M87 is a monster galaxy, the nearest giant elliptical to the Milky Way and also the nearest “active galaxy”: a galaxy whose nucleus appears to be superbright, emitting vast amounts of matter and radiation.
These AGs baffled astronomers for a long time, but now we understand the general scenario that leads to these galaxies having active nuclei. Every major galaxy has a supermassive black hole in its core, and as matter falls in it forms a flattened disk, called the accretion disk. The inner part of the disk is incredibly hot, and a witch’s brew of forces operates there. They combine to focus a titanic jet of matter and energy that screams out from the vicinity of the black hole.
M87 has a jet. A big one!
This newly released image was taken in the radio part of the spectrum using a technique called interferometry, where the abilities of widely-separated telescopes can be combined. In this case, ‘scopes from around the planet were used to make a virtual ‘scope with an effective aperture the size of the Earth. It’s called the Very Long Baseline Array, or VLBA. In this image, the resolution is incredible; objects only one milliarcsecond can be separated. That’s like reading the letters on a coin located 200 kilometers away!
Usually, AGs emit two jets, one in each direction. However, M87’s jet is aimed
almost directly more or less at us. That makes the counterjet difficult to see. Weirdly, relativity effects add in, making the counterjet almost invisible. It is just barely detectable in this observation.
Interestingly, the researchers who took these observations found that the jet is moving at only a few percent the speed of light, much slower than previously thought. This means that the relativistic effects aren’t particularly strong… so why is the counterjet so faint? Good question, and the answer isn’t obvious. Maybe the counterjet is intrinsically faint, or there isn’t as much material in it. This doesn’t throw all the old theories out the window – there are other explanations – but it does mean that we have a lot to learn about the inner workings of these complex beasts, and the galaxies which spawn them.
Speaking of which, M87 is amazing. It’s about 50 million light years away, and sits in the center of the Virgo cluster, a collection of many thousands of galaxies. Even at that distance it’s bright enough to see with small binoculars. M87 is so big because it probably ate lots of smaller galaxies, slowly growing in the process. The central black hole in the galaxy is about 3 billion times the mass of the Sun, almost a thousand times the mass of the Milky Way’s central black hole.
Years ago I worked on a series of Hubble ultraviolet images of the jet – there is so much energy in the jet we thought there might be antimatter in it, which would create a weird molecule called positronium which emits in the UV. We didn’t find any (we didn’t really expect to, but it was an interesting observation, and I was able to use it to map how the UV detectors behaved at different temperatures), but working on that data gave me a chance to study this very cool galaxy. Now when I see it though my own telescope, I have a little more appreciation for what looks like a simple fuzzy patch of light.