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On the multiphase structure of the turbulent neutral interstellar medium

Abstract : Star formation in galaxies is strongly linked to the physical processes that govern the evolution of the interstellar medium. Stars form by gravitational collapse of dense and cold structures in molecular clouds but the process that leads to the formation of these over-densities is still unclear. One key element seems to be related to the efficiency of the formation of cold clouds of neutral hydrogen (HI) also called the Cold Neutral Medium (CNM).Several studies have aimed at understanding the production of the CNM through the condensation of the Warm Neutral Medium (WNM) in a turbulent and thermally unstable flow using numerical simulations. In general, these studies indicate the presence of a significant fraction of the mass being in the thermally unstable regime, (i.e., with a temperature mid-way between the CNM and WNM stable states). However, the thermodynamical conditions of the gas remain largely unexplored from the observational point of view.To go further, and really compare with numerical simulation that are, for now, under-constrained by observation, it is mandatory to map the column density structure of each phase and study the spatial variations of their centroid velocity and velocity dispersion. This calls for methods that can extract the information of each HI phase from fully sampled 21 cm emission data only.An original Gaussian decomposition algorithm, named ROHSA, is presented is this thesis. Based on a multi-resolution process from coarse to fine grid, and using a regularized non-linear least-square criterion to take into account simultaneously the spatial coherence of the emission and the multiphase nature of the gas, this method allows us to infer a spatially coherent vision of the three-phase neutral ISM.A detailed analysis is then presented on a high latitude HI field centred on the North Ecliptic Pole. In particular we provide new constraints on the thermodynamical properties of the WNM, and the statistical properties of the turbulent cascade acting in the fluid. Finally, we discuss under which condition the condensation mode of the thermal instability can grow in this medium and converge toward the cold phase of the neutral ISM, the CNM.
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Submitted on : Tuesday, May 19, 2020 - 4:34:25 PM
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Antoine Marchal. On the multiphase structure of the turbulent neutral interstellar medium. Astrophysics [astro-ph]. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLS249⟩. ⟨tel-02612863⟩



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