Casting light on cold dust

The citizen science project Galaxy Zoo aimed to use humans to classify galaxies found in various surveys according to their morphology. In addition to this, a forum and emails allowed users to discuss the more weird and wonderful things that popped up. Among them were the ‘green peas’ a brand new class of galaxies and “Hanny’s Voorwerp” a strange feature observed by Galaxy Zoo classifier and Dutch School Teacher Hanny Van Arkel. The feature appears to be a galaxy sized glow in the background dust of intergalactic space. With not a star in sight, it definitely isn’t a galaxy, so the culprit for the glow has to be elsewhere, and so far attention has focused on nearby galaxy IC 2497.

The hypothesis goes that IC 2497 has in the past acted as a quasar, a galaxy with a very active central black hole chewing on the bodies of stars after stars after stars. The vast amounts of accretion lead to jets of matter and radiation bursting out of the poles of the black hole and into intergalactic space, meeting and shocking the background gas. In this scenario, Hanny’s Voorwerp is the afterglow of a quasar outburst that switched off 100,000 years ago.

Some researchers are uncomfortable with this explanation. They don’t like the idea of quasars switching on and off that quickly – though we know modern nearby galaxies have indeed switched off from their quasar or active galactic nuclei past (should they have had them) – otherwise there would be a Voorwerp zone, a point in the collective evolution of galaxies where the engines were closing down and the afterglows would be common. We do see the black holes of nearby (and even this) galaxies switching on and off as they nibble on a passing star, but that’s a stochastic process caused by there being so few stars around available to munch on.

Now radio observations taken by the European Very Long Baseline Interferometry (eVLBI) network at wavelengths of 18cm and the Multi-Element Radio Linked Interferometer Network (MERLIN) at both 18 and 6cm are adding to evidence that something else is going on.

The observations seem to reveal that in the heart of the assumed to be switched off galaxy, there is an active galactic nucleus putting out radio and other emissions in roughly the direction of the Voorwerp. The line of sight between the radio receivers and the nucleus is shrouded in dust, making precise determinations more difficult than would otherwise be, and for this reason there isn’t sufficient evidence to say there is definitely enough ultraviolet or x-ray radiation coming off the nucleus to create the Voorwerp. It may even be the case that the AGN has switched off its ionising rays and turned to emitting radio waves for the time being, leaving behind an ionisation echo, where the atoms torn asunder into ions and electrons by shortwave radiation are coming together again and releasing excess energy as light. This produces a characteristic recombination spectrum, which has been observed and is also held up as evidence that the Voorwerp hasn’t been lit by shocks.

Data on the Voorwerp and other candidate voorwerpjes are coming in from different telescopes and wavelengths, including the Hubble Space Telescope, whose findings are presently being analysed and have themselves uncovered a few surprises for future release.

UPDATE: Hanny (who tweets here) and Chris Lintott (who tweets here) of the Sky at Night and Galaxy Zoo, both feature in a radio interview.


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