Planck unveils Galactic surprises

Carbon monoxide seen by Planck
The clouds of carbon monoxide as seen by Planck. Image credit: ESA/Planck collaboration

Planck has unveiled more surprises about our own Galaxy, bringing scientists closer to being able to understand the structure of the Universe. The new results are being presented this week at an international conference in Bologna, Italy. The results include previously unknown clouds of cold gas, and a mysterious haze of microwave emission near the centre of the Galaxy.

Planck is surveying the sky at microwave wavelengths, with a primary science goal of observing the Cosmic Microwave Background - the afterglow of the Big Bang. But in the course of its cosmological quest Planck is also making exquisite maps of anything in the foreground, which includes our own Galaxy. Astronomers from around the world are busily poring over these maps, with some of the intermediate results being presented for the first time this week.

George Efstathiou, at the University of Cambridge and the Planck Survey Scientist, said “Although the Planck satellite was designed to detect structure created in the very early Universe, it is a powerful probe of emission from our own Galaxy. Measurements of Galactic emission, as well as being interesting in their own right, need to be identified and removed before we can perform the full cosmological analysis.” Most of the emission from our Galaxy is due to hot gas and dust between the stars, or from very energetic electrons moving through magnetic fields.

Galactic Haze as seen by Planck
The Galactic Haze as seen by Planck. Image credit: ESA/Planck collaboration

Planck’s instruments are able to pick up the emission from very cold clouds of gas scattered throughout the Milky Way. The main constituent of these clouds is hydrogen, though this is very difficult to observe as hydrogen molecules do not emit much light. Instead, astronomers identify the gas clouds by detecting emission from carbon monoxide molecules, which although rarer is much easier to detect.

“Planck was never designed to detect carbon monoxide, but it turns out to be a very useful bonus”, commented Clive Dickinson of the University of Manchester. “Maps of this gas from ground-based telescopes take a long time to make, and are normally limited to the places where we expect to find it. Now Planck has picked them out in locations all over the sky.” The full-sky map of carbon monoxide will provide a vital resource for astronomers using other telescopes, pointing out new places to look for stars in the process of forming.

An even more unusual result is the detection of a mysterious haze of microwave emission near the Galactic centre. A relatively well understood phenomenon, called ‘synchrotron emission’, involves very fast electrons travelling through magnetic fields. The haze, however, seems to be caused by an unexpectedly energetic population of electrons, the origin of which is uncertain.

There are a number of possible explanations for the haze, though none have been confirmed. It could be that there are more supernovae occurring in the centre of the Galaxy than previously thought, or that the flow of material from the Galactic centre is stronger for some reason. A more intriguing possibility is that the electrons could be the result of the annihilation of dark matter particles. If true, this would provide a new way for tracing this unusual constituent of the Universe.

“The results achieved thus far by Planck on the Galactic haze and on the carbon monoxide distribution provide us with a fresh view on some interesting processes taking place in our Galaxy,” says Jan Tauber, ESA’s Project Scientist for Planck. “The lengthy and delicate task of foreground removal provides us with prime datasets that are shedding new light on hot topics in Galactic and extragalactic astronomy alike. We look forward to characterising all foregrounds and then being able to reveal the CMB in unprecedented detail.”