Free Astronomy Magazine July-August 2015

SPACE CHRONICLES absorb light from the quasar and slow- ly re-emit it over many thousands of years. Their unmi- stakable emerald hue is caused by ionised oxygen, which glows green. These ghostly struc- tures are so far from the galaxy’s heart that it would have taken light from the quasar tens of thousands of years to reach them and light them up. So, al- though the quasars themselves have turned off, the green clouds will continue to glow for much longer before they too fade. Not only are the green filaments far from the centres of their host gal- axies, they are also immense in size, spanning tens of thousands of light- years. They are thought to be long tails of gas formed during a violent pastmer- ger between gal- axies — this event would have caused strong gravitation- al forces that would rip apart the galac- tic participants. Despite their tur- bulent past, these ghostly filaments are now leisurely orbiting within or around their new host galaxies. These Hubble images Galactic mergers do not just alter the forms of the previously serene galaxies involved; they also trigger extreme cosmic phenomena. Such a merger could also have caused the birth of a quasar, by pouring mate- rial into the gal- axies’ supermassive black holes. The first object of this type was found in 2007 by Dutch schoolteach- er Hanny van Ar- kel. She discovered the ghostly struc- ture in the on- line Galaxy Zoo project, a project enlisting the help of the public to classify more than a million galaxies catalogued in the Sloan Digital Sky Survey (SDSS). The bizarre feature was dubbed Hanny’s Voorwerp (Dutch for Hanny’s object). These objects were found in a spin-off of the Galaxy Zoo project, in which about 200 volunte- ers examined over 16,000 galaxy ima- ges in the SDSS to identify the best candidates for clouds similar to Hanny's Voorwerp. A team of re- searchers analysed these and found a total of twenty galaxies that had gas ionised by quasars. T his diagram illustrates how mysterious glowing streamers of gas are formed and then illuminated around colliding galaxies. 1-Two spiral galaxies move close enough together to exert gravitational tidal forces on each other. 2- A large tail of stars and gas is pulled out of the smaller galaxy. 3-A black hole in the center of the larger galaxy is fuel- ed by infalling gas from the collision and glows as a quasar. A beam of radiation from the quasar ionizes a portion of the extragalactic tidal tail. 4-After the quasar dies down the ionized tidal streamer continues to glow as forensic evidence of the quasar outburst that happened many thousands of years earlier. [NASA, ESA, and A. Feild (STScI)] n show bright, braided and knotted streams of gas, in some cases con- nected to twisted lanes of dark dust.