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Absorbers in the spectrum of background objects probe the circumgalacticmedium (CGM) surrounding galaxies, but its physical properties remainunconstrained. We use the cosmological hydrodynamical simulation TNG50 tostatistically trace the origins of HI Ly-$\alpha$ absorbers around galaxies at$z = 0.5$ with stellar masses ranging from 10$^8$ to 10$^{11}$ M$_\odot$. Weemulate observational CGM studies by considering all gas within a line of sightvelocity range of $\pm 500$ km s$^{-1}$ from the central, to quantitativelyassess the impact of other galaxy haloes and overdense gas in the IGM thatintersect sightlines. The impact of satellites to the total absorber fractionis most significant at impact parameters $0.5 R_{\rm vir} < b < R_{\rm vir}$and satellites with masses below typical detection limits ($M_* < 10^8$M$_\odot$) account for 10 (40) per cent of absorbers that intersect anysatellite bound to $10^{10}$ and $10^{11}$ $(10^9)$ M$_\odot$ centrals. Afterconfirming outflows are more dominant along the minor axis, we additionallyshow that at least 20 per cent of absorbers exhibit no significant radialmovement, indicating that absorbers can also trace quasi-static gas. Themetallicity of absorbers also depends on the azimuthal angle, but this signalis largely driven by enriched inflowing and quasi-static gas. Our work showsthat determining the stellar mass of galaxies at $z_{\rm abs}$ is essential toconstrain the physical origin of the gas traced in absorption, which in turn iskey to characterising the kinematics and distribution of gas and metals in theCGM.

The physical origins of gas in the circumgalactic medium using observationally-motivated TNG50 mocks