An unexpected use of large optical telescope: Imaging the small scale structure of the diffuse interstellar medium.

Optical images in true colours of the cirrus field obtained with MegaCam on the CFHT. Image credits: MATLAS collaboration, Pierre-Alain Duc.

By combining multi-wavelength data obtained from space with Planck and WISE, and from the ground with MegaCam on CFHT, a team of researchers has revealed the structure of the diffuse interstellar medium over several square degrees with unprecedented details. In particular, this study reveals the statistical properties of interstellar turbulence over a wide range of scales, from 0.01 to 10 pc.

The main innovation of this work is the use of a large optical telescope (the CFHT) to study the structure of the interstellar medium at high resolution and on a large area on the sky, something that is very challenging to obtain with more classical observations done in the infrared. This mapping of these interstellar cirrus clouds located within a few hundred parsec from the Sun can be done because interstellar dust grains scatter starlight. This scattered light has been detected for decades by optical telescopes. Here it is the first time that is exploited scientifically to study the structure of interstellar clouds which is composed of faint filamentary structures of various sizes. The result obtained benefit from specific image processing and data acquisition techniques developed in the context of the MATLAS Large Program at CFHT that aims at detecting faint diffuse emission around galaxies.

Combined power spectrum : black is Planck radiance, red is WISE and blue is MegaCam. The units of the y axis are arbitrary; each power spectrum was scaled in order to match the others. For each power spectrum, we show data points corresponding to scales larger than the beam and where the power is above the noise component. The data points shown here are noise subtracted and divided by the beam function. The best fit gives P(k) ~ k^(2.9±0.1). Figure from Miville-Deschênes et al. 2016.

One advantage of the high angular resolution provided by the CFHT data is to eventually reach the angular scale at which turbulent energy dissipates. Understanding the exact process by which kinetic energy is dissipated and heats gas is essential. It is key in the formation of dense structures that lead to the formation of stars. For instance, recent studies based on Herschel observations of molecular clouds have revealed the presence of filaments with widths of 0.1 pc that seem constant whatever their mass. This observational fact has been attributed to the energy dissipation process, namely ambipolar diffusion (friction between neutrals and ions). The present study shows that the dissipation scale in the interstellar medium is smaller than 0.01 pc which brings important constraints on the exact process resposible for this dissipation.

These results emphasize the fact that scattered light from cirrus, an important source of pollution for deep imaging destined to mapping diffuse structures around massive galaxies, is carrying potentially precious information about the nature of the physical processes involved in the evolution of matter in our own galaxy.

Additional information

CNRS/INSU press release (In French only)
Scientific paper


Media contact
Mary Beth Laychak
Canada-France-Hawaii Telescope
(808) 885-3121

Science contacts
Marc-Antoine Miville-Deschênes
IAS (CNRS/Université Paris Sud/Université Paris-Saclay)
Tel: 01 69 85 85 79

Pierre-Alain Duc
AIM (CEA/CNRS/Université Paris Diderot/Université Paris-Saclay)
Tel:01 69 08 92 68