by Yuguang Chen & Joan Schmelz

Paper: Accurate Oxygen Abundance of Interstellar Gas in Mrk 71 from Optical and Infrared Spectra
Chen, Yuguang, et al., 2023/04, Nature Astronomy, tmp, 87.

Far-IR measurements from the SOFIA telescope have been combined with optical data to effectively rule out temperature fluctuations as the primary cause of the Abundance Discrepancy Factor (ADF) in the metallicity of galaxy Markarian 71. This finding provides a framework for accurately measuring metallicities in galaxies across cosmic history, a crucial requirement for understanding galaxy evolution.

The heavy element content (metallicity) of the Universe is a record of the total star formation history. Gas-phase metallicity in galaxies and its evolution with time are of particular interest as tracers of the accretion and outflow processes. However, metallicities from the widely used electron-temperature method are typically about two times lower than the values based on the recombination-line method. This ADF is well known and is commonly ascribed to bias due to temperature fluctuations, i.e., an unresolved multithermal distribution of gas.

The electron-temperature method compares the fluxes of collisionally excited auroral ([O III] 4363) and nebular ([O III] 4959, 5007) lines. The ratio of these lines is sensitive to the electron temperature, allowing for the determination of the metallicity within the ionized regions. If temperature fluctuations are present, the measured electron temperature will be biased to high-temperature gas, and the metallicity will be underestimated. The recombination-line method analyzes fainter lines (O II 4649 multiplet), which are emitted when electrons recombine with ions. The intensities are less sensitive to electron temperature, and are thus, not biased by temperature fluctuations.

The integral field spectroscopic capability of the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) on SOFIA provided a unique opportunity to spatially map the far-IR [O III] 52 μm emission line. Unlike its optical counterparts, the [O III] 52 μm emission is collisionally excited, but insensitive to temperature fluctuations.

If temperature fluctuations were the primary cause of the ADF, the metallicity measured from the [O III] 52 μm emission should be consistent with recombination lines and about two times higher than the metallicity derived from the [O III] 4959 emission. To test this hypothesis, researchers compared the far-IR FIFI-LS map with optical [O III] emission maps from the Keck Cosmic Web Imager for Markarian 71. They also incorporated archival data of [O III] 88 μm emission from Herschel to eliminate the dependency on electron density and paint a more comprehensive picture of the metallicity landscape in Markarian 71.

The findings revealed an inconsistency with the temperature-fluctuation hypothesis. The [O III] 52 μm metallicity measurements were approximately two times lower than the recombination lines, aligning instead with the [O III] 4959 Å metallicity. This unexpected discovery suggests that our understanding of the recombination lines might not be as accurate as previously thought. Consequently, these results challenge the prevailing perception of the ADF and temperature fluctuations. Currently, researchers are working to confirm this discovery by examining a larger sample of nearby star-forming regions, which should provide further insights into the cause of the ADF and whether it might be dependent on host-galaxy properties.

Using the unique contribution of the SOFIA telescope, this study was able to rule out the long-standing hypothesis that the ADF is primarily due to temperature fluctuations, at least for Markarian 71. This methodology demonstrates the potential for using multi-wavelength data from ground- and space-based observatories, including ALMA and JWST, to obtain accurate metallicities for galaxies across cosmic history. This finding has significant implications for future studies of galaxy metallicities, especially those targeting high-redshift galaxies during the epoch of reionization.

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