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Constraints on ISM Conditions in Infrared-Luminous Galaxies: Ionized Gas and OH⁺ in NGC 6764 from SOFIA FIFI-LS Far-Infrared Spectroscopic Observations

Presentation #105.03 in the session Molecular Clouds and the ISM — iPoster Session.

Published onJun 29, 2022
Constraints on ISM Conditions in Infrared-Luminous Galaxies: Ionized Gas and OH⁺ in NGC 6764 from SOFIA FIFI-LS Far-Infrared Spectroscopic Observations

The warm ionized component of the interstellar medium (ISM) stirs during galaxy mergers, disrupting dust and gas from their quiescent states, triggering star formation and generating extraordinarily bright and unusual objects including starburst galaxies, luminous infrared galaxies and active galactic nuclei. Recent observations of a small set of local galaxies with the Herschel Space Observatory have revealed that the ionization of molecular species in the ISM is a result of cosmic rays, produced by dramatic shocks and outflows from these extreme bursts of star formation. However, it is not understood whether such high cosmic ray energetics are commonplace or rare. We use spectroscopy from the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) to examine NGC6764 and eight other galaxies in detail. We find evidence of three transitions of OH+ and additional neutral species in the spectra of our sample objects. We argue an excess of cosmic rays is responsible for the ionization of these species and is an important physical mechanism influencing ISM chemistry and galaxy evolution. We construct far-infrared spectral energy distributions to characterize star formation and constrain interstellar dust conditions by estimating dust masses, luminosities and star formation rates, gaining new insights into how star formation proceeds. The SAO REU program is funded in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. AST-2050813, and by the Smithsonian Institution.

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