Direct Insights into Observational Absorption Line Analysis Methods of
  the Circumgalactic Medium Using Cosmological Simulations
  We study the circumgalactic medium (CGM) of a z=0.54 simulated dwarf galaxy
using hydroART simulations. We present our analysis methods, which emulate
observations, including objective absorption line detection, apparent optical
depth (AOD) measurements, Voigt profile (VP) decomposition, and ionization
modeling. By comparing the inferred CGM gas properties from the absorption
lines directly to the gas selected by low ionization HI and MgII, and by higher
ionization CIV and OVI absorption, we examine how well observational analysis
methods recover the "true" properties of CGM gas. In this dwarf galaxy, low
ionization gas arises in sub-kiloparsec "cloud" structures, but high ionization
gas arises in multiple extended structures spread over 100 kpc; due to complex
velocity fields, highly separated structures give rise to absorption at similar
velocities. We show that AOD and VP analysis fails to accurately characterize
the spatial, kinematic, and thermal conditions of high ionization gas. We find
that HI absorption selected gas and OVI absorption gas arise in totally
distinct physical gas structures, calling into question current observational
techniques employed to infer metallicities and the total mass of "warm-hot" CGM
gas. We present a method to determine whether CIV and OVI absorbing gas is
photo or collisionally ionized and whether the assumption of ionization
equilibrium is sound. As we discuss, these and additional findings have strong
implications for how accurately currently employed observational absorption
line methods recover the true gas properties, and ultimately, our ability to
understand the CGM and its role in galaxy evolution.
