Studying the absorption signatures of H I Lyman-\alpha\ in the warm-hot circumgalactic medium with TNG50

Abstract

In this study, we investigate the spectral signatures of neutral hydrogen Lyman-$\alpha$ 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 Ly$\alpha$ 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 $10^{11.7} M_{\odot} \leq M_{200} \leq 10^{12.3} M_{\odot}$, we find $f_c \approx 0.8$ for $\log N_{\rm HI} \geq 13$, where CBLAs span a total column-density range $\log N_{\rm HI}=11.6-15.4$. Therefore, CBLAs provide a significant contribution to the overall H I optical depth in the CGM with $\sim 50\%$ of the CGM absorbers being dominated by CBLA absorption. Furthermore, we find that CBLAs trace warm-hot gas in a temperature range $T=10^{5.2-6.4}$ K, which accounts for $\sim 7\%$ of the overall baryon budget in the TNG50 galaxies and $\sim 25\%$ of the total CGM mass. Finally, we identify a population of strong CBLAs that exhibit substantial H I column densities up to $\log N_{\rm HI}=14.9$. This population represents a new absorber class tracing massive warm-hot circumgalactic structures at large radial distances. 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.