Using cosmological simulations and synthetic absorption spectra to
  assess the accuracy of observationally derived CGM metallicities
  We used adaptive mesh refinement hydrodynamic cosmological simulations of a
$z=1$ Milky Way-type galaxy and a $z=0$ Dwarf galaxy and generated synthetic
quasar absorption-line spectra of their circumgalactic medium (CGM). Our goal
is to assess whether standard observational spectroscopic analysis methods
accurately reproduce intrinsic column densities, metallicities [Si/H], and
hydrogen densities $n_{H}$, in simulated absorption-line systems. Without
knowledge of the intrinsic simulated properties (blind study), we analysed
synthetic COS and HIRES spectra with fixed $S/N=30$ to determine the column
densities, metallicity, and $n_{H}$, using Voigt profile fitting combined with
Markov-Chain Monte-Carlo single-phase {\sc Cloudy} modelling techniques. To
quantify the intrinsic simulated absorbing gas properties, we objectively
determined which gas cells along a line of sight (LOS) contribute to detected
absorption in the spectra and adopt the unweighted geometric mean of these gas
cell properties. For this pilot study, we performed this experiment for five
LOS in the Milky-Way galaxy and five LOS in the Dwarf galaxy. We found an
average agreement between the "observed" and intrinsic metallicity
overestimated within $0.8\sigma$ or $0.2$ dex for the "Milky-Way"' CGM and
overestimated within $1.4\sigma$ or $0.2$ dex for the Dwarf galaxy CGM. We
found that the spectroscopically-derived $n_{H}$ are underestimated within
$0.8\sigma$ or $0.4$ dex of the intrinsic $n_{H}$ for the "Milky-Way" CGM and
overestimated within $0.3\sigma$ or $0.3$ dex for the Dwarf galaxy CGM. The
overall agreement suggests that, for single-phase ionisation modelling of
systems where there is substantial spread in gas properties, global metallicity
measurements from quasar absorption line studies are capturing the average
metallicity and ionisation parameters in a given astrophysical environment.
