Recovering interstellar gas properties with HI spectral lines: A comparison between synthetic spectra and 21-SPONGE

TL;DR

This study evaluates how well synthetic HI spectral lines from simulations can recover interstellar gas properties and compares these with actual observations from the 21-SPONGE survey, highlighting strengths and limitations of current methods.

Contribution

Introduces a new uniform spectral line decomposition method using AGD and compares synthetic spectra with real data to assess the accuracy of gas property recovery.

Findings

High completeness of spectral line recovery at high Galactic latitudes

Discrepancies in component counts and broad low-optical depth components between synthetic and observed data

Synthetic spectra show high spin temperature components not observed in real data

Abstract

We analyze synthetic neutral hydrogen (HI) absorption and emission spectral lines from a high- resolution, three-dimensional hydrodynamical simulation to quantify how well observational methods recover the physical properties of interstellar gas. We present a new method for uniformly decomposing HI spectral lines and estimating the properties of associated gas using the Autonomous Gaussian Decomposition (AGD) algorithm. We find that Hi spectral lines recover physical structures in the simulation with excellent completeness at high Galactic latitude, and this completeness declines with decreasing latitude due to strong velocity-blending of spectral lines. The temperature and column density inferred from our decomposition and radiative transfer method agree with the simulated values within a factor of < 2 for the majority of gas structures. We next compare synthetic spectra with observations from the 21-SPONGE survey at the Karl G. Jansky Very Large Array using AGD. We find more components per line of sight in 21-SPONGE than in synthetic spectra, which reflects insufficient simulated gas scale heights and the limitations of local box simulations. In addition, we find a significant population of low-optical depth, broad absorption components in the synthetic data which are not seen in 21-SPONGE. This population is not obvious in integrated or per-channel diagnostics, and reflects the benefit of studying velocity-resolved components. The discrepant components correspond to the highest spin temperatures (1000 < Ts < 4000 K), which are not seen in 21-SPONGE despite sufficient observational sensitivity. We demonstrate that our analysis method is a powerful tool for diagnosing neutral ISM conditions, and future work is needed to improve observational statistics and implementation of simulated physics.