X-ray scaling relations of early-type galaxies in IllustrisTNG and a new way of identifying backsplash objects

We investigate how feedback and environment shapes the X-ray scaling relations of early-type galaxies (ETGs), especially at the low-mass end. We select central-ETGs from the TNG100 box of IllustrisTNG that have stellar masses log(10)(M-*/M-circle dot) is an element of [10.7, 11.9]. We derive mock X-ray luminosity (L-X,L- 500) and spectroscopic-like temperature (T-sl,T- 500) of hot gas within R-500 of the ETG haloes using the MOCK-X pipeline. The scaling between L-X,L- 500 and the total mass within 5 effective radii (M-5Re) agrees well with observed ETGs from Chandra. IllustrisTNG reproduces the observed increase in scatter of L-X,L- 500 towards lower masses, and we find that ETGs with log(10)(M-5Re/M-circle dot) <= 11.5 with above-average L-X,L- 500 experienced systematically lower cumulative kinetic AGN feedback energy historically (vice versa for below-average ETGs). This leads to larger gas mass fractions and younger stellar populations with stronger stellar feedback heating, concertedly resulting in the above-average L-X,L- 500. The L-X,L- 500-T-sl,T- 500 relation shows a similar slope to the observed ETGs but the simulation systematically underestimates the gas temperature. Three outliers that lie far below the L-X-T-sl relation all interacted with larger galaxy clusters recently and demonstrate clear features of environmental heating. We propose that the distinct location of these backsplash ETGs in the L-X-T-sl plane could provide a new way of identifying backsplash galaxies in future X-ray surveys.