Stars shine light on Milky Way’s magnetic secrets
Our Milky Way is a flat, spiral galaxy containing several hundred billion stars. In between are giant gas clouds, out of which these stars are created. The gas clouds – and the stars themselves – are composed of ionised matter (i.e. atoms with a positive or negative charge). This gives the Milky Way its magnetic field, which plays a crucial role in things such as star formation. “In space, this magnetic field curves and pulls like a rubber band on ionised matter,” says MAGALOPS(opens in new window) project coordinator Marijke Haverkorn(opens in new window) from Radboud University(opens in new window) in the Netherlands. “This can delay, for example, the collapse of a gas cloud into a star.”
Detecting the galactic magnetic field
A key challenge of course is that astronomers cannot see this galactic magnetic field. One way to detect magnetic fields could be to measure their influence on light, and in particular polarised light from stars. “While waves in water only move up and down, light waves in principle can move in many directions,” explains Haverkorn. “While most light waves move in all of these directions, polarised light only goes in one direction.” One influence of magnetic fields is that they can make some starlight polarised. By measuring this, we can derive that there is a magnetic field with a certain direction. This was the goal of MAGALOPS, which was supported by the European Research Council(opens in new window). The project built on custom optical polarimetric observations, supported by data supplied by the European Space Agency’s Gaia mission(opens in new window) containing distances to stars.
Measuring polarised light from stars
The project had two key components. First, available data on polarisation was processed to try to build up a 3D image of the galactic magnetic field. “We proved the validity of a new method that inverts polarisation measurements into something that tells you where gas clouds are and the direction of the magnetic fields,” adds Haverkorn. The project team also sought to determine the strengths of magnetic fields, and to identify the kinds of observations needed to accurately calculate this. The other part of the project involved modelling. “There are lots of different kinds of indirect observations,” remarks Haverkorn. “These include radio observations of gas clouds and optical measurements of stars. None however can determine what the magnetic field is like by themselves.” The project therefore built models capable of combining different sources of information. For this, software was developed within the international IMAGINE(opens in new window) project. “This work is still ongoing,” notes Haverkorn.
Our galaxy and the universe beyond
Haverkorn believes that MAGALOPS has established the groundwork for future international collaboration. The project team is working with Brazilian, Swedish and Japanese partners to survey polarised light from millions of stars. “We now know how to interpret and analyse this data,” she says. “We know it is possible to build up a 3D map of the galactic magnetic field.” This will contribute to a better overall understanding of the galaxy itself. “The magnetic field is such an active and dynamic component,” remarks Haverkorn. “This data is essential for models of star formation, so we can explain why our starry night looks the way it does.” Astronomers looking beyond our own galaxy will also find this research crucial. “For them, the galactic magnetic field is a nuisance,” adds Haverkorn. “By knowing what this magnetic field looks like, they can subtract it from their data, to look at the universe beyond.”
