The nature of the multi-phase and multi-scale ISM

The interstellar medium is far from a homogenous atmosphere for our Galaxy. It is a turbulent, multiphase environment, with dramatic variation in temperature, density, magnetization, ionization, velocity, composition and abundance. How matter evolves in a Galaxy like the Milky Way, from diffuse and hot gas to dense and gravitationally stable structures and then stars, is not clear. In particular how energy, mass and momentum is transported from Galactic scales down to AU scales and in between the different phases of the ISM is still to be understood. This involves understanding the details of energy injection mechanisms at large scales, the interstellar turbulence cascade, phase transitions, various instabilities, as well as the impact of gravity and stellar feedback in the context of the large­scale structure of the Galactic ISM.

We have made progress in understanding the role and importance of the complex ISM physics in the star formation process by studying these features in groups of clouds in the Galactic disk or on the plane of the sky. Moreover, massive numerical simulations allow the exploration of the complex nonlinear physics of the ISM. Nevertheless one difficulty rests in obtaining clear and general observational results about the nature of the different phases and their interactions, and of interstellar magnetized turbulence in the Galactic context. The study of the ISM is made difficult because no one tracer provides a full view of the physical properties and because of the entanglement of information along the line of sight. Creative techniques have to be invented to take into account projection effects in estimating the real 3D physical properties of the ISM.

A revolution is underway that will dramatically change our way of studying the ISM. Over the last decade, astronomy has seen an explosion of precise, deep, large­ area surveys across the electromagnetic spectrum, including polarization. With this wealth of information we are now able to make maps of the ISM in three dimensions, parsing out overlapping structures on the sky into discrete features. While three dimensional maps are useful alone, it is the synthesis of depth information with intrinsic ISM diagnostics that is opening new frontiers and new questions.

A Ψ2 program at
Institut d'Astrophysique Spatiale 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 informal talks in the morning and open time for discussion and collaborative work in the rest of the day.

In this Ψ2 program we aim to bring together pioneering researchers to pose, explore, and answer paramount questions about the nature and evolution of the multi­phase ISM from which stars form. The participants will be selected in order to foster exploration in data combination, and synthesis with numerical simulations and theory.

The program will highlight the four following topics, one per week.

Dust as a tracer of structure

  • How do intrinsic properties of dust grains, in emission and absorption, relate to the rest of structure of the Galaxy and ISM?
  • How do we construct the best extinction maps?
  • How do we construct the best 3D map of ISM density?

The multi-phase ISM

  • Do the formation of ISM structures occur in discrete phases?
  • How are their morphologies related to intrinsic properties?
  • What is the best statistical way to describe the ISM density structure ?
  • How filamentary is the ISM ?
  • What physical processes are shaping it (MHD turbulence, phase transition, etc.) ?

The magnetized ISM

  • What is the 3D magnetic structure of the ISM and how does it interact with the density field?
  • What gives rise to the curl-free/divergence-free asymmetry in the magnetized ISM?

Interstellar turbulence

  • What can higher-dimension, multi-wavelength data tell us about the origin of turbulent motions (gravity, infall, feedback)?
  • What metrics of the high-dimensional ISM most strongly constrain the properties of magnetized turbulence, energy injection and energy dissipation mechanisms?


The multidimensional cycle of interstellar matter through its various phases must be 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. Insight on the kinematics, temperature, density structure and magnetic field properties of the different ISM phases can be obtained by the combination of various observational tracers (21 cm, synchrotron, H-alpha, molecular lines, dust) with numerical simulations.

In this context, the program will assemble participants from different areas:

  • experts in the analysis of large area data sets : 21­cm, CO, Hα, radio continuum, dust, Pan­STARRS, SDSS, Planck, WISE, APOGEE and beyond.
  • experts in numerical simulations and theory of the spatially­ resolved 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 is to put together a sufficiently large number of observers and theoreticians for a significant amount of time in order to provoke a real exchange between the two communities. By confronting a large number of observational and theoretical viewpoints / constraints, this extensive work in common will enable specific theoretical aspects to emerge. This collaborative work should also lead to the identification of new observational probes that could be used to test some aspects of the ISM theory.

The program will also bring together experts of different type of observations to find new ways to look at the data in a combined manner, in the context of theory. 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.


Application to the program is now closed.