  Gas metallicity is a key quantity used to determine the physical conditions
of gaseous clouds in a wide range of astronomical environments, including
interstellar and intergalactic space. In particular, considerable effort in
circumgalactic medium (CGM) studies focuses on metallicity measurements,
because gas metallicity serves as a critical discriminator for whether the
observed heavy ions in the CGM originate in chemically-enriched outflows or in
more chemically-pristine gas accreted from the intergalactic medium. However,
because the gas is ionized, a necessary first step in determining CGM
metallicity is to constrain the ionization state of the gas which, in addition
to gas density, depends on the ultraviolet background radiation field (UVB).
While it is generally acknowledged that both the intensity and spectral slope
of the UVB are uncertain, the impact of an uncertain spectral slope has not
been properly addressed in the literature. This Letter shows that adopting a
different spectral slope can result in an order of magnitude difference in the
inferred CGM metallicity. Specifically, a harder UVB spectrum leads to a higher
estimated gas metallicity for a given set of observed ionic column densities .
Therefore, such systematic uncertainties must be folded into the error budget
for metallicity estimates of ionized gas. An initial study shows that empirical
diagnostics are available for discriminating between hard and soft ionizing
spectra. Applying these diagnostics helps reduce the systematic uncertainties
in CGM metallicity estimates.
