The hot interstellar medium in normal elliptical galaxies

I present a complete morphological and spectral X-ray analysis of the hot interstellar medium in 54 normal elliptical galaxies in the Chandra archive. I isolate their hot gas component from the contaminating point source emission, and adaptively bin the gas maps with a new adaptive binning technique using weighted Voronoi tesselations. A comparison with optical images and photometry shows that the gas morphology has little in common with the starlight. In particular, I observe no correlation between optical and X-ray ellipticity, contrary to expectations for hydrostatic equilibrium.

Instead, I find that the gas in general appears to be very disturbed, and I statistically quantify the amount of asymmetry. I see no correlations with environment, but a strong dependence of asymmetry on radio and X-ray AGN luminosities, such that galaxies with more active AGN are more disturbed. Surprisingly, this AGN-morphology connection persists all the way down to the weakest AGN, providing strong morphological evidence for AGN feedback in normal elliptical galaxies. I conclude that the hot gas in elliptical galaxies is generally not in hydrostatic equilibrium; instead, it is continually disturbed by intermittent outbursts of the central AGN.

I extract radial temperature profiles, revealing surprisingly complex structures with positive and negative gradients, or even combinations of both. I find that the outer temperature gradient is determined by galaxy environment, while the inner temperature profiles shows a strong correlation with radio luminosity and a weaker with stellar mass. While our data are consistent with compressive heating in cooling flow models or supernova heating, AGN feedback is a more likely explanation. I suggest that the change of sign for the temperature gradient indicates either how localized different AGN heat, or where AGN heating becomes unimportant.

Despite the disturbed gas morphology, I also report on the discovery of a tight correlation, the X-ray gas fundamental plane (XGFP), linking temperature, half- light radius, and average surface brightness as T X 0( [Special characters omitted.] , reducing the large scatter in the closely related luminosity- temperature relation. The XGFP has a small intrinsic width of only 0.07 dex, and represents a new constraint on the hydrodynamic history of the gas.