Thanks to new sophisticated techniques, space missions and new observation instruments, Astronomy has entered a new era where not only the position in the sky, but also the distance of all heavenly bodies from us can be determined with precision. The components of our cosmic home, our Galaxy – stars, gas, magnetic fields – can finally be mapped in three dimensions.
The space between the stars is not empty. It is filled with dust, the grains of which align with the local magnetic field, and emit light. Light which, moreover, has the property of being linearly polarized, i.e. of having a preferred direction in space which overshadows the faint polarization of the cosmic background radiation – the “ashes” of the Big Bang – thus confusing our image of the first moments of the Universe.
These grains also absorb some of the starlight that passes through them, just as a polaroid filter would, imprinting information about the magnetic fields they live in on the polarization of the emerging light. Magnetic fields are very important to the evolution of our Galaxy, regulating the formation of new stars, shaping galactic structures and turning gas flows into cosmic accelerators stronger than CERN.
The polarization of starlight is therefore key: it contains information about the Galaxy’s all-important magnetic fields, and is the ‘duster’ that can help us ‘clean up’ our picture of the early Universe – as long as we could observe the polarization of several stars, and study it carefully to extract all the information it conveys.
The PASIPHAE experiment
This is precisely the object of the experiment PASIPHAan international scientific collaboration between the University of Crete and the Institute of Astrophysics of the Foundation for Technology and Research in Greece, the Inter-University Center for Astronomy and Astrophysics in India, the South African Astronomical Observatory in South Africa, Caltech in California, USA, and of the University of Oslo in Norway.
PASIPHAE’s goal is to measure the polarization of millions of stars across almost the entire sky. And now, we can get a first taste of the possibilities of this ambitious undertaking.
A team of researchers, with first author Dr. Vincent Pelgrims (until recently a postdoctoral researcher in the PASIPHAE project at the ITE Institute of Astrophysics and now a Marie Curie Fellow at the Interuniversity High Energy Institute in Belgium) demonstrated the power of the polarimetric data and algorithms he developed for PASIPHAE, using observations made with the polarimeter RoboPol at the Skinaka Observatory in Crete.
The scientists measured the polarization of more than 1,500 stars in a region of the sky more than 15 times the size of the full moon, combined them with the distances measured for each star by the European Space Agency’s Gaia satellite, and a ground-breaking algorithm that developed, mapped the magnetic fields in this direction of the sky.
“It is the first time that such a large volume of the Galactic magnetic field has been mapped in three dimensions,” enthuses Dr. Pilgrims.
“We found several dust clouds in this region of the Galaxy and were able to determine both their distances – up to thousands of light-years away – and their polarimetric properties, revealing the magnetic field that permeates these clouds.”
The team publishes the first tomographic map of the Galactic magnetic field over a large enough region of the sky, which it presents today in the journal Astronomy & Astrophysics.
“It is a great achievement, and an important step in the direction of 3D mapping of our Galaxy and its magnetic field”, says Vassiliki Pavlidou, professor of the Department of Physics at the University of Crete and collaborating faculty member of the Institute of Astrophysics of the ITE, who is also co-author of the publication.
“The structure of the Galactic magnetic field is currently not known with great precision. This hinders progress in many research areas, such as the study of ultra-high energy cosmic rays. The possibility of a detailed three-dimensional mapping of the Galactic magnetic field for the first time will lead to discoveries in all fields connected with it and is extremely important”, adds Ms. Pavlidou.
“This is just the beginning”
“This work is only the beginning”, adds Konstantinos Tassis, professor of the Physics Department of the University of Crete and collaborating faculty member of the Institute of Astrophysics of the ITE, co-author of the publication and scientific manager of the PASIPHAE program.
“Imagine such a map – but for most of the sky! This 3D atlas of the Milky Way’s magnetic field will become a reality in the coming years, with the help of the special WALOPs instruments that will begin mapping the polarization of stars in the sky this year.”
The 3D map produced by the team is shown in the video.
The PASIPHAE experiment is supported by the European Research Council (ERC) of the European Union, the Stavros Niarchos Foundation (SNF), the Infosys Foundation, the National Science Foundation in the United States of America, and the National Research Foundation in South Africa.
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