1995A&A...298..726H

The following is a study of the irregular clumpy galaxy VV 523 (NGC 3991 - UGC 6933 - IRAS 11549+3237). High angular resolution BRI CCD images were obtained using the 2m, Bernard Lyot telescope at the Observatoire du Pic du Midi. Long slit spectra with dispersions of 33 and 260A/mm were obtained with the Carelec spectrograph using the 1.93m telescope at the Observatoire de Haute Provence. Three emitting regions A, B and C are resolved along the slit. A wavelet analysis enabled detection of about twenty clumps and also measurements of their size (about 300pc) and brightness. The measurements were interpreted using the evolutionary population synthesis models of Mas-Hesse and Kunth. The model was constrained by matching the metallicity, Hβ equivalent widths, and colour measurements. We found that we were dealing with instantaneous starbursts rather than continuous star formation. Our data are consistent with an Initial Mass Function slope of 3 and the ages of the starbursts were found to be between 5 and 6.6 mega-years. The model may be used to predict the visible, infrared and UV properties observed. The burst region total mass is estimated to be about 4 10⁷M_☉ while the rotational mass of the galaxy is 2.7x10¹⁰M_☉. A correlation is found between the location on the galaxy and the colour of the clumps; our spectroscopic data show that it corresponds to a variation in starburst ages. This feature may be interpreted as a consequence of a self-propagating, star formation mechanism.