In 2007, a Dutch school teacher called Hanny van Arkel discovered an extraordinary object while combing though images for the Galaxy Zoo project to classify galaxies. The object appears as a bright green blob close to a spiral galaxy in the constellation of Leo Minor and soon became known as Hanny’s Voorwerp (meaning Hanny’s object in Dutch).

Hanny’s Voorwerp is astounding because astronomers have never seen anything like it. Although galactic in scale, it is clearly not a galaxy because it does not contain any stars. Detailed spectrographic observations since its discovery suggest that it is a giant cloud of gas that is glowing an unusual green colour.

That raises an obvious question: what is causing the gas to glow?

Today, Hayden Rampadarath at the Joint Institute for VLBI in Europe, based in the the Netherlands and a multitude of friends, provide an answer. Their data comes from a new study of the nearby spiral galaxy IC 2497 using a couple of very long baseline interferometers to study the region at various wavelengths.

Their conclusion is that, like many galaxies, IC 2497 contains a massive black hole at its centre. The infall of matter into the black hole generates a cone of radiation emitted in a specific direction. The great cloud of gas that is Hanny’s Voorwerp just happens to be in the firing line. The black hole radiation is ionising the gas, causing it to glow green.

What has confused the issue is that another cloud of dust and gas sits between us on Earth and IC 2497 and this prevents us from directly seeing the black hole (or the active galactic nucleus as astronomers call it).

That seems a sensible interpretation. And certainly more likely than another idea put forward last year. This proposed that some 10,000 years ago, IC 2497 suddenly underwent a dramatic outburst of quasar-like radiation and then became quiet. What we see today from this cloud of gas some 10,000 light years from IC 2497, is simply a reflection of this outburst. In other words, Hanny’s Voorwerp is a quasar light echo.

That was unsatisfactory because it resolved one mystery merely by posing another: what could cause an entire galaxy to flare up briefly and suddenly?

The evidence that IC 2497 is still active today puts to rest this idea. It also explains why Voorwerps are so rare: the radiation cones from active galactic nuclei are highly directional so only occasionally do unlucky gas clouds get caught in the cross fire.

Ref: arxiv.org/abs/1006.4096: Hanny’s Voorwerp: Evidence Of AGN Activity And A Nuclear Starburst In The Central Regions Of IC 2497