Multiple superbubbles in the starburst nucleus of NGC 5253? Implications for mass loss from dwarf galaxies

We have obtained a long ROSAT HRI observation of the nearby dwarf starburst and Wolf-Rayet galaxy NGC 5253. Our aim was to resolve the source of the soft thermal X-ray emission seen by the ROSAT PSPC, proposed to be a luminous superbubble by Martin & Kennicutt. Instead of a single superbubble, we find a complex of at least five sources of X-ray emission, associated with the massive clusters of young stars at the centre of NGC 5253. The individual 0.1-2.4 keV X-ray luminosities of the components lie in the range 2-7 x 10(37) erg s(-1). Three of the components are statistically extended beyond the HRI point spread function, the largest having a FWHM of 8(-4)(+10) arcsec, equivalent to 160(-80)(+200) pc. We consider the origin of the observed X-ray emission, concentrating on the sources expected to be associated with the starburst region: superbubbles, supernovae, supernova remnants and massive X-ray binaries. To assess the X-ray luminosity of a young superbubble blown by a single massive cluster of stars, we perform hydrodynamical simulations with realistic time-varying mass and energy injection rates. The predicted soft X-ray luminosity for a superbubble blown by an M-star = 10(5) M. cluster during its Wolf-Rayet period agrees very well. with the luminosities inferred for the extended components seen by the HRI. We conclude that the extended X-ray components are most likely young superbubbles blown by individual young clusters in the starburst region. Although we do not rule out a contribution to the observed soft X-ray emission by supernova remnants and massive X-ray binaries, we argue that most of the emission comes from a few young superbubbles blown by the young stellar clusters in the starburst region. We discuss in detail the implications of multiple superbubbles for the efficiency of mass and metal ejection from dwarf galaxies by starburst-driven galactic winds. We suggest that the presence of multiple stellar clusters in starbursting dwarf galaxies and the resulting multiple superbubbles will reduce the total ISM mass ejected from dwarf galaxies, compared with the current models which consider only the blowout of a single superbubble.