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Mineo, Stefano (2011): X-ray emission from star-forming galaxies. Dissertation, LMU München: Faculty of Physics
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Abstract

In this dissertation we study the properties of high-mass X-ray binaries (HMXBs) and hot inter-stellar medium in star-forming galaxies and their relation with the star formation rate (SFR), based on the data from Chandra, Spitzer, GALEX and 2MASS public archives. We constructed a large sample of galaxies for which we collected homogeneous sets of multiwavelength measurements in X-ray, ultraviolet (UV), far-infrared (FIR) and near-infrared (NIR) bands. The sample includes 45 star-forming galaxies in total, divided in two sub-samples: the primary sample, consisting of 29 nearby galaxies, having distance < 40 Mpc, so that Chandra can resolve their X-ray point-like source population; the high-SFR sample, including 16 more distant galaxies that allowed us to extend the dynamical range of SFRs by approximately two orders of magnitude. In this sample we detected 1057 compact X-ray sources, of which ~300 are expected to be background active galactic nuclei (AGN). The majority of remaining ~700 sources are young systems associated with star-formation in the host galaxy. Based on their high X-ray luminosities and analogy with the X-ray populations in the Milky Way and few other very nearby galaxies, we conclude that they are high-mass X-ray binaries, powered by accretion of matter from a massive donor star onto a compact object - a black hole or a neutron star. Such a large number of sources allowed us to perform the most detailed study of the population of HMXBs and its dependence on various properties of the host galaxy, as well as to obtain a very accurate calibration of the X-ray luminosity-SFR relation. The study of the population of HMXBs is based on their X-ray luminosity functions (XLF). To this end, we took a special care to minimize the contamination by LMXBs, background AGN and to control the incompleteness of the Chandra source lists. The shape of the HMXB luminosity function is similar in different galaxies with the power law indexes having rms=0.25 with respect to the average value of ~1.6. The XLF normalizations, on the contrary, show significantly larger dispersion with the rms=0.34 dex around the A-SFR law. Combining the data of all galaxies, which include ~700 X-ray sources, we produced the average XLF of high-mass X-ray binaries in nearby star-forming galaxies. Its statistical accuracy exceeds by far that achieved in any of the previous studies of the HMXB luminosity function. The HMXB XLF has a single power law shape in a broad luminosity range of logLx~35-40 and shows a moderately significant evidence for the high luminosity break or cut-off at logLx~40. We did not find any statistically significant features at the Eddington luminosity limits of neutron stars or a 10 Msun black hole. With the knowledge of the relation between the number of high-mass X-ray binaries and star formation rate of the host galaxy, we estimated that the fraction of compact objects that went through an X-ray active phase at least once in their lifetime, powered by accretion of matter from a massive donor star in a binary system is fx~0.2. This constrains the mass distribution of the secondary in massive binaries. For an independent mass distribution of the secondary, the power law index must be flatter than 0.3. In particular, an independent mass distribution of a Kroupa or Salpeter type is strongly excluded. Assuming that the masses of components in a binary are not independent, our results are consistent with the flat mass ratio distribution. For comparison, we obtained a similar estimate for the fraction of compact objects that become X-ray sources powered by accretion from a low-mass donor star in an LMXB. Based on the scaling-laws by Gilfanov (2004), the fraction of compact objects, X-ray active in LMXBs, is small, fx~1e-6, demonstrating that LMXBs are extremely rare objects. This result is in line with the conclusions of the binary population studies. The collective luminosity of high-mass X-ray binaries is a good tracer of the recent star formation activity in the host galaxy: L_XRB(0.5-8 keV)(erg/s) = 2.5 10^{39} SFR (Msun/yr) The rms of points around this relation is 0.4 dex. The observed dispersion is unlikely to be caused by any of the obvious contaminating factors such as CXB or LMXB sources and is likely to have a physical origin. In addition to the emission from XRB population, the X-ray emission from star-forming galaxies includes a hot diffuse gas component with a mean characteristic temperature of 2-3 10^{6} K. We show that its X-ray luminosity correlates with the star formation rate of the host galaxy. Finally we demonstrate that the total X-ray luminosity of a galaxy scales with the star formation rate: L_tot(0.5-8 keV)(erg/s) = 4.5 10^{39} SFR(Msun/yr) with a dispersion sigma = 0.32 dex. We obtained consistent scale factors for nearby galaxies from the resolved sample and galaxies from the high-SFR sample. Among the latter (eight out of 16) are Chandra Deep Field North galaxies, located at the red-shifts of z~0.2-1.3. This proves that the total X-ray luminosity of a galaxy is a powerful tool to measure the star formation rate in distant galaxies.