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

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 IllustrisTNG-100 box that have stellar masses ${\log }_{10}(M_{\ast }/{\mathrm{M}}_{\odot })\in [10.7,11.9]$. We derive mock X-ray luminosity ($L_{\mathrm{X},500}$) and spectroscopic-like temperature ($T_{\mathrm{sl},500}$) of hot gas within $R_{500}$ of the ETG haloes using the MOCK-X pipeline. The scaling between $L_{\mathrm{X},500}$ and the total mass within 5 effective radii ($M_{5R_{\mathrm{e}}}$) agrees well with observed ETGs from Chandra. IllustrisTNG reproduces the observed increase in scatter of $L_{\mathrm{X},500}$ towards lower masses, and we find that ETGs with ${\log }_{10}(M_{5R_{\mathrm{e}}}/{\mathrm{M}}_{\odot })⩽11.5$ with above-average $L_{\mathrm{X},500}$ 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 $L_{\mathrm{X},500}$. The $L_{\mathrm{X},500}$--$T_{\mathrm{sl},500}$ relation shows a similar slope to the observed ETGs but the simulation systematically underestimates the gas temperature. Three outliers that lie far below the $L_{\mathrm{X}}$--$T_{\mathrm{sl}}$ 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 $L_{\mathrm{X}}$--$T_{\mathrm{sl}}$ plane could provide a new way of identifying backsplash galaxies in future X-ray surveys.

16 pages, 10 figures. Submitted to MNRAS