X-rays from Central Starburst Galaxies - An Analysis of ROSAT Observations
Starbursts represent an important phase in galaxy evolution. Mass and energy input from starburst regions into the interstellar medium (ISM) by stellar winds and supernovae have a strong impact on the chemical composition of the ISM in galaxies. Large-scale outflows from starburst galaxies are important for the evolution of the intergalactic medium in galaxy clusters. From theory different triggering mechanisms of starbursts have been discussed, like e.g. a bar potential in giant spirals or gravitational interaction between galaxies. Central activity in giant gas-rich spirals has been observed in the form of a nuclear or circumnuclear starburst as well as an active galactic nucleus (AGN). This work presents the observed X-ray properties of a small sample of four starburst galaxies differing in distance, morphology, and external influence. The study serves to investigate the complexity of starburst activity, the influence of different external factors, and the detectable consequences for the starburst host systems in the X-ray regime. The observations have been carried out by the German X-ray satellite ROSAT. NGC 4410 consists of two close gravitationally interacting galaxies. The western spiral component NGC 4410a is the brightest X-ray sources in the galaxy sample indicating the influence of interaction on central activity, as observed in the majority of luminous IRAS galaxies. For the eastern companion only an upper limit of its X-ray luminosity can be specified. The X-ray observations of the face-on Virgo galaxy NGC 4303 are discussed in connection with gas dynamics caused by the bar potential as trigger mechanism of the observed central starburst. The dominant X-ray source is the nucleus, additional sources are at the tips of the bar and within the spiral arms, consistent with hydrodynamical simulations of the gas component in barred galaxies. A common phenomenon of spirals with central starbursts is observed in NGC 4569 and NGC 2903. In this work I present the first observations of hot X-ray gas several kpc above the galactic disk in spirals with intermediate inclination, i.e. NGC 2903 and NGC 4569. The properties of the X-ray halo gas in these two systems are discussed in comparison with reported X-ray halos of edge-on galaxies. In X-rays NGC 2903 reveals a one-sided gaseous outflow from the center, extending more than 5 kpc into the galactic halo. In contrast to this isolated galaxy, NGC 4569 experiences strong interaction with the Virgo ICM. Its HI and Halpha structure clearly indicate the influence of the ICM. Most strikingly, the one-sided western X-ray outflow coincides with a giant Halpha feature up to 10 kpc above the disk. Such an extended feature has not been observed in any other starburst galaxy of comparable morphology so far.