No stellar age gradient inside supergiant shell LMC 4

The youngest stellar populations of a 'J'-shaped region (400 pc strip E-W across LH 77 and 850 pc S-N) inside the supergiant shell (SGS) LMC 4 (with a diameter of 1.4 kpc) have been analysed with CCD photometry in B,V passbands. Isochrone fitting to the colour-magnitude diagrams yields ages in the range from 9 Myr to 16 Myr without correlation with the distance to the LMC 4 centre. We construct the luminosity function and the mass function of five regions to ensure that projection effects don't mask the results. The slopes lie in the expected range (gamma in [0.22;0.41] and Gamma in [-1.3;-2.4] respectively, with the Salpeter value of Gamma = -1.35). After our calculations a total of 5-7 10^3 supernovae has dumped the energy of 10^54.5 erg over the past 10 Myr into LMC 4, in fact enough to tear the original star-forming cloud apart in the time span between 5 and 8 Myr after the starformation burst, initiated by a large scale triggering event. We conclude that LMC 4 can have been formed without a contribution from stochastic self-propagating star formation (SSPSF).