Isabelle Grenier: studying the interaction between cosmic rays and interstellar gas
Isabelle Grenier is an astrophysicist, a member of the Astrophysics, Instrumentation and Modelling Laboratory (AIM - Univ. Paris-Saclay, CNRS, CEA, Univ. Paris Cité) and a professor of Astrophysics at Université Paris Cité. She specialises in high-energy stars and the interstellar medium, and won the 2022 CNES-Astrophysics prize from the French Academy of Sciences. She has been working for several years now at the Fermi space observatory, studying interstellar gas and cosmic rays.
Isabelle Grenier's passion for the sky and the stars began back in 1969, when the Apollo 11 mission to the Moon was launched. She was eleven years old at the time. "I was lucky enough that year to be in the class of a really passionate teacher, who made us work all year on the Moon and the solar system. This quite literally propelled me into the stars, and I have never come back down," recalls Isabelle Grenier. Driven throughout her schooling by the desire to study the sky, she naturally turned to Physics, enrolled at the École Normale Supérieure of Cachan (ENS Cachan, now ENS Paris-Saclay) and in 1982 joined the Astrophysics laboratory at the French Alternative Energies and Atomic Energy Commission (CEA), which has since become the Astrophysics, Instrumentation and Modelling laboratory (AIM - Univ. Paris-Saclay, CNRS, CEA, Univ. Paris Cité), to begin a PhD thesis on gamma rays, tracers of the interstellar medium, messengers of pulsars and other energetic objects.
From COS-B to Fermi: astronomical observation in gamma rays
"In my career I was lucky enough to come in at the right time to accompany the adventure of the discovery of the gamma-ray sky with several telescopes," explains the researcher. Indeed, during her thesis Isabelle Grenier started work on the data from the COS-B satellite, which allowed scientists to map the sky and list the sources of gamma-ray emission. She then started analysing the data collected by the Compton Gamma-Ray Observatory (CGRO) satellite before joining the prefiguration team at the Fermi space gamma-ray observatory. "When I started my PhD thesis work, most astronomers were looking at the sky with radio, optics or X-rays. To devote your work to gamma rays was perceived as iconoclastic at the time. Even when we got our first results, it took time to convince people and have them accepted," adds Isabelle Grenier. Today, things have obviously changed and there is no doubt that the work carried out and the numerous results obtained by the researcher have largely contributed to this.
Producing the catalogue of gamma-ray sources
Isabelle Grenier therefore devoted her first years of research to the analysis of the data collected by COS-B and then by CGRO. "My work consisted of studying the gamma-ray emissions observed, locating and characterising point sources (pulsars, supernova remains, extragalactic black holes) and in particular trying to understand the origin of the unidentified sources, which accounted for two thirds of the several hundred that we had identified at the time, and for which no counterpart could be found in other forms of light," explains the researcher.
And for good reason, as 80% of the gamma rays in the sky do not come from these objects but from a luminous background produced by the interaction of cosmic rays with the interstellar gas in the galaxy. "Identifying a source on this luminous background is like trying to distinguish a very faint star in a hyper-bright Milky Way, or identifying a needle from the strands of a haystack," adds the researcher. "We needed to know more about the amount of interstellar gas to be able to separate cosmic ray emissions from those produced by other sources. This is why, during my PhD thesis, I spent a year at Columbia University in New York to help map the molecular gas in the galaxy," she says. In 1998, Isabelle Grenier joined the team in charge of prefiguring the Fermi space observatory, armed with this double expertise - gamma sources and interstellar gas - and her experience with the COS-B and CGRO satellites.
Launching the Fermi adventure
When NASA called her in 2000 to announce that the new Fermi gamma-ray space telescope project had been authorised, a new chapter in Isabelle Grenier's life began. "It was an extraordinary opportunity to be able to accompany this observatory, from the first phases of definition to the current use of the observations," enthuses the researcher. After convincing NASA of the relevance of the scientific targets pursued with Fermi and of the possible performance of the instruments, the small team - which has grown since then - to which Isabelle Grenier belongs was entrusted with numerous different missions. "In addition to the construction phase from 2000 to 2008, we piloted the implementation of analysis tools, worked on the formatting of data to make it usable by astronomers who are not experts in particle physics, and estimated and verified the performance of the telescope with the instrumentalists, etc." This international collaboration was particularly effective, as was shown by the availability of the first sky maps just a few days after the instruments were switched on.
Mapping the distribution of interstellar gas and cosmic rays
After the launch of Fermi in 2008, the researcher, who took part in the production of the gamma-ray source catalogue, focused more and more on the study of the interactions between cosmic rays and interstellar gas thanks to gamma rays. "This approach is doubly interesting because it allows us, once we know the amount of gas in a cloud, to estimate the flux of cosmic rays that pass through it. Or conversely, if we know the flux of cosmic rays in a region, to measure the amount of gas. We discovered a large amount of dark gas that had completely escaped other observations. The theory predicted the existence of a discreet layer of gas at the transition between the atomic and molecular phases of the clouds, but estimated it would be negligible in quantity. We could never have imagined that it accounted for almost 20% of the gas in our galaxy and that some clouds were richer in dark gas than in molecular gas seen in radio waves!" explains Isabelle Grenier.
This discovery sheds new light on the cycle of gas in a galaxy and provides a better understanding of how the interstellar medium is structured to produce stars. "We have also been able to map the distribution of cosmic rays in our galaxy and show that they are everywhere, forming a kind of fairly homogeneous fog," adds the researcher. These results have also enjoyed a major impact among astrophysicists, who are now more and more convinced that cosmic rays have a significant dynamic influence on the evolution of galaxies. "Now in order to understand the impact of these cosmic rays, we first have to understand how they propagate in the different environments of a galaxy. This is what we are trying to do today, by comparing observations and simulations of galaxies," says Isabelle Grenier.
Towards a more and more scientific culture
It is perhaps because she does not forget the teacher to whom she owes her passion for the stars that Isabelle Grenier has also always wanted to contribute to the dissemination of scientific culture, which she is concerned to see weakening. "I am convinced that, more than ever, our society needs scientists to get out of their laboratories and speak to the world, to reinforce the taste for rationality, the desire to ask questions and the ability to admit the limits of their knowledge," concludes the researcher.