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Recovering Interstellar Gas Properties with HI Spectral Lines: A Comparison between Synthetic Spectra and 21-SPONGE

Author(s): Murray, Claire E; Stanimirovic, Snezana; Kim, Chang-Goo; Ostriker, Eve C; Lindner, Robert R; et al

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Abstract: We analyze synthetic neutral hydrogen (H I) 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 H I spectral lines and estimating the properties of associated gas using the Autonomous Gaussian Decomposition (AGD) algorithm. We find that H I 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 < T-s < 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 interstellar medium conditions, and future work is needed to improve observational statistics and implementation of simulated physics.
Publication Date: 1-Mar-2017
Electronic Publication Date: 2-Mar-2017
Citation: Murray, Claire E, Stanimirovic, Snezana, Kim, Chang-Goo, Ostriker, Eve C, Lindner, Robert R, Heiles, Carl, Dickey, John M, Babler, Brian. (2017). Recovering Interstellar Gas Properties with HI Spectral Lines: A Comparison between Synthetic Spectra and 21-SPONGE. ASTROPHYSICAL JOURNAL, 837 (10.3847/1538-4357/aa5d12
DOI: doi:10.3847/1538-4357/aa5d12
ISSN: 0004-637X
EISSN: 1538-4357
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Type of Material: Journal Article
Journal/Proceeding Title: ASTROPHYSICAL JOURNAL
Version: Final published version. Article is made available in OAR by the publisher's permission or policy.

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