Astron. Astrophys. 360, 447-456 (2000)

5. Conclusions

We have analyzed spatial and spectral data from the barred late-type spiral galaxy NGC 4303 in the soft X-ray regime. Several separate X-ray sources can be observed in the core and disk of the galaxy. The locations of the sources correspond to several H  II regions and indicate a concentration of gas at the center and at the ends of the galactic bar, in agreement with numerical simulations of gas dynamics in a barred potential.

The low spatial resolution of the PSPC observation of NGC 4303 does not allow a distinction of several individual X-ray sources within the object. The best fit of the soft X-ray spectrum taking into account the information from the high resolution HRI observation is a combination of a RS component with a temperature of 0.3 keV and a power-law component with a spectral index of 2.6. The total 0.1-2.4 keV X-ray luminosity amounts to 4.710 erg s^-1, in agreement with other comparable barred galaxies with a nuclear starburst, like e.g. NGC 4569 (Tschöke et al. in preparation). A pure starburst model for the nucleus of NGC 4303 would require a special explanation for the unusually high / ratio.

The combination of the flux fraction of the separate sources, the spectral information, and the comparison with the H luminosity from the core leads to the following picture: the soft X-ray emission originates from a composition of several distinct emission regions. The central source consists of a low luminous AGN and a circumnuclear starburst. The disk sources are dominated by SNRs and superbubbles in star forming regions preferably at the ends of the bar and along the eastern spiral arm. Several HMXBs may contribute to the X-ray flux.

The disk X-ray sources are coincident with some of the most luminous H  II regions in the galaxy. The estimated total SFR from the X-ray flux is 1-2  yr^-1. Most H  II regions are not detectable in the X-ray, like most H sources in the eastern boomerang-shaped arm. The enhanced star formation in NGC 4303 may have been caused by some kind of interaction although the H  I morphology of the galaxy does not support very strong perturbation. If a dwarf galaxy has fallen in and merged with NGC 4303 in the past, the bar may have been produced with the subsequent triggering of the star formation at the center and in the spiral arms. The accreted dwarf galaxy would be resolved and not directly detectable.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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