Molecules, magnetic fields and Intermittency in coSmic Turbulence – Following the energy trail.

Project Information

MIST

Grant agreement ID: 742719

DOI

10.3030/742719

Project closed

EC signature date 30 May 2017

Start date 1 October 2017

End date 30 September 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 500 000,00

EU contribution

€ 2 500 000,00

Coordinated by

ECOLE NORMALE SUPERIEURE
France

Periodic Reporting for period 5 - MIST (Molecules, magnetic fields and Intermittency in coSmic Turbulence – Following the energy trail.)

Reporting period: 2023-10-01 to 2024-09-30

Summary of the context and overall objectives of the project

Stars are the long-lived building blocks of galaxies. While most of them formed, almost all together, 10 billion years ago, new stars are still forming now, at a much lower pace. Star formation is the outcome of simultaneous accretion, driven by gravity, and ejection, powered by the energy of stars and black holes.
A most vexing concern is that everywhere, in the Early Universe as in our own Galaxy now, star formation is far less efficient than predicted by theory.
What is the missing piece in the puzzle?
Turbulence is a critical player, and the far-reaching question addressed in the MIST project is: can we measure the fraction of the large-scale gravitational energy stored (and eventually lost) in turbulence generated by the violent interplay between accretion and ejection?

The MIST project provides insights on a fundamental property of turbulence, its space-time intermittency, present in laboratory experiments, in the solar wind, in the physics of plasma confinement in the perspective of nuclear fusion. The challenge of the prodigious range of scales coupled by turbulence is also shared by climate sciences where turbulence in the oceans and in the atmosphere controls their exchange of matter and energy.
Astronomy is an important driver of advanced technology (interferometry, spectroscopy in the submillimeter range, space missions).
Last, human beings are curious of the Universe and are often grateful to researchers who dedicate their life to acquire this knowledge. As one of the 2020 Nobel prize of physics winners said " It is important to get people excited about research and astronomy has a special role to play". Humanity would not have evolved as it did without a knowledge of their sky.

The goal of the MIST project is therefore to follow the energy trail of baryonic matter in the star formation process from its source, the gravitational energy of the largest scales to its fate, its loss in radiative cooling.
The project involves complementary steps: (1) observations of lines emitted by molecular species, specifically formed and/or excited in turbulent dissipation bursts, (2) numerical simulations of magneto-hydrodynamical turbulence dedicated to dissipation scales to determine the nature of the dissipation structures, (3) modelling of the thermal and chemical properties of these structures, (4) novel statistical methods developed for the available observables to disclose the location of dissipation structures in the velocity field of cosmic turbulence.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Work performed:
(1) In the irradiated shock models, new types of shocks have been shown to exist in the presence of UV photons. A new and robust algorithm has been developed to find steady-state solutions, to cross sonic points and identify non physical trajectories.
(2) In the UV self-irradiated shock models, the complex radiative transfer of Lyman alpha and Lyman beta photons is treated self-consistently with a doubly iterative method between the shock evolution and the radiative transfer. Calculations in quadruple precision are necessary.
(3) In these two shock models, non-equilibrium chemistry is a prerequisite since the timescales of the dissipative dynamics are far smaller than the time it takes to reach chemical equilibrium.
(4) In the numerical simulations of compressible magneto-hydrodynamical turbulence dedicated to dissipation, algorithms have been developed to control and estimate the numerical dissipation.
(5) The work with the WST is opening new perspectives for the analysis of observations and their comparison with numerical simulations.
(6) The size and topology in space and velocity of the smallest-scale molecular structures that we are detect with interferometers in diffuse molecular gas are opening new field of investigation on the emergence of molecules in atomic gas.
(7) The ALMA discovery of emission and absorption lines of CH+ in high-redshift starburst galaxies is a breakthrough because this unique species is shedding a new light on dissipative processes in the process of galaxy growth.
Main results:
- the achievement of a complete census of the CH+ emission regions in several starburst galaxy at cosmic noon,
- finalizing the predictive power of intermediate velocity shock models in terms of H2 and CH+ emission lines,
- numerical simulations of turbulence in multi-phasic media
- coupling chemistry and dissipation in the dedicated numerical simulations at the 300 au scale.
Figure caption: Left: Regions of dissipation extrema in numerical simulations of magnetized turbulence due to compressible viscosity (blue), solenoidal viscosity (green) and ohmic resistivity (red).
Upper right: Dissipation rate in a 2-dimensional simulation of hydrodynamic turbulence with chemistry and radiative cooling (CHEMSES code. Lower right: CO abundance in the same simulation. The black contours show the dissipation extrema (here, shocks). CO is generated in the wake of the dissipation structures.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The discovery with the ALMA interferometer of CH+ emission lines broader than 1000 km/s in the circumgalactic medium of starburst galaxies at redshifts up to z~6 is a new milestone. They are the signposts of kiloparsec-scale shocks highlighting the long-sought interface between accretion and ejection i.e. infalling matter and outflows.

At the other end of the scales accessible to interferometers, i.e. close to the dissipation scales, the CO lines are used to trace the velocity field in diffuse molecular gas. Elongated and sharp-edged structures are detected. A spatial coherence, never seen before, is found in the orientation of these CO-edges at the milliparsec- and parsec-scale.

Numerical simulations of compressible magnetized turbulence dedicated to dissipation scales show that dissipation is concentrated in sheet-like fractal structures filling a tiny fraction of the volume (see Figure). Rotational discontinuities and current sheets play a major role, in spite of the high compressibility of turbulence.

The state-of-the-art Paris-Durham shock model has been augmented to treat the combined impact of mechanical and UV irradiation energy inputs. The energy radiated by specific molecules, such as H2 and CH+, is of major interest to the MIST project. The CH+ abundance in these shocks is enhanced by UV-photons while that of CO and H2O is reduced.

In UV self-irradiated shocks, UV self-generation turns on sharply at shock velocities of 30 km/s. These shocks produce enhanced abundances of molecules such as CO, HCO+ and CH+ while H2O is destroyed. Atomic and molecular lines reprocess the quasi totality of the kinetic energy, allowing the connection of observable line emission to the driving kinetic energy source.

A collaboration with mathematicians is opened to apply their method - e.g. the Wavelet Scattering Transform (WST) - to interstellar matter. This approach is inspired by deep neural networks but, unlike those, the statistical descriptors are obtained without any learning step, an essential asset as we have one unique sky. The WST describes sky maps with only a few tens of coefficients.

The final steps of the MIST project - bridging theory and observations - include two-dimensional simulations of hydrodynamic turbulence using the CHEMSES code, with fully resolved viscous dissipation, time-dependent heating, cooling, chemistry and excitation of H2 (see Figure). Molecules are produced and excited in the wake of strong dissipation ridges.

Regions of extrema of dissiation in magnetized turbulence and CO production