The X-ray morphology of elliptical galaxies.

Abstract

The hot interstellar medium found in most elliptical galaxies can offer a new window into the understanding of the dynamics of these objects. Using both low and high angular resolution X-ray data from ROSAT, the behavior of the hot gas can be examined from the centers to the outer regions of these objects. Because of the low photon counts found in most ROSAT observations, we have developed extensive optimization and Monte Carlo simulation techniques to define the shape and the corresponding uncertainties of the isophotes of diffuse X-ray sources. If cooling gas is flowing towards the center of an elliptical galaxy and if angular momentum is conserved, disks of X-ray emitting material should form that will cause a flattening in the observed X-ray isophotes of the galaxy as one looks towards the center. However, we find that the predicted flattening does not, in general, occur. This implies that there might be transfer of angular momentum or turbulence in the hot gas in elliptical galaxies. We have developed the X-ray counterpart to the optical analysis of model-subtracted images, performing the analysis on a sample of elliptical shell galaxies. Any structure in the hot interstellar medium of an elliptical galaxy would imply a very recent (${<}10\sp{8.5}$ years) merger event, since structure in a dissipationless gas will only last for a sound-crossing time or so. We have shown that there is statistical evidence that the X-ray emission from these galaxies is not consistent with a uniform $\beta$ model, but we detected no direct correlations between optical substructure and X-ray morphology. The analysis technique created to examine the shape of X-ray emission in elliptical galaxies has been expanded to find not just a single ellipticity for an image, but to detect ellipticity gradients. Our current data make it difficult to detect shifts in morphology in all but the brightest galaxies, but as better observations are performed, this technique will take advantage of the new technologies to best determine the nuances in the morphology of elliptical galaxies.