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