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

Abstract

ABSTRACT 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 $\log _{10}(M_{\ast }/\mathrm{M_{\odot }})\in [10.7, 11.9]$. We derive mock X-ray luminosity (LX, 500) and spectroscopic-like temperature (Tsl, 500) of hot gas within R500 of the ETG haloes using the MOCK-X pipeline. The scaling between LX, 500 and the total mass within 5 effective radii ($M_{5R_{\rm e}}$) agrees well with observed ETGs from Chandra. IllustrisTNG reproduces the observed increase in scatter of LX, 500 towards lower masses, and we find that ETGs with $\log _{10} (M_{5R_{\rm e}}/\mathrm{M_{\odot }}) \leqslant 11.5$ with above-average LX, 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 LX, 500. The LX, 500–Tsl, 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 LX–Tsl 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 LX–Tsl plane could provide a new way of identifying backsplash galaxies in future X-ray surveys.