Huge Jets into the Cosmic Web
In previous posts, I’ve written about supermassive black holes, the colossal objects in the centres of galaxies that can be millions or even billions of times the mass of our Sun. These critters are actually physically very small, extraordinary amounts of material crushed into an infinitesimal volume smaller than a pinprick. Depending on their mass though, black holes have a larger boundary called the “event horizon” which is the maximum distance from the black hole within which nothing can escape. The biggest event horizon I can think of is the one surrounding the multi-billion solar mass black hole in the centre of the Messier 87 galaxy, which amazingly is only of comparable size to our solar system. Black holes, however, are also surrounded by disks of hot material that is spiraling into its doom. Likely due to a complex interaction between the rotation of that material and the magnetic fields threading the disk, some fraction of that material can be expelled perpendicular to the disk at fantastically high speed. These jets of material, moving at an appreciable fraction of the speed of light, can blast through the gas surrounding the central regions of galaxies, and extend well beyond the galaxies themselves. Indeed, these plumes of fast moving gas were one of the most surprising discoveries of radio astronomy, that many innocuous-looking galaxies had pairs of lobes of radio emission extending hundreds of thousands of lightyears beyond and sometimes billowing out to diffuse features many times larger than the parent galaxy.
In today’s news, a surprising new radio lobe has been discovered by a Dutch graduate student using the LOFAR radio observatory in the Netherlands, which is the largest yet detected. You can see in the scale of these radio lobes in attached image, where the thin jet associated with the galaxy in the centre is clearly related to the puffy lobes further out. (The radio emission comes from electrons zipping around magnetic field lines.) For a sense of scale, the size of the lobes is a whopping 15 million light years long, and since we don’t know by how much the jet itself is tilted toward or away from us, the true length is probably even longer. The result actually has some interesting connotations about how large-scale structure in the universe is maintained. The galaxy in question resides within a filament of the so-called “Cosmic Web” of dark and regular matter that arose as the universe evolved and cooled, and led to the formation of galaxies. Gravity should collapse the filaments of the web but the outward pressure its constituent material will counteract it. How such material is kept “warm” and able to provide a reasonable amount of pressure has been something of a mystery, but this result shows that jets from supermassive black holes can extend well into their surrounding host filaments, providing the kind of energy transfer needed to support them. It’s rather amazing that something so “small” like a supermassive black hole, which in the attached image is much smaller than the dot in the very centre, can be responsible for producing something so humungous like a radio jet, which then in turn provides the energy needed to maintain structures on scales even larger than those shown in the image. Whew! Here’s a link to the arXiv preprint for those who’d like to read up more on this interesting result. (The paper will be soon published in Astronomy & Astrophysics.)
