NASA’s Hubble Space Telescope has photographed a set of wispy, goblin-green objects that are the ephemeral ghosts of quasars that flickered to life and then faded. The glowing structures have looping, helical, and braided shapes. “They don’t fit a single pattern,” said Bill Keel of the University of Alabama in Tuscaloosa, who initiated the Hubble survey. The ethereal wisps outside the host galaxy are believed to have been illuminated by powerful ultraviolet radiation from a supermassive black hole at the core of the host galaxy. The most active of these galaxy cores are called quasars, where infalling material is heated to a point where a brilliant searchlight shines into deep space. The beam is produced by a disk of glowing superheated gas encircling the black hole. “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,” Keel said. “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.” Keel said that one possible explanation is that pairs of co-orbiting black holes are powering the quasars, and this could change their brightness, like using the dimmer switch on a chandelier.
The quasar beam caused the once invisible filaments in deep space to glow through a process called photoionization. Oxygen atoms in the filaments absorb light from the quasar and slowly re-emit it over many thousands of years. Other elements detected in the filaments are hydrogen, helium, nitrogen, sulfur, and neon. “The heavy elements occur in modest amounts, adding to the case that the gas originated in the outskirts of the galaxies rather than being blasted out from the nucleus,” Keel said. The green filaments are believed to be long tails of gas pulled apart like taffy under gravitational forces resulting from a merger of two galaxies. Rather than being blasted out of the quasar’s black hole, these immense structures, tens of thousands of light-years long, are slowly orbiting their host galaxy long after the merger was completed. The ghostly green structures are so far outside the galaxy that they may not light up until tens of thousands of years after the quasar outburst and would likewise fade only tens of thousands of years after the quasar itself does. That’s the amount of time it would take for the quasar light to reach them.
The first “green goblin” type of object was found in 2007 by Dutch schoolteacher Hanny van Arkel. She discovered the ghostly structure in the online Galaxy Zoo project. The project has enlisted the public to help classify more than a million galaxies cataloged in the Sloan Digital Sky Survey (SDSS) and moved on to add galaxies seen in Hubble images probing the distant universe. The bizarre feature was dubbed Hanny’s Voorwerp, Dutch for Hanny’s object. In follow-up observations from Kitt Peak National Observatory and the Lick Observatory, his team found 20 galaxies that had gas that was ionized by radiation from a quasar rather than from the energy of star formation. And the clouds extended more than 30,000 light-years outside the host galaxies.
Credit: Astronomy Magazine