Intrinsic Soft X-ray Emission from the Hot Circumgalactic Medium in IllustrisTNG

We study the intrinsic 0.5--2 keV emission of the hot circumgalactic medium (CGM) in the IllustrisTNG simulation TNG100-2 at redshifts 0, 0.1, and 0.2. We calculate the emission of gas cells with T at least 10 6 K in the spherical shell between 0.15R 500 and R 500 and compare samples selected by their star-formation activity. X-ray luminosity increases with halo mass, although the class-dependent fits cover different mass intervals. Among galaxies with positive luminosity and star formation rate (SFR), the high-activity sample retains a positive association after a linear adjustment for halo mass. The residual association is weaker in the quiescent subset with positive luminosity and SFR. Within 10 11.75<M 500/M<10 12.25, the high-activity sample has more centrally concentrated emission and a median luminosity about five times that of the quiescent sample. The two populations have similar median hot-gas masses despite their different X-ray luminosities and radial profiles. These differences are consistent with variations in the spatial distribution and thermodynamic state of the emitting gas, although the samples are not matched in halo mass. Our results suggest that soft X-ray emission probes the structure of the hot CGM as well as its gas content, offering a way to investigate how star formation and feedback shape gaseous halos.