NASA says the Hubble Space Telescope has captured dramatic images of a set of wispy, goblin-green objects, the ephemeral "ghosts" of quasars that once flickered into life and then faded.

The ethereal, glowing wisps photographed by Hubble in eight separate active galaxies are thought to have been illuminated, possible just briefly, by a blast of ultraviolet radiation from quasars, giant black holes at the cores of the galaxies that emit massive amounts of energy as they consume gas from their cosmic surroundings.

"However, the quasars are not bright enough now to account for what we're seeing; this is a record of something that happened in the past," says Bill Keel of the University of Alabama at Tuscaloosa, who initiated the Hubble investigation of the distant objects.

"The glowing filaments are telling us that the quasars were once emitting more energy, or they are changing very rapidly, which they were not supposed to do."

One possible explanation, he says, is that the quasars may be in fact co-orbiting black holes, which could change the quasar's brightness as they circle each other, acting something like a cosmic dimmer switch.

The once-invisible filaments in deep space have been made to glow green, as seen in the Hubble images, by a process known as photoionization, where oxygen atoms in the filaments absorb energy from the quasar and slowly re-emit it as light for many thousands of years.

That the quasars themselves are not visible could be explained by the possible tens of thousands of years it took their energy to reach and ionize the far-flung gas filaments, the researchers say.

So, although the quasars responsible have "turned off," the oxygen clouds will continue to glow for much longer before they also fade, NASA says.

The green filaments are thought to be long tendrils of gas stretched out under gravitational forces created in a merger of two galaxies, now slowly orbiting their host galaxies long after the mergers were finished.

In additional the being far from the centers of their host galaxies, the green filaments are massive, spanning tens of thousands of light years, NASA says.

"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 says.  "Potentially, you can say we're seeing it 1.5 billion years after a smaller gas-rich galaxy fell into a bigger galaxy."