Direct Far-Infrared Metal Abundances (FIRA) I: M101
  Accurately determining gas-phase metal-abundances within galaxies is critical
as metals strongly affect the physics of the interstellar medium (ISM). 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 abundances
(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 ($\sim$ 1.5$\sigma$) in the radial
abundance-gradients derived by the FIR/free-free-normalized vs.
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.
