  Diagnosing the physical and chemical conditions within star-forming galaxies
(SFGs) is of paramount importance to understanding key components of galaxy
formation and evolution: star-formation, gas enrichment, outflows, and
accretion. Well established optical emission-line diagnostics used to discern
such properties (i.e., metal content, density, strength/shape of ionizing
radiation) will be observationally inaccessible for the earliest galaxies,
emphasizing the need for robust, reliable interstellar medium (ISM) diagnostics
at ultraviolet (UV) wavelengths. Calibrating these UV diagnostics requires a
comprehensive comparison of the UV and optical emission lines in nearby SFGs.
Optical integral field unit (IFU) surveys have revealed the inhomogeneous
nature of the ISM in SFGs, which leads to non-systematic biases in the
interpretation of unresolved sources. Spatial variations are especially
important to consider at UV wavelengths, where the strongest emission features
originate from only the highest excitation regions of the nebula and are
challenging to distinguish from competing high-ionization sources (e.g.,
shocks, AGN, etc.). Since surveys collecting large-scale optical integral field
unit (IFU) spectroscopy are already underway, this white paper calls for an IFU
or multi-object far-UV (FUV) spectroscopic instrument with high sensitivity,
high spatial resolution, and large field of view (FoV). Given the impact of
large-scale optical IFU surveys over the past decade, this white paper
emphasizes the scientific need for a comparable foundation of
spatially-resolved far-UV spectroscopy survey of nearby galaxies that will lay
the foundation of diagnostics critical to the interpretation of the distant
universe.
