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 * /M ) ∈ [10.7, 11.9]. We derive mock X-ray luminosity (𝐿 X,500 ) and spectroscopic-like temperature (𝑇 sl,500 ) of hot gas within 𝑅 500 of the ETG haloes using the MOCK-X pipeline. The scaling between 𝐿 X,500 and the total mass within 5 effective radii (𝑀 5𝑅 e ) agrees well with observed ETGs from Chandra. IllustrisTNG reproduces the observed increase in scatter of 𝐿 X,500 towards lower masses, and we find that ETGs with log 10 (𝑀 5𝑅 e /M ) 11.5 with above-average 𝐿 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 X,500 . The 𝐿 X,500 -𝑇 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 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.