3D chemical structure of diffuse turbulent ISM

Abstract

Context. The amount of data collected by spectrometers from radio to ultraviolet (UV) wavelengths opens a new era where the statistical and chemical information contained in the observations can be used concomitantly to investigate the thermodynamical state and the evolution of the interstellar medium (ISM). Aims. In this paper, we study the statistical properties of the HI-to-H2 transition observed in absorption in the local diffuse and multiphase ISM. Our goal is to identify the physical processes that control the probability of occurrence of any line of sight and the origins of the variations of the integrated molecular fraction from one line of sight to another. Methods. The turbulent diffuse ISM is modeled using the RAMSES code, which includes detailed treatments of the magnetohydrodynamics, the thermal evolution of the gas, and the chemistry of H2. The impacts of the UV radiation field, the mean density, the turbulent forcing, the integral scale, the magnetic field, and the gravity on the molecular content of the gas are explored through a parametric study that covers a wide range of physical conditions. The statistics of the HI-to-H2 transition are interpreted through analytical prescriptions and compared with the observations using a modified and robust version of the Kolmogorov-Smirnov test. Results. The analysis of the observed background sources shows that the lengths of the lines of sight follow a flat distribution in logarithmic scale from ~100 pc to ~3 kpc. Without taking into account any variation of the parameters along a line of sight or from one line of sight to another, the results of one simulation, convolved with the distribution of distances of the observational sample, are able to simultaneously explain the position, the width, the dispersion, and most of the statistical properties of the HI-to-H2 transition observed in the local ISM. The tightest agreement is obtained for a neutral diffuse gas modeled over ~200 pc, with a mean density n̅H̅ = 1−2 cm−3, illuminated by the standard interstellar UV radiation field, and stirred up by a large-scale compressive turbulent forcing. Within this configuration, the 2D probability histogram of the column densities of H and H2, poetically called the kingfisher diagram, is remarkably stable and is almost unaltered by gravity, the strength of the turbulent forcing, the resolution of the simulation, or the strength of the magnetic field Bx, as long as Bx < 4 μG. The weak effect of the resolution and our analytical prescription suggest that the column densities of HI are likely built up in large-scale warm neutral medium and cold neutral medium (CNM) structures correlated in density over ~20 pc and ~10 pc, respectively, while those of H2 are built up in CNM structures between ~3 and ~10 pc. Conclusions. Combining the chemical and statistical information contained in the observations of HI and H2 sheds new light on the study of the diffuse matter. Applying this new tool to several atomic and molecular species is a promising perspective to understanding the effects of turbulence, magnetic field, thermal instability, and gravity on the formation and evolution of molecular clouds.

Modeling the statistics of the cold neutral medium in absorption-selected high-redshift galaxies

“…Recent numerical simulations are able to follow the detailed evolution of H 2 directly and resolve the small-scale CNM (e.g., Nickerson, Teyssier, & Rosdahl 2019;Bellomi et al 2020). However, the CNM cross section depends heavily on resolution and on the poorly constrained feedback mechanisms from star formation and supernovae.…”

Spectral index of synchrotron emission: insights from the diffuse and magnetised interstellar medium

“…In order to understand the effect of an energy-dependent spectral slope at a quantitative level, we first considered a slab with a fixed, spatially uniform B ⊥ exposed to a flux of CRe and we showed that, thanks to high-precision observational estimates of β at low frequencies (Mozdzen et al 2017), it is possible to discard some realisations of the CRe spectrum that would imply unrealistically high values of B ⊥ . Then, we used two snapshots of 3D MHD simulations (Bellomi et al 2020) with different median magnetic field strength, and studied the synchrotron emission according to the CRe spectrum by Orlando (2018). We computed the distribution of β for three frequency ranges, at low, mid, and high frequencies (115−189 MHz, 467−672 MHz, and 833−1200 MHz, respectively).…”

“…To correctly describe the thermal state of the diffuse ISM, we have included the heating induced by the photoelectric effect on interstellar dust grains and secondary electrons produced during cosmic-ray propagation, and the cooling induced by emission of Lyman-α photons, the fine-structure lines of OI and CII, and the recombination of electrons onto grains. All these processes, described in Appendix B of Bellomi et al (2020), are modelled with the analytical formulae given by Wolfire et al (2003).…”

The “Maggie” filament: Physical properties of a giant atomic cloud

“…Numerical studies show that the condensation process toward a stable CNM phase depends on the properties of turbulence in the WNM (Seifried et al 2011;Saury et al 2014;Bellomi et al 2020). These studies suggest that highly supersonic turbulence that is driven by compressive modes regulates the formation of a stable CNM phase as turbulent compression and decompression of the gas will be much more rapid than the cooling timescale.…”