Studying the absorption signatures of HI Lyα in the warm-hot circumgalactic medium with TNG50

In this study, we investigate the spectral signatures of neutral hydrogen Lyman-α absorption arising from the warm-hot gas component of the circumgalactic medium (CGM) around z=0 Milky Way (MW)-like galaxies using the high-resolution TNG50 cosmological simulation. We used synthetic absorption spectra to identify and characterise coronal broad Lyα absorbers (CBLAs), which represent H I absorption features produced by the warm-hot CGM at temperatures above 10^5 K. Our study implies that CBLAs have a significant absorption cross-section, f_c, around MW-like galaxies. Based on an analysis of 75 sightlines intersecting the CGM of 15 galaxies in the mass range 10^ 11.7 M_⊙ łeq M_ 200 łeq 10^ 12.3 M_⊙, we find f_c≈ 0.8 for łog N_ HI ≥ 13, where CBLAs span a total column-density range łog N_ HI i optical depth in the CGM with ∼ 50 % of the CGM absorbers being dominated by CBLA absorption (in terms of H, i column density). Furthermore, we find that CBLAs trace warm-hot gas in a temperature range T=10^5.2-6.4 K, which accounts for ∼ 7% (median value) of the overall baryon budget in the TNG50 galaxies and ∼ 25 % of the total CGM mass. Finally, we identify a population of strong CBLAs that exhibit substantial H, i column densities up to łog N_ HI =14.9. This population represents a new absorber class that traces massive, extended circumgalactic structures composed of warm-hot gas even at large radial distances. They arise in regions of increased gas density and long path lengths, and their radial velocities indicate contributions from both inflowing and outflowing warm-hot gas, consistent with warm-hot intergalactic medium-like CGM components. In conclusion, our study demonstrates that CBLAs represent an important absorber class that needs to be considered when interpreting the H, i absorption signatures from the multi-phase CGM of MW-like galaxies at low redshift.