Astronomers stumble upon mysterious filaments

While studying the earliest light from the Universe with the LOFAR telescope, astronomers discovered unexpected structures in our galaxy. These structures are probably very close to the solar system. University of Groningen astronomers Saleem Zaroubi and Vibor Jelić, together with several colleagues, have had two papers describing their serendipitous finding accepted for publication.

‘We have found something that nobody has ever seen, but we don’t really know what it is’, says astronomy professor Saleem Zaroubi. He and postdoc researcher Vibor Jelić were using the LOFAR radio telescope to study the first light produced in the Universe, in what is known as the Epoch of Reionization . This is the period in which the first stars and galaxies were formed, about 500 million years after the Big Bang, and their light ionized the all-pervasive neutral hydrogen in the Universe.

In order to study this ancient light, astronomers first have to get rid of the highly contaminating radiation from our own Milky Way. ‘We looked at a part of the sky where this contamination was minimal’, explains Zaroubi. But even then, such a measurement is still very challenging.

When they used a special technique to measure magnetic fields, an unexpected pattern emerged in one of the observed areas. ‘This technique uses the polarization of synchrotron radiation’, explains Zaroubi. Synchrotron radiation is generated when high-energy electrons interact with a magnetic field, in this case the magnetic field of the Milky Way. The result came as a surprise. Jelić: ‘The polarization of the synchrotron emission was much stronger than expected, but we also found a strange pattern of straight filaments in the data.’

Straight filaments, like ‘uncooked spaghetti’, as Jelić describes them, are not a common feature in the Universe. So they double checked all their data and the analytical procedures, but couldn’t find any flaws to explain the strange findings. ‘We didn’t find a similar pattern in other patches of the sky that we observed’, adds Zaroubi.

The next step was even more surprising. They compared their results to measurements taken by the Planck satellite which measures, among other things, dust emission in our Galaxy. ‘We looked at the dust emission in the same area’, says Zaroubi. ‘And to our surprise, we saw features that strongly correlated with our observations.’ So the pattern must be real.

But what can it be? ‘Looking at our data, it is most likely a filamentary structure of some sorts, aligned with the magnetic field of our Galaxy and containing both dust and gases, including neutral hydrogen’, Zaroubi explains. ‘This structure is positioned somewhere between our Solar system and the source of the synchrotron radiation, probably quite near to us, about 300 light years at most. Furthermore, the shape we see suggests it is moving.’

However, this still leaves a lot unexplained, like the straightness of the filaments. Jelić: ‘Turbulence, which is prevalent in our Galaxy, cannot produce such ordered structures.’ He therefore wants to find out more about the mysterious filaments. ‘We will look in the same region in the sky for signals at different frequencies, using data from the Planck satellite and the Westerbork radio telescope. And we’ll run simulations to see how these ordered structures could arise.’

One reason to do this is quite simply because it is exciting to investigate a mystery. But there is a more practical reason as well, says Zaroubi: ‘We need to understand what is happening, because this phenomenon is interfering with our measurements of the Epoch of Reionization. Only when we understand what is happening, can we filter out the interference.’ And finally, Jelić concludes, ‘it will tell us something new about our own Galaxy!’

References:

1. Jelić et al., ‘Linear polarization structures in LOFAR observations of the interstellar medium in the 3C196 field’, 2015, Astronomy & Astrophysics, in press.

2. Zaroubi et al., ‘Galactic interstellar filaments as probed by LOFAR and Planck’, 2015, MNRAS Letters, in press.