Examining quasar absorption-line analysis methods: the tension between
  simulations and observational assumptions key to modelling clouds
  A key assumption in quasar absorption line studies of the circumgalactic
medium (CGM) is that each absorption component maps to a spatially isolated
"cloud" structure that has single valued properties (e.g. density, temperature,
metallicity). We aim to assess and quantify the degree of accuracy underlying
this assumption. We used adaptive mesh refinement hydrodynamic cosmological
simulations of two $z=1$ dwarf galaxies and generated synthetic quasar
absorption-line spectra of their CGM. For the SiII $\lambda 1260$ transition,
and the CIV $\lambda\lambda1548, 1550$ and OVI $\lambda\lambda1031, 1037$
fine-structure doublets, we objectively determined which gas cells along a
line-of-sight (LOS) contribute to detected absorption. We implemented a fast,
efficient, and objective method to define individual absorption components in
each absorption profile. For each absorption component, we quantified the
spatial distribution of the absorbing gas. We studied a total of 1,302
absorption systems containing a total of 7,755 absorption components. 48% of
SiII, 68% of CIV, and 72% of OVI absorption components arise from two or more
spatially isolated "cloud" structures along the LOS. Spatially isolated "cloud"
structures were most likely to have cloud-cloud LOS separations of
0.03$R_{vir}$, 0.11$R_{vir}$, and 0.13$R_{vir}$ for SiII, CIV, and OVI,
respectively. There can be very little overlap between multi-phase gas
structures giving rise to absorption components. If our results reflect the
underlying reality of how absorption lines record CGM gas, they place tension
on current observational analysis methods as they suggest that
component-by-component absorption line formation is more complex than is
assumed and applied for chemical-ionisation modelling.
