The Hubble Space Telescope has captured new images of the strange cosmic ghosts lurking at the outside edges of some distant galaxies—dubbed "green goblins." These are twisted, gnarled clouds of energized material tens of thousands of light-years long, deformed by the immense gravitational forces of the nearby galaxy. The most likely explanation is itself unlikely: These ethereal phantoms are the result of rapidly changing or dissipating quasars, which is something that quasars aren't supposed to do.
To understand, we need to look into the very heart of a quasar, which is a black hole.
The most powerful black holes are hardly black at all. Instead, they come under cover of the brightest, most intense light sources in the universe: quasars. These are the eruptions of electromagnetic energy (as both x-rays and visible light) that result as infalling material around a black hole comes together in an extended energetic crunch, the result of intense friction and gravitational stresses.
A single quasar might emit energy thousands of times more powerful than the entire Milky Way, which consists of many billions of stars. Yet that same quasar might be as small as our own solar system, itself barely a grain of a grain of sand compared to its host galaxy. The energy density is baffling, like a star on the head of a pin.
Quasars are, however, fleeting—features of young, active galaxies. If they weren't fleeting, quasars would be all we could see of space. But it's the suddenness of the change required to create these quasar phantoms that's unexpected.
One explanation, according to Hubble astronomer Bill Keel, is that the phantoms are the result of quasars surrounding co-orbiting black holes, which could produce relatively quick changes and act is a sort of cosmic quasar dimmer switch. This arrangement might occur as the result of a galactic collision.
The phantoms are likely made up of gases that originated on the outside of the colliding galaxies in question. They would have become green goblins as oxygen atoms within the clouds absorbed light from the nearby quasar through the process of photoionization. The clouds then re-emit the radiation in turn, but over the course of many thousands of years, acting as sort of quasar documentarians.
"We see these twisting dust lanes connecting to the gas, and there's a mathematical model for how that material wraps around in the galaxy," Keel explains. "Potentially, you can say we're seeing it 1.5 billion years after a smaller gas-rich galaxy fell into a bigger galaxy."
The green goblins were originally found 2007 by Dutch schoolteacher Hanny van Arke, who was taking part in the galaxy classification crowdsourcing project Galaxy Zoo. The odd structures were then dubbed Hanny's Voorwerp, Dutch for "Hanny's object," though "green goblin" has a bit more ring to it.