Spatial and spectral comparisons of soft X-rays from galaxies of different morphological types

Abstract

Encounters between galaxies have long been recognised as having a possible influence over a galaxy's evolution. Nevertheless for years such encounters were regarded as rare events, with models incapable of producing the tidal tails, ripples and shells observed in classical mergers. Numerical simulations and advances in observational techniques have emphasised the significance of mergers as a delayed formation or evolution process for a sizable number of galaxies. Many of the initial worries over the theory of ellipticals being the remnants of merged disk systems have since been answered. X-ray analysis of normal galaxies demonstrated a dichotomy between the nature of X-ray emission associated with spirals and that of ellipticals. This thesis compares spectral and spatial information for normal galaxies to that of galaxies experiencing environmental encounters in order to clarify the evolutionary relationship between spirals, mergers and ellipticals. As a pilot study we present statistical analysis for luminosity data derived from Einstein observations. Our analysis shows that the properties of galaxies experiencing environmental encounters are intermediate between those of normal
spiral and elliptical galaxies. We present spectral analysis of deep exposure ROSAT PSPC observations of 27 spirals, 35 ellipticals and 18 galaxies classified as peculiar. Single-temperature models (Raymond Smith, bremsstrahlung or power
law) give unacceptable fits (x>1.5) for many cases in all morphological subsets, unless a free abundance of heavy elements was allowed for, in which case very sub-solar values were derived (<0.5 solar). Two-temperature models give substantially
better fits, for all morphological types. Lower emission temperatures are measured for both spirals and ellipticals in comparison to Einstein results. We suggest this is a consequence of the complexity of the galaxy spectra and the softer energy band of ROSAT. In order to better characterise the soft X-ray emission from galaxies, we present seven high signal-to-noise composite spectra. Elliptical galaxies are separated into four groups according to their offset in X-ray luminosity offsets from the LXcXLB one-to-one line. Our spectral analysis allows up to three components: a very soft component (VSC) a soft component (SC) and a hard component (HC). Nevertheless not all of them are detected in every group. This work shows a correlation between X-ray excess (increasing Lx/LB) and the contribution from the soft component. We confirm the presence of a third component at kT0.2 keV (the VSC) in ellipticals with the faintest X-ray luminosities (lowest Lx/Lp). We
present composite spectra for spirals, SOs and galaxies experiencing environmental encounters. Each spectrum reveals distinct combinations of the aforementioned components.
We present statistical analysis of luminosities derived from our ROSAT sample. Our results once more empasise the intermediate characteristics of galaxies experiencing environmental encounters in comparison to normal spirals and ellipticals. We also present a catalogue of PSPC iso-intensity contour maps superimposed over optical images for galaxies included within our sample, to show the spatial distribution of X-ray relative to the optical light.