An interferometric study of the blue compact dwarf galaxy IZW 18 ?

We present high spatial resolution observations of the blue compact dwarf galaxy IZW 18 performed in the Hα line with a scanning Fabry-Perot interferometer at the CFH telescope. Morphological structure of the galaxy in Hα and in the red continuum is investigated. We also analyse the velocity field of the ionized gas. Besides the two compact HII components of the main body we find a population of small HII regions in its surroundings whose diameter distribution and Hα luminosity 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 Hα maxima, three new red condensations have been discovered. They have no clear Hα counterparts. The velocity field in IZW 18 shows peculiar motions superimposed on a quite regular background implying solid-body rotation with a gradient of about 70 km s −1 kpc −1. The Hα line profiles exhibit an asym-metric structure, except for the NW main compact component. At least part of this asymmetry could result from accreted and/or expelled surrounding gas from the main star-forming core(s) of the galaxy. Contrary to previous suggestions that the southwest and northeast 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 velocity as IZW 18. In such a context the optical ridge that appears to be an isolated morphological structure has a shape that may result from the gravitational interaction with the Zwicky's " flare " if this latter is a neighbour extreme dwarf object.