Multiphase Neutral Interstellar Medium: Analyzing Simulation with H I 21cm Observational Data Analysis Techniques

TL;DR

This study compares methods for analyzing the neutral interstellar medium using simulated and observed H I 21cm spectra, revealing discrepancies in temperature distributions and discussing potential causes for these differences.

Contribution

It introduces a physically motivated model for T_B-v and τ-v distributions and evaluates Gaussian decomposition methods against simulations, highlighting observational-simulation tensions.

Findings

Methods recover gas distribution up to ~2500 K in simulations.

Observations show more thermally unstable gas than simulations.

Results are robust against noise, beam size, and resolution effects.

Abstract

Several different methods are regularly used to infer the properties of the neutral interstellar medium (ISM) using atomic hydrogen (H I) 21cm absorption and emission spectra. In this work, we study various techniques used for inferring ISM gas phase properties, namely the correlation between brightness temperature and optical depth $(T_B(v)$, $\tau(v))$ at each channel velocity ($v$), and decomposition into Gaussian components, by creating mock spectra from a 3D magnetohydrodynamic simulation of a two-phase, turbulent ISM. We propose a physically motivated model to explain the $T_B(v)-\tau(v)$ distribution and relate the model parameters to properties like warm gas spin temperature and cold cloud length scales. Two methods based on Gaussian decomposition -- using only absorption spectra and both absorption and emission spectra -- are used to infer the column density distribution as a function of temperature. In observations, such analysis reveals the puzzle of large amounts (significantly higher than in simulations) of gas with temperature in the thermally unstable range of $\sim$200 K to $\sim$2000 K and a lack of the expected bimodal (two-phase) temperature distribution. We show that, in simulation, both methods are able to recover the actual gas distribution in the simulation till temperatures $\lesssim2500$~K (and the two-phase distribution in general) reasonably well. We find our results to be robust to a range of effects such as noise, varying emission beam size, and simulation resolution. This shows that the observational inferences are unlikely to be artifacts, thus highlighting a tension between observations and simulations. We discuss possible reasons for this tension and ways to resolve it.