A Fundamental Plane Relation for the X-ray Gas in Normal Elliptical Galaxies

We report on the discovery of a new correlation between global parameters of the hot interstellar gas in elliptical galaxies. We reanalyze archival Chandra data for 30 normal early-type systems, removing the contributions of resolved and unresolved point sources to reveal the X-ray morphology of the hot gas. We determine the half-light radius, RX , and the mean surface brightness, IX , from the gas surface brightness profiles. A spectral analysis determines the temperature, TX , of the gas within 3 optical effective radii. We find that the galaxies lie on an X-ray Gas Fundamental Plane (XGFP) of the form TX ∝ R 0.28 X I 0.22 X . This is close to, but distinct from, a simple luminosity-temperature relation. The intrinsic width of the XGFP is only 0.07 dex, nearly identical to that of the stellar (optical) fundamental plane (SFP). This is surprising since X-ray gas masses are typically ∼ 10 of the stellar masses. We show that the XGFP is not a simple consequence of the virial theorem or hydrostatic equilibrium, and that it is essentially independent of the SFP. The XGFP thus represents a genuinely new constraint on the hydrodynamical evolution of elliptical galaxies. Subject headings: galaxies: cooling flows—galaxies: elliptical and lenticular, cD—galaxies: ISM—Xrays: galaxies—X-rays: ISM