TY - JOUR
T1 - Recovering Interstellar Gas Properties with Hi Spectral Lines
T2 - A Comparison between Synthetic Spectra and 21-SPONGE
AU - Murray, Claire E.
AU - Stanimirović, Snežana
AU - Kim, Chang Goo
AU - Ostriker, Eve Charis
AU - Lindner, Robert R.
AU - Heiles, Carl
AU - Dickey, John M.
AU - Babler, Brian
N1 - Funding Information:
This work was supported by the National Science Foundation (NSF) Early Career Development (CAREER) Award AST-1056780. C.E.M. acknowledges support by the NSF Graduate Research Fellowship and the Wisconsin Space Grant Institution. S.S. thanks the Research Corporation for Science Advancement for their support. The work of E.C.O. and C.-G. K. was supported by NSF grant AST-1312006. The National Radio Astronomy Observatory is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc. The Arecibo Observatory is operated by SRI International under a cooperative agreement with the NSF (AST-1100968), and in alliance with Ana G Mendez-Universidad Metropolitana, and the Universities Space Research Association.
Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.
PY - 2017/3/1
Y1 - 2017/3/1
N2 - 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 < 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 interstellar medium conditions, and future work is needed to improve observational statistics and implementation of simulated physics.
AB - 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 < 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 interstellar medium conditions, and future work is needed to improve observational statistics and implementation of simulated physics.
KW - ISM: clouds
KW - ISM: structure
KW - radio lines: ISM
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U2 - 10.3847/1538-4357/aa5d12
DO - 10.3847/1538-4357/aa5d12
M3 - Article
AN - SCOPUS:85014913044
SN - 0004-637X
VL - 837
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 55
ER -