Direct Far-infrared Metal Abundances (FIRA). I. M101

C. Lamarche, J. D. Smith, K. Kreckel, S. T. Linden, Noah S Rogers, E. Skillman, D. Berg, E. Murphy, R. Pogge, G. P. Donnelly, R. Kennicutt, A. Bolatto, K. Croxall, B. Groves, C. Ferkinhoff

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


Accurately determining gas-phase metal abundances within galaxies is critical as metals strongly affect the physics of the interstellar medium. To date, the vast majority of widely used gas-phase abundance indicators rely on emission from bright optical lines, whose emissivities are highly sensitive to the electron temperature. Alternatively, direct-abundance methods exist that measure the temperature of the emitting gas directly, though these methods usually require challenging observations of highly excited auroral lines. Low-lying far-infrared (FIR) fine structure lines are largely insensitive to electron temperature and thus provide an attractive alternative to optically derived abundances. Here, we introduce the far-infrared abundance (FIRA) project, which employs these FIR transitions, together with both radio free-free emission and hydrogen recombination lines, to derive direct, absolute gas-phase oxygen abundances. Our first target is M101, a nearby spiral galaxy with a relatively steep abundance gradient. Our results are consistent with the O++ electron temperatures and absolute oxygen abundances derived using optical direct-abundance methods by the CHemical Abundance Of Spirals (CHAOS) program, with a small difference (∼1.5σ) in the radial abundance gradients derived by the FIR/free-free-normalized versus CHAOS/direct-abundance techniques. This initial result demonstrates the validity of the FIRA methodology-with the promise of determining absolute metal abundances within dusty star-forming galaxies, both locally and at high redshift.

Original languageEnglish (US)
Article number194
JournalAstrophysical Journal
Issue number2
StatePublished - Feb 1 2022

Bibliographical note

Funding Information:
We thank the anonymous referee for the insightful comments and detailed suggestions that helped to improve this manuscript. We also thank Eva Schinnerer and Karin Sandstrom for contributions to acquiring the PPAK data, as well as Bo Peng and Karla Arellano-Córdova for helpful discussions that have improved this manuscript. C.L. acknowledges support from NASA ADAP grant 80NSSC18K0730. K.K. gratefully acknowledges funding from the German Research Foundation (DFG) in the form of an Emmy Noether Research Group (grant No. KR4598/2-1, PI Kreckel).

Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.


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