The Hot Interstellar Medium in Normal Elliptical Galaxies. III. The Thermal Structure of the Gas

Abstract

This is the third paper in a series analyzing X-ray emission from the hot interstellar medium in a sample of 54 normal elliptical galaxiesobserved by Chandra. We focus on a subset of 36 galaxies with sufficientsignal to compute radial temperature profiles. We distinguish fourqualitatively different types of profile: positive gradient (outwardlyrising), negative gradients (falling), quasi-isothermal (flat), andhybrid (falling at small radii and rising at larger radii). We measurethe mean logarithmic temperature gradients in two radial regions: from 0to 2 J-band effective radii RJ (excluding the central pointsource), and from 2 to 4 RJ. We find the outer gradient to beuncorrelated with intrinsic host galaxy properties, but stronglyinfluenced by the environment: galaxies in low-density environments tendto show negative outer gradients, while those in high-densityenvironments show positive outer gradients, suggesting the influence ofcircumgalactic hot gas. The inner temperature gradient, however, islargely unaffected by the environment, but strongly correlated withintrinsic host galaxy characteristics: negative inner gradients are morecommon for smaller, optically faint, low radio luminosity galaxies,whereas positive gradients are found in bright galaxies with strongerradio sources. There is no evidence for bimodality in the distributionof inner or outer gradients. We propose three scenarios to explain theinner temperature gradients: (1) weak AGNs heat the ISM locally, whilehigher luminosity AGNs heat the system globally through jets inflatingcavities at larger radii; (2) the onset of negative inner gradientsindicates a declining importance of AGN heating relative to othersources, such as compressional heating or supernovae; or (3) the varietyof temperature profiles are snapshots of different stages of atime-dependent flow, cyclically reversing the temperature gradient overtime.