X-ray Population Study of the Draco Dwarf Spheroidal Galaxy

Abstract

This thesis presents the analysis of five XMM-Newton observations of the Draco dwarf spheroidal galaxy (dSph) in X-rays. We performed source detection sepa- rately for each image in each observation and for the total band mosaic image and produced a catalogue of 70 X-ray sources. The sources were classified through cross-correlation with catalogues of other wavelengths, spectral analysis, hardness ratio diagrams, and variability studies. We classified 18 sources as active galac- tic nuclei (AGNs) and 9 sources as galaxies and galaxy candidates. Six sources were classified as foreground stars and one source as a foreground contact binary system in the Milky Way. Four sources were classified as Low-mass X-ray bi- nary (LMXB) candidates. One of them shows a significant pulsation, while the others were classified based on the X-ray-to-optical flux ratio and their positionthe colour-magnitude diagram. We also found a symbiotic star, a candidate forcataclysmic variable, and a binary system in the Draco dSph. Based on hardness ratios we classified 9 hard sources, which can be AGNs or LMXBs, in the fieldthe Draco dSph. In the next step, the X-ray luminosity functions (XLFs) of the X-ray sourcesthe energy ranges of 2.0 − 10 keV and 0.5 − 2 keV were derived. The XLFs indicate that X-ray sources in the field of the Draco dSph are strongly contaminated by AGNs. The 0.2−5 keV XLF shows an excess of about ten sources, which are most likely objects in Draco dSph. The studies of XLFs were continued by deriving the XLFs of five other dwarf galaxies, Phoenix, Fornax, Leo I, UMi, and UMa II, based on the observations of XMM-Newton in the energy range of 0.5 − 2 keV. We studied the star forma- tion history of each galaxy using literature, and showed that there is a correlation between the population of X-ray binaries and the recent star formation history of dwarf galaxies. The normalised XLF of Phoenix and Leo I dSphs represent the XLF of dwarf galaxies before completely leaving the star forming ages. The XLF shows that dwarf galaxies in this stage contain a population of LMXBs with luminosities of log~ 34 − 35 erg/s. The XLFs of old dSph (Draco, UMi, and UMa II) show that their X-ray sources have very low luminosity ranges log ~ 32 − 1034 erg /s . Therefore, it seems that they are dominated by white-dwarf binaries and/or faint transient LMXBs. In the case of the Fornax dSph, we found an unexpected luminosity range for its X-ray sources, inconsistent with the recent star formation history of the galaxy. We suggested a possible correlation between the nature of the X-ray sources of Fornax and the structure of the galaxy (its dark matter halo). However, more studies of the nature of X-ray sources in this galaxy are necessary. In the last part of the studies, we estimated dark matter halo mass that is required to keep the LMXBs of the Draco dSph, on the basis of the escape velocity of the LMXBs. We considered dark matter halo masses, suggested by different models, and showed that this galaxy could retain its LMXBs with speeds of 70 km/s assuming a dark halo mass of 10^9 solar masses.