The COS CGM Compendium. IV. Effects of Varying Ionization Backgrounds on
  Metallicity Determinations in the z < 1 Circumgalactic Medium
  Metallicity estimates of circumgalactic gas based on absorption line
measurements typically require photoionization modeling to account for unseen
ionization states. We explore the impact of uncertainties in the extreme
ultraviolet background (EUVB) radiation on such metallicity determinations for
the z < 1 circumgalactic medium (CGM). In particular, we study how
uncertainties in the power-law slope of the EUV radiation,
$\mathrm{\alpha_{EUVB}}$, from active galactic nuclei affect metallicity
estimates in a sample of 34 absorbers with HI column densities between 15.25 <
log ($\mathrm{N_{HI}}$ / $\mathrm{cm^{-2}}$) < 17.25 and measured metal ion
column densities. We demonstrate the sensitivity of metallicity estimates to
changes in the EUV power-law slope of active galactic nuclei,
$\mathrm{\alpha_{EUVB}}$, at low redshift (z < 1), showing derived absorber
metallicities increase on average by approximately 0.3 dex as the EUV slope is
hardened from $\mathrm{\alpha_{EUVB}}$ = -2.0 to -1.4. We use Markov Chain
Monte Carlo sampling of photoionization models with $\mathrm{\alpha_{EUVB}}$ as
a free parameter to derive metallicities for these absorbers. The current
sample of absorbers does not provide a robust constraint on the slope,
$\mathrm{\alpha_{EUVB}}$, itself; we discuss how future analyses may provide
stronger constraints. Marginalizing over the uncertainty in the slope of the
background, we find the average uncertainties in the metallicity determinations
increase from 0.08 dex to 0.14 dex when switching from a fixed EUVB slope to
one that freely varies. Thus, we demonstrate that EUVB uncertainties can be
included in ionization models while still allowing for robust metallicity
inferences.
