  In the Halo21 absorption modeling challenge we generated synthetic absorption
spectra of the circumgalactic medium (CGM), and attempted to estimate the
metallicity, temperature, and density (Z, T, and nH) of the underlying gas
using observational methods. We iteratively generated and analyzed three
increasingly-complex data samples: ion column densities of isolated uniform
clouds, mock spectra of 1--3 uniform clouds, and mock spectra of
high-resolution turbulent mixing zones. We found that the observational
estimates were accurate for both uniform cloud samples, with Z, T, and nH
retrieved within 0.1 dex of the source value for >90% of absorption systems. In
the turbulent-mixing scenario, the mass, temperature, and metallicity of the
strongest absorption components were also retrieved with high accuracy.
However, the underlying properties of the subdominant components were poorly
constrained because the corresponding simulated gas contributed only weakly to
the H I absorption profiles. On the other hand, including additional components
beyond the dominant ones did improve the fit, consistent with the true
existence of complex cloud structures in the source data.
