Galaxy Zoo: 'Hanny's Voorwerp', a quasar light echo?

@article{Lintott2009GalaxyZ,
  title={Galaxy Zoo: 'Hanny's Voorwerp', a quasar light echo?},
  author={Chris J. Lintott and Kevin Schawinski and William C. Keel and Hanny van Arkel and Nicola Bennert and Edward M. Edmondson and Daniel Thomas and Daniel J. B. Smith and Peter D. Herbert and Matt J. Jarvis and Shanil N. Virani and Daniel Andreescu and Steven Bamford and Kate R. Land and P. G. Murray and Robert C. Nichol and Jordan Raddick and An{\vz}e Slosar and Alexander S. Szalay and Jan vandenBerg},
  journal={Monthly Notices of the Royal Astronomical Society},
  year={2009},
  volume={399},
  pages={129-140}
}
We report the discovery of an unusual object near the spiral galaxy IC 2497, discovered by visual inspection of the Sloan Digital Sky Survey (SDSS) as part of the Galaxy Zoo project. The object, known as Hanny’s Voorwerp, is bright in the SDSS g band due to unusually strong [O III]4959, 5007 emission lines. We present the results of the first targeted observations of the object in the optical, ultraviolet and X-ray, which show that the object contains highly ionized gas. Although the line… 
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