AN INTERFEROMETRIC STUDY OF THE BLUE COMPACT DWARF GALAXY IZW-18

We present high spatial resolution observations of the blue compact dw arf galaxy IZW 18 performed in the H alpha line with a scanning Fabry- Perot interferometer at the CFH telescope. Morphological structure of the galaxy in H alpha and in the red continuum is investigated. We als o analyse the velocity field of the ionized gas. Besides the two compa ct HII components of the main body we find a population of small HII r egions in its surroundings whose diameter distribution and H alpha lum inosity function are consistent with those observed in dwarf irregular galaxies. In the main body of the galaxy besides of the NW and SE red continuum peaks which are displaced with respect to the H alpha maxim a, three new red condensations have been discovered. They have no clea r H alpha counterparts. The velocity field in IZW 18 shows peculiar mo tions superimposed on a quite regular background implying solid-body r otation with a gradient of about 70 km s(-1) kpc(-1). The H alpha line profiles exhibit an asymmetric structure, except for the NW main comp act component. At least part of this asymmetry could result from accre ted and/or expelled surrounding gas from the main star-forming core(s) of the galaxy. Contrary to previous suggestions that the south-west a nd north-east extensions of this galaxy are diffused emission produced by bipolar emitting gas we provide evidence that they are HII regions powered by star formation sites. The redshift of the Zwicky's ''flare '' has been measured for the first time and corresponds to the same ve locity as IZW 18. In such a context the optical ridge that appears to be an isolated morphological structure has a shape that may result fro m the gravitational interaction with the Zwicky's ''flare'' if this la tter is a neighbour extreme dwarf object.