The Interstellar Institute (I2) is a CNRS International Research Network. Its goal is to produce new scientific breakthroughs and to lead the study of complex physics of diffuse baryonic matter, focusing on theoretical and data science approaches, rather than on designing instrumentation and conducting observations. I2 runs a yearly 4-week intensive working session combining its 27 core members with a similar number of visiting scientists to foster new ideas and interactions. These sessions are done in Paris, the world’s most active centre for interstellar physics. I2 members collaborate throughout the year on projects developed at the summer intensive sessions.

Scientific scope


Intensive working sessions

The members of I2, along with visiting scientists, meet for 4 weeks each year at the Institut Pascal (IPa) in Orsay. The 50-60 participants at every annual working session is composed of the members of I2, complemented by a mix of visiting scientists and scientists from the larger ISM group of the Paris area. As only a few attendees attend all 4 weeks, there are about 25 attendees at any one time. These intensive working sessions follow the structure of the IPa scientific program, with most of the time devoted to collaborative work. These are quite different from (e.g.) an Apsen Center for Physics program in that a group of people returns year after year, forming a committed, vibrant scientific core, and establishing the scientific trust so critical to the creation of new work. It is also quite different from a repeating international group meeting; in I2 the returning scientists are not part of a single scientific project or experiment, and are often in disagreement about core concepts of the ISM. The I2 is an open structure with many new people attending the program every year bringing new perspectives.

The Interstellar Medium beyond 3D July 2017 - Institut d'Astrophysique Spatiale, Université Paris-Saclay, France.

The Milky-Way in the age of Gaia Oct 2018 - Institut d'Astrophysique Spatiale, Université Paris-Saclay, France.

The self-organised star formation process Sept-Oct 2019 - Institut Pascal, Université Paris-Saclay, France.

The Grand Cascade July 2021 - Institut Pascal, Université Paris-Saclay, France.

Combining expertise

The I2 brings together scientists from different backgrounds, all related to the complex physics of diffuse matter. We believe that breakthrough in understanding the processes that guide the evolution of baryons in the Universe will be made by bringing together knowledge on different scales, different physical conditions and different modelling, numerical and theoretical expertise. The goal of I2 is to build a group of people that would last long enough to establish common languages in several sub-groups, long enough for new collaborations and new vantage points to emerge. This can only be done with enough time, that is why I2 members meet in person during the summer intensive sessions for long periods of time over several years. We believe this will lead to numerous new scientific insights that cannot be obtained otherwise. Specifically, I2 is currently composed of a mixture of faculty (16 permanent staff level) and fellows (~11 student & postdoc level). These 27 people constitute the current I2 membership. Members are free to explore any topic. They were selected based on their investment in, and success with, the seed programs and to provide a broad range of perspectives and backgrounds. To be part of this collective, each member commits to attend the annual program every year for the next 3 year period, and uses II as a secondary affiliation on publications. Membership is expected to rotate over a period of years.

I2 members


Our approach to achieving scientific breakthroughs in the study of the ISM is deliberately open, based on our three years of experience. We have found that by putting the right mixture of people together in the right environment and structure, wholly new ideas form that dramatically advance the field.

The topics explored in the I2 programs range from the study of the origin of the filamentary structure of the ISM, to the properties of the Galactic magnetic field, to the structure of multi-phase interstellar turbulence. These projects used a wide range of approaches from numerical simulations, to hyper-spectral data, to deep learning and new segmentation algorithms. In many cases it is the hybridisation across theories and methods that have yielded new insights. More than twenty papers have already been published or submitted following the work that happened at the three sessions, and that number will increase as we are aware of several papers in preparation and many ongoing projects.

To illustrate the scientific scope of I2, here are a few examples of recent studies that benefited from the collaborative interactions at the first two programs. These works show the combination of hyper-spectral data, numerical simulation and advanced data science techniques typical of the work done by this group.

Clark et al. (2019) have addressed a 20 year old problem related to the physical nature of intensity features observed in velocity channels of 21 cm emission of Galactic HI. Up to now it was thought that they were dominated by signatures of the turbulent velocity field. These authors have shown that they are in fact dominated by density structures of the HI cold phase which changes significantly the properties of interstellar turbulence that can be deduced from the data.