Starship Asterisk*

[b][i]NASA, ESA, Galaxy Zoo Team and W. Keel (University of Alabama, USA)[/i][/b]

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The NASA/ESA Hubble Space Telescope has imaged a set of enigmatic quasar ghosts — ethereal green objects which mark the graves of these objects that flickered to life and then faded. The eight unusual looped structures orbit their host galaxies and glow in a bright and eerie goblin-green hue. They offer new insights into the turbulent pasts of these galaxies.

The ethereal wisps in these images were illuminated, perhaps briefly, by a blast of radiation from a quasar — a very luminous and compact region that surrounds a supermassive black hole at the centre of a galaxy. Galactic material falls inwards towards the central black hole, growing hotter and hotter, forming a bright and brilliant quasar with powerful jets of particles and energy beaming above and below the disc of infalling matter.

In each of these eight images a quasar beam has caused once-invisible filaments in deep space to glow through a process called photoionisation. Oxygen, helium, nitrogen, sulphur and neon in the filaments absorb light from the quasar and slowly re-emit it over many thousands of years. Their unmistakable emerald hue is caused by ionised oxygen, which glows green.

These ghostly structures 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, although the quasars themselves have turned off, the green clouds will continue to glow for much longer before they too fade. ...

• arXiv.org > astro-ph > arXiv:1408.5159 > 21 Aug 2014 (v1), 02 Mar 2015 (v2)

bystander wrote:

... once-invisible filaments ...