The heavy element content (‘metallicity’) of the Universe is a record of the total star formation history. Gas-phase metallicity in galaxies, as well as its evolution with time, is of particular interest as a tracer of accretion and outflow processes. However, metallicities from the widely used electron temperature (Te) method are typically approximately two times lower than the values based on the recombination line method. This ‘abundance discrepancy factor’ is well known and is commonly ascribed to bias due to temperature fluctuations. We present a measurement of oxygen abundance in the nearby (3.4-Mpc) system, Markarian 71, using a combination of optical and far-infrared emission lines to measure and correct for temperature fluctuation effects. Our far-infrared result is inconsistent (>2σ significance) with the metallicity from recombination lines and, instead, indicates little to no bias in the standard Te method, ruling out the long-standing hypothesis that the abundance discrepancy factor is explained by temperature fluctuations for this object. Our results provide a framework to accurately measure metallicity across cosmic history, including with recent data reaching within the first billion years, with the James Webb Space Telescope and the Atacama Large Millimeter Array.
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The raw Keck Cosmic Web Imager data in this work are available in the Keck Observatory Archive: https://www2.keck.hawaii.edu/koa/public/koa.php. The reduced Far Infrared Field-Imaging Line Spectrometer data are available in the Stratospheric Observatory for Infrared Astronomy data archive: https://irsa.ipac.caltech.edu/Missions/sofia.html. The Photodetector Array Camera and Spectrometer data before transient correction are available in the Herschel data archive: http://archives.esac.esa.int/hsa/whsa/. Additional data can be provided upon reasonable request.
This study uses publicly available software/packages, including PyNeb43, Astropy47, emcee48, the Keck Cosmic Web Imager Data Reduction Pipeline (https://github.com/Keck-DataReductionPipelines/KCWI_DRP.git), SOSPEX (https://github.com/darioflute/sospex), Montage (http://montage.ipac.caltech.edu) and the Stratospheric Observatory for Infrared Astronomy Data Reduction Pipeline (https://github.com/SOFIA-USRA/sofia_redux). For the Keck Cosmic Web Imager data, the post-Data Reduction Pipeline background subtraction and stacking code is maintained at https://github.com/yuguangchen1/kcwi. Additional analysis code can be provided upon reasonable request.
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This work was based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration (NASA). The observatory was made possible by the generous financial support of the W. M. Keck Foundation. We recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are fortunate to have the opportunity to conduct observations from this mountain. Results in this paper are based on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is jointly operated by the Universities Space Research Association, Inc. (NASA Contract NAS2-97001) and the Deutsches SOFIA Institut (DLR Contract 50-OK-0901 to the University of Stuttgart). Herschel is an ESA space observatory with science instruments provided by a European-led principal investigator consortia and important participation from NASA. We thank the W. M. Keck Observatory staff, the SOFIA observatory staff and the Herschel Data Archive for making this study possible. Financial support for this work was provided by NASA (Award 08_0071 issued by the Universities Space Research Association, Inc.). R.S. acknowledges the support of NASA (NASA Hubble Fellowship Grant HST-HF2-51469.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated (NASA Contract NAS5-26555)). J. Sutter acknowledges funding from STScI (Grant JWST-GO-02107.006.A). R.M. acknowledges support from the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement with Associated Universities, Inc.
The authors declare no competing interest.
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Chen, Y., Jones, T., Sanders, R. et al. Accurate oxygen abundance of interstellar gas in Mrk 71 from optical and infrared spectra. Nat Astron 7, 771–778 (2023). https://doi.org/10.1038/s41550-023-01953-7
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