| Summary: | We present a study of the cold atomic hydrogen (H i) content of molecular clouds simulated within the SILCC-Zoom project for solar neighbourhood conditions. We produce synthetic observations of H i at 21 cm, including H i self-absorption (HISA) and observational effects. We find that H i column densities, NHI, of ≳1022 cm-2 are frequently reached in molecular clouds with H i temperatures as low as ∼10 K. Hence, HISA observations assuming a fixed H i temperature tend to underestimate the amount of cold H i in molecular clouds by a factor of 3-10 and produce an artificial upper limit of NHI around 1021 cm-2. We thus argue that the cold H i mass in molecular clouds could be a factor of a few higher than previously estimated. Also, NHI PDFs obtained from HISA observations might be subject to observational biases and should be considered with caution. The underestimation of cold H i in HISA observations is due to both the large H i temperature variations and the effect of noise in regions of high optical depth. We find optical depths of cold H i around 1-10, making optical depth corrections essential. We show that the high H i column densities (≳1022 cm-2) can in parts be attributed to the occurrence of up to 10 individual H i-H2 transitions along the line of sight. This is also reflected in the spectra, necessitating Gaussian decomposition algorithms for their in-depth analysis. However, also for a single H i-H2 transition, NHI frequently exceeds 1021 cm-2, challenging one-dimensional, semi-analytical models. This is due to non-equilibrium chemistry effects and the fact that H i-H2 transition regions usually do not possess a one-dimensional geometry. Finally, we show that the H i gas is moderately supersonic with Mach numbers of a few. The corresponding non-thermal velocity dispersion can be determined via HISA observations within a factor of ∼2. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.
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