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 Si II λ1260 transition, and the C IV λλ1548, 1550 and O VI λλ1031, 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 1302 absorption systems containing a total of 7755 absorption components. 48 per cent of Si II, 68 per cent of C IV, and 72 per cent of O VI 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 0.03R_vir (1.3 kpc), 0.11R_vir (4.8 kpc), and 0.13R_vir (5.6 kpc) for Si II, C IV, and O VI, respectively. There can be very little overlap between multiphase 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-ionization modelling.