Gaia and the Galactic Interstellar Medium

The interstellar medium (ISM) is a fascinating place! The more we look at it, the more its complexity is revealed. The evolution of matter in a galaxy is indeed guided by complex physics, including magneto-hydrodynamics turbulence, gravity and stellar feedback. The result is a multi-phase medium, with extreme contrasts in temperature and density, out of which stars form. Our understanding of the star formation process has progressed immensely in the last decades thanks to exquiste hyperspectral data, extinction measurements, and continuum data including polarization, combined with enlightning numerical simulations. One inherent difficulty of this research field is the fact that the multidimensional cycle of interstellar matter through its various phases is usually interpreted from its projection onto the 2D sky. This calls for specific data inversion techniques, comparison of observational data with synthetic observations from numerical simulations, and the combination of multiple observational probes.

The new Gaia catalog (to be released in April 2018) is about to add a totally new dimension to ISM physics : the third one!

ESA’s Gaia mission was launched in 2013 with the aim of providing unprecedented positional measurements for about one billion stars in the Milky Way and the Local Group. It will provide global astrometry, absolute parallaxes and proper motions, with the addition of all-sky homogeneous multi-colour photometry and spectroscopy. These unique capabilities go well beyond, but are complementary to, the science cases being addressed by ground based surveys such as RAVE, SDSS, Pan-Starrs, APOGEE, LSST, etc. On practically all aspects of Galactic evolution, the exploitation of the Gaia data is expected to lead to scientific breakthroughs: measurement of the rotation curve, the gravitational potential, the structure of our Galaxy, its mass growth, etc. With the new Gaia catalog we can now gain insight on the kinematics, temperature, density structure and magnetic field properties of the different ISM phases by combining it with data from various observational tracers (21 cm, synchrotron, H-alpha, molecular lines, dust).

This program is about using the new Gaia data to infer three-dimensional information of the distribution of matter in 6D phase-space. It will bring together experts of the Gaia data release and data analysis, and experts on both the stellar and interstellar physics, theoreticians and observers alike. The goal is to foster collaborations and create new analysis tools benefiting from the new Gaia data.

A Ψ2 program at Paris-Saclay University in Orsay, France

The Ψ2 programs bring scientists in a single location for a signficant amount of time. Unlike typical conferences, the daily schedule is composed of a few informal talks and lots of open time for discussion and collaborative work.

In this Ψ2 program we aim to bring together pioneering researchers to exploit the new Gaia data to pose, explore, and answer paramount questions about the nature and evolution of the multi­phase ISM from which stars form.

The program will focus on the following topics throughout the four week program.

Star formation and ISM properties

This Gaia Ψ2 program follows on ISM3D held in July 2017 that also focused on the complex physics of interstellar matter evolution. In addition, this Gaia program will look specifically at the properties of interstellar matter in three dimensions and on the link between ISM conditions and star formation which is still poorly known. This will be a significant addition to ISM3D, requiring new collaborators with expertise in star formation, stellar spectra and diffuse interstellar bands.

3D extinction mapping

The DR2 release of the Gaia data will include over a billion stars together with their parallax and, for ~5-10% of the sources, an estimate of the interstellar extinction they have suffered. This dataset is a game changer for 3D extinction techniques as it provides a strong prior on many extinction structures in 3D and removes degeneracy amongst possible solutions. The goal is to explore new ways of combining existing data sets with Gaia DR2 in order to map our Galaxy’s ISM in striking detail.

Linking density, velocity and magnetic fields

The combination of the Gaia data with data from various observational tracers (21 cm, synchrotron, H-alpha, molecular lines, dust) opens the possibility to do kinetic tomography and gain insight on the kinematics, temperature, density structure and magnetic field properties of the different ISM phases in 3 dimensions. This will allow better comparison with numerical simulations in trying to understand the physical processes that are shaping the ISM (MHD turbulence, phase transition, etc.).

The Gaia Data release 2

Gaia experts will be present during the whole program for participants to familiarize themselves with the new DR2 data. Hands-on sessions to discover and manipulate the DR2 catalog will be organised. No prior knowledge of Gaia data is expected. This is a very timely opportunity for part of the ISM3D community to meet again, build on new collaborations established last July and move forward on the detailed mapping of where we are in the Galaxy.


The program will assemble participants from different areas:

  • experts of the Gaia data : scientists involved in various parts of the Gaia DR2.
  • experts in the analysis of large area data sets : 21­cm, CO, Hα, radio continuum, dust, Pan­STARRS, SDSS, Planck, WISE, APOGEE and beyond.
  • experts in stellar photometry, stellar spectra and diffuse interstellar bands
  • experts in numerical simulations and theory of the multi-phase interstellar medium of the Galaxy.
  • ISM domain experts with background and interest in data mining, statistics, visualization, and other data science tools.

The goal here is to invent new data exploration and visualization methods that, by combining different data sets, would lead to an improved vision of the 3D properties of the multi­phase ISM. Participants will be selected in order to foster interaction between these different communities.