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X-rays from Irregular Dwarf Galaxies
The Connection between Coronal Gas and High Energetic Stellar Events

dc.contributor.advisorKerp, Jürgen
dc.contributor.authorKappes, Michael
dc.description.abstractRadio observations of irregular dwarf galaxies (dIrr) in the 21 cm-line emission reveal cavities in their interstellar medium which are denoted as holes, bubbles or shells. The origin of these structures is still a matter of debate. A frequently cited model (hereafter called standard model) for the creation of these cavities is tested in the thesis.
The standard model describes the creation of cavities by high energetic stellar events (i.e. supernovae, stellar winds). These events produce hot (coronal) gas at typical temperatures around log(T[K])=6.0 which expands into the galactic ISM, forming a cavity filled with this coronal gas. The cavity and hence the coronal gas can either break out of the neutral gas disk or it stays confined by the swept up material, depending on the energetic input, scale height of the disk and the gravitational potential of the galaxy.
Since coronal gas cools by emission of soft X-ray photons (below a photon energy of about 1 keV) the first part of the thesis presents XMM-Newton and CHANDRA X-ray observations of eight dIrrs comprising Holmberg I, IC 2574, Sextans A, NGC 1569, NGC 4214, NGC 4449 and NGC 5253. The standard model is tested by evaluating the diffuse soft X-ray emission of these galaxies making use of a newly developed data processing which rejects the unrelated signal of solar flares in the data output of the X-ray telescope cameras. Furthermore, the vignetting effect of the XMM-Newton mirrors is corrected by newly developed exposure maps based on deep X-ray observations taken from archival data making use of the in-flight performance of the instruments. This correction is more reliable than the dedicated exposure maps especially in the inner 10 arcmin of the field of view. After the data reduction the detected extended soft X-ray emission is analyzed spectrally taking into account the X-ray emission of the Milky Way and the extragalactic background sources.
It turns out that in four of eight cases extended diffuse soft X-ray emission i s present in the disk/halo interface of the galaxies. The spectral data shows that only in one of the four cases the coronal gas is bound to the galaxy while the remaining galaxies expelled the gas into the intergalactic medium. On smaller angular scales only a few shells are associated with point like plasma sources. With respect to the observational result it is concluded that the standard model applies only for a few shells in the whole galaxy sample. Hence alternative models of the origin of shells such as ram-pressure effects or impacting high-velocity clouds should be further investigated in the future.
The second part of the thesis concerns the X-ray point source population of the dIrrs presenting an improved point source detection method which identifies fainter sources than the commonly used search algorithms. Using hardness ratios and observations in lower frequency bands the X-ray point source population can be separated into accreting sources and plasma sources while the distinction between low-mass (LMXB) and high-mass X-ray binaries (HMXB) is still rough.
The spatial distribution of the X-ray binary population depending on their luminosity is flat for small (r < 4 kpc) dIrrs while for the larger galaxies the most luminous sources are concentrated towards the galactic center. Moreover the star formation rate derived from the optical blue luminosity correlates with the number of HMXB which is qualitatively in agreement with the findings of Grimm et al. (2003) for late-type galaxies. It is suggested that these results point to a non-uniform starformation history of dIrrs.
dc.rightsIn Copyright
dc.subjectGalaxy evolution
dc.subjecthot gas
dc.subjectInterstellar Medium
dc.subjectIntergalactic Medium
dc.subjectNeutral gas
dc.subject.ddc520 Astronomie, Kartografie
dc.subject.ddc530 Physik
dc.titleX-rays from Irregular Dwarf Galaxies
dc.title.alternativeThe Connection between Coronal Gas and High Energetic Stellar Events
dc.typeDissertation oder Habilitation
dc.publisher.nameUniversitäts- und Landesbibliothek Bonn
ulbbnediss.affiliation.nameRheinische Friedrich-Wilhelms-Universität Bonn
ulbbnediss.fakultaetMathematisch-Naturwissenschaftliche Fakultät
dc.contributor.coRefereeKlein, Ulrich

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