Where the Forest Goes Dark: Halo-centered Characterization of High Column Density Systems with IllustrisTNG

High column density systems of neutral hydrogen (HCDs) --- Lyman limit systems (LLSs), sub-damped and damped Ly α absorbers (DLAs) --- contaminate both the 3D and the 1D statistics of the Ly α forest. Most DLAs are masked out from the analyses, but cleaning algorithms are not perfect, and weaker LLSs are individually undetectable, so the residual contamination is absorbed into nuisance parameters that dilute the cosmological constraining power. Simulations provide the ideal setting to study this effect, allowing absorption features to be directly traced back to the gas producing them. We identify high-column-density structures on the gas cells of the TNG50 hydrodynamical simulation, deblend them in velocity space, and split every sightline into HCD-only and forest-only spectra. Applied to halos at z 3 out to 50 virial radii, it yields each class's covering fraction against impact parameter b and halo mass. The impact parameter defines the type of absorption appearing in the spectrum: DLAs give way to sub-DLAs at b about 0.11,R 200c, sub-DLAs to LLSs at 0.3, and LLSs to the Ly α-forest at 0.5, which covers 78% of sightlines at R 200c. In units of R 200c the sequence is nearly mass-independent over three decades in mass, so the virial radius sets the scale of the neutral gas distribution. A single Voigt component recovers the column density of essentially every damped system, but not of LLSs, whose absorption features often arise from several separate contributions from gas structures along the sightline. Our detailed characterization of HCDs gives the first step to the construction of advanced techniques to directly forward-model their contribution to Ly α spectra.