in brightness from 1999 to 2006. It brightens and fades with time, but in 2005 it suddenly increased in brightness so much that it actually blasted out 90 times the energy as its parent black hole!

What could cause such a horrendous explosion of luminosity? No one knows. Maybe a wandering cloud of gas drifted into the jet and got slammed by the relativistically-moving particles, heating up and pouring out high-energy light. Or perhaps the magnetic fields that focus the jet somehow got compressed, squeezing the material in the beam and causing it to glow ever-brighter. It’s hard to say.

But this tremendous surge in energy has some interesting implications for astronomers who study galactic shenanigans. We call galaxies like M87 active for obvious reasons. There are different kinds of active galaxies, and the most energetic of them are called blazars. We think these are galaxies where we are looking straight down the throat of the jet. We see hugely energetic gamma rays from blazars as well as light from across the spectrum. The gamma rays are so tightly focused that if the beam is aimed even slightly away from us we don’t see them.

Blazars are known for being highly variable in their light, fluctuating by a factor of ten or more in brightness over time. It’s always been thought that these changes in brightness are tied to the way the black hole is feeding; if a big ol’ clump of gas suddenly gets caught up near the hole, the jet gets a surge in energy. But now we have reason to question this! The knot that suddenly brightened so much in M87’s jet was a long way from the black hole, far outside the maw of the beast – a million times farther out than where we think the big energy changes happen near the black hole. So it’s possible, even likely, that there are at least two ways a blazar (or any active galaxy) can suddenly blast out huge explosions of energy.

Science is like that. It’s a bit ironic, I suppose: active galaxies caused astronomers headaches for decades. They were all so different from each other! Then it dawned on folks that maybe what we were seeing was the same phenomenon, but seen from different angles (rather like the parable of the blind men and the elephant). It unified the models, and things got a lot simpler. Yay!

And now we see once again that Nature is cleverer than we are, still able to throw a monkey in our wrench. We assumed jets were variable because their engine – the black hole – was eating matter at different rates, but now we see that something else can happen far away from that central engine, and whatever this process is it can dramatically change the appearance of the jet.

I personally like it when things like this crop up. While it can make analysis harder, it makes life more interesting! There are always more mysteries to solve, more puzzles to piece together. In this case we didn’t even know there were more pieces at all!

And so the game goes on.

Credit: NASA, ESA, and J. Madrid (McMaster University)