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Astrophysicists led by Romanian researcher map out the light energy contained within the Milky Way

on 28 July 2017

In a research breakthrough, a team of scientists has for the first time derived a complete picture of the distribution of all light energy contained within our Galaxy. The research was done by an international team of scientists from the Astronomical Institute of the Romanian Academy (Romania), the University of Central Lancashire (UK) and the Max Planck Institute for Nuclear Physics in Heidelberg (Germany).

Prof. Dr. Cristina C. Popescu, from the Astronomical Institute of the Romanian Academy and the University of Central Lancashire, and Vice-President of Commission J1 of the International Astronomical Union, is the Principal Investigator of the research article that has recently been published in the renowned volume of the Monthly Notices of the Royal Astronomical Society.

Ever since our ancestors raised their eyes to the dark sky, the glow of our Galaxy, the Milky Way, has been a source of inspiration. What we see as a band of fuzzy light on the night sky from dark locations is nothing but the disk of the galaxy where our Sun resides. But what would we see if we were able to travel towards the centre of the Milky Way? What would an alien civilisation living at the opposite end of our galaxy see? All these questions can now be answered thanks to a recent research study that predicts the variation with position of the glow inside the Milky Way.

Through this new study, researchers wanted to understand what is the make-up of our Galaxy, which in turn, can give us unique insight into how stars in galaxies in general form and evolve over cosmic time. To do this they aimed to build a picture of how photons emitted by stars in our Galaxy are distributed in energy and space throughout the Milky Way. This process was challenging for several reasons: we have not yet detected the large majority of stars in our Galaxy that produce the photons; stellar photons are continuously absorbed or scattered by interstellar dust which redistributes their energy and direction of propagation; it was not fully understood how the interstellar dust is distributed in space, its amount or properties.

Cristina-Popescu smallDespite the challenges, their endeavour was successful. "We have not only determined the distribution of light energy in the Milky Way, but also made predictions for the stellar and interstellar dust content of the Milky Way", said Prof. Cristina C. Popescu.

Their research results were possible using a novel method that involves computer calculations that self-consistently track the destiny of all photons in the Galaxy, including the photons that are emitted by interstellar dust as heat radiation, and make predictions for how the Milky Way should appear in ultraviolet, visual and heat radiation. The predictions were then compared with observation of the all-sky emission, which is the total emission of the Milky Way rather than just emission from individual sources. In particular, the team used the recent images provided by the European Space Agency's Planck Space Observatory, in which Romania is heavily involved through the support of the Romanian Space Agency (ROSA). These images were essential in constraining the model calculations.

Determining how light is distributed in our Galaxy is crucial for understanding the make-up of our Milky Way, including the origin and propagation mechanisms of photons that do not originate in stars, like the very high energy gamma-ray photons. The current research shows how stellar photons affect the production of gamma-ray photons emitted by the so-called cosmic rays.

Cosmic rays are electrons and protons which criss-cross interstellar space at almost the speed of light. Because of the colossal energies they carry, they control key processes like star and planet formation and the processes governing galactic evolution. They promote chemical reactions in the interstellar space, leading to the formation of complex and ultimately life critical molecules.

Although counter-intuitive, it is the least energetic photons, in the form of heat photons in the infrared, and the coldest gas particles, at temperatures of only a few tens of degrees above zero absolute, that control the production of the highest energy photons in the Universe, the gamma-rays. Dr. Richard Tuffs from the Max Planck Institute for Nuclear Physics, one of the authors of the paper, said: "Working backwards through the chain of interactions and propagations, one can work out the original source of the cosmic rays”.

The modelling of the distribution of light in the Milky Way was strongly interdisciplinary, bringing together optical/infrared astrophysics and astro-particle physics. Dr. Mark Rushton, from the Astronomical Institute of the Romanian Academy and co-author of the paper said: “This work required a collaboration between specialists in modelling emission from galaxies in terms of low-energy processes and collaborators from the Max Planck Institute for Nuclear Physics, who utilises techniques developed in particle physics to measure and interpret high-energy emissions from cosmic rays.”

Main image caption: An all-sky image of the Milky Way, as observed by the Planck Space Observatory in the infrared. The data contained in this image were used in the current research and were essential in constraining the results for the light energy of our Galaxy. Credit: ESA, HFI and LFI consortia. 

The scientific article can be accessed at the following links: 

https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stx1282

https://arxiv.org/abs/1705.06652