Observing the circumgalactic medium of simulated galaxies through synthetic absorption spectra

We explore the multiphase structure of the circumgalactic medium (CGM) probed by synthetic spectra through a cosmological zoom-in galaxy formation simulation. We employ a Bayesian method for modelling a combination of absorption lines to derive physical properties of absorbers with a formal treatment of detections, including saturated systems, and non-detections in a uniform manner. We find that in the lines of sight (LOS) passing through localized density structures, absorption lines of low, intermediate, and high ions are present in the spectrum and overlap in velocity space. Low, intermediate, and high ions can be combined to derive the mass-weighted properties of a density-varying peak, although the ions are not cospatial within the structure. By contrast, LOS that go through the hot halo only exhibit detectable HI and high ions. In such LOS, the absorption lines are typically broad due to the complex velocity fields across the entire halo. We show that the derived gas density, temperature, and metallicity match closely the corresponding HI mass-weighted averages along the LOS. We also show that when the data quality allows, our Bayesian technique allows one to recover the underlying physical properties of LOS by incorporating both detections and non-detections. It is especially useful to include non-detections, of species such as NV or Ne VIII, when the number of detections of strong absorbers, such as HI and OVI, is smaller than the number of model parameters (density, temperature, and metallicity).