University of Groningen Structure and dynamics of giant low surface brightness galaxies

Giant low surface brightness (GLSB) galaxies are commonly t hought to be massive, dark matter dominated systems. Howeve r, this conclusion is based on highly uncertain rotation curves. We present here a new study of two prototypical GLSB galaxies: M alin 1 and NGC 7589. We re-analysed existing H I observations and deriv ed new rotation curves, which were used to investigate the di stributions of luminous and dark matter in these galaxies. In contrast to previous findings, the rotation curves of both galaxies show a steep rise in the central parts, typical of hi gh surface brightness (HSB) systems. Mass decompositions with a dark m tter halo show that baryons may dominate the dynamics of the inner regions. Indeed, a “maximum disk” fit gives stellar mass-tolight ratios in the range of values typically found for HSB ga laxies. These results, together with other recent studies, suggest that GLSB galaxies are systems with a double structure: an in ner HSB early-type spiral galaxy and an outer extended LSB disk. We also tested the predictions of MOND: the rotation curve of NGC 7589 is reproduced well, whereas Malin 1 represents a cha llenging test for the theory.