Have you ever heard of an object called ‘Hanny’s Voorwerp?’ It’s a thin wispy ghost-like blob at the edge of a Galaxy. It was discovered by Dutch schoolteacher Hanny van Arkel in 2007 as she was classifying galaxies as part of the Galaxy Zoo project. Since then, astronomers have been studying its origin, as it was the first of a brand new phenomenon in astronomy.
This past week, a new data set of wispy trails at the edge of Galaxies have been released as part of a Hubble study by Bill Keel of the University of Alabama, Tuscaloosa. The new Hubble images show several wispy structures at the edge of galaxies with striking similarities to Hanny’s Voorwerp.
To understand what is causing the ghost-like features to appear, we have to look at the host Galaxies, and one striking property that they share: They all host Active Galactic Nuclei (AGN). An AGN is a central region in a Galaxy that has a much higher than normal luminosity over the electromagnetic spectrum, generally due to a super massive black hole that is rapidly accreting material or two merging black holes rapidly orbiting one another. When they become highly energetic, we call them Quasars, the most luminous type of AGN.
The clouds are glowing because of a process called photoionization. Oxygen in the clouds is being ionized by intense UV light from the active nucleus of the host galaxy. when the ions recombine to form new Oxygen atoms over thousands of years, they emit infrared radiation that we can see from Earth. But here’s the interesting part: Looking at the host galaxies, the AGN isn’t currently bright enough to account for what we are seeing. This means that we are likely seeing an artefact of the past. The associated AGN recently flared up, producing huge amounts of UV light, which took thousands of years to reach the distant wispy gas clouds where it triggered photoionization, and then the slow recombination of ions produced the green glow over many more millennia.
One possible explanation for the brightening and dimming of the AGN is if the Galaxy contains orbiting black holes. At certain points in the orbit, the flow of material into the black holes would vary, and more energy could be released in intervals lasting a few thousand years as the black holes move closer to one another, interact, and then swing further away again. This would also explain the other interesting part, that the wisps of gas are actually outside the host galaxy. It means that sometime in the past, the host galaxies were part of a merger with a smaller gas-rich galaxy which contained a central black hole. As the two merged, streaks of gas from the smaller galaxy were tossed outward by the centripetal force as the smaller galaxy swung into the larger one. As the smaller galaxy was absorbed and the central black hole moved inward, the gaseous streaks remained on the outskirts. And finally, when the smaller galaxy’s black hole began closely interacting with that of the larger galaxy, the increased radiation sparked photoionization in the leftover gaseous wisps.
By looking at the spectroscopic make-up of the wispy clouds, we find the elements Oxygen, Hydrogen, Helium, Nitrogen, Sulphur, and Neon. The merging galaxies had to have been very rich in gas in order to produce the observed wisps. The same kind of photoionization happens around newly formed O and B stars along the spiral arms of galaxies, causing them to glow in infrared. This phenomenon can be seen in nebulae in our own Galaxy, such as Messier 42, the Orion nebula.