Chuck Bednar for redOrbit.com – @BednarChuck
Ethereal wisps captured recently by the Hubble Space Telescope are the ghosts of quasars that flickered into life before fading. They were most likely illuminated by ultraviolet radiation from a supermassive black hole at the core of their host galaxies, NASA revealed on Thursday.
Survey leader Bill Keel from the University of Alabama, Tuscaloosa and his colleagues explain that the ephemeral structures come in looping, helical, and braided shapes, and are caused by the heating of infalling material to a point where an extremely bright light shines out into space.
Photoionization could help explain the ghostly objects
Keel noted that the objects might provide new insights into the unusual behavior of galaxies that have energetic cores, including quasars that produce a beam from the superheated, glowing disk of gas that encircles the black hole. What Hubble found were the ghosts of dead quasars.
“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,” he said of the objects, which were located around eight active galaxies. “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, Keel said, is that the quasars are being powered by pair of co-orbiting black holes that could cause fluctuations in their brightness. As a result of the quasar beam, the formerly-invisible deep space filaments began to glow through photoionization, a process which causes oxygen atoms in the filaments to absorb and slowly re-emit light from the quasar.
Other elements detected in the filaments are hydrogen, helium, nitrogen, sulfur, and neon, the researchers said. The heavy elements occur in modest amounts, Keel noted, which adds evidence to the notion that the gas originated from the outskirts of the galaxy, not the nucleus.
These glowing green objects take a long time to form
It is believed that the green filaments are long strands of gas that were pulled apart under the gravitational forces from a merger between two galaxies, in much the same way that a person would stretch taffy, NASA explained. However, instead of being forcibly ejected from the black holes of the quasar, they are slowly orbiting around their host galaxy following the merger.
“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 said. “Potentially, you can say we’re seeing it 1.5 billion years after a smaller gas-rich galaxy fell into a bigger galaxy.”
These phantom-like green structures are located so far outside of their galaxies that they might not even become illuminated until tens of thousands of years after the quasar outburst, the US space agency added. Likewise, they would not fade until tens of thousands of years after the actual quasar does, since it would take that long for the light to reach them.
Lack of brightness could be explained by a second black hole
Keel and his team found a total of 20 galaxies with gas that had been ionized by radiation from a quasar, including eight that were more energetic than expected based on the amount of radiation that they would have received from the host quasar. In some cases, the host quasars were as little as one-tenth the brightness needed to provide enough energy to photoionize the gas.
The researchers believe that the fluctuations in brightness are governed by the rate at which the material is falling onto the central black hole, and that this variability could be explained by the presence of two massive black holes circling each other in the host galaxy’s center following a merger. This would disrupt the steady flow of infalling gas, causing the accretion rate to increase abruptly and resulting in unexpected blasts of radiations.
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