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 H linewithascanningFabry-PerotinterferometerattheCFHtele- scope. Morphological structure of the galaxy in H and in the redcontinuumisinvestigated.Wealsoanalysethevelocityeld of the ionized gas. Besides the two compact HII components of the main body we nd a population of small HII regions in its surroundings whose diameter distribution and H 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 H maxima, three new red condensations have been discov- ered. They have no clear H counterparts. The velocity eld in IZW 18 shows peculiar motions superimposed on a quite regu- lar background implying solid-body rotation with a gradient of about 70 km s 1 kpc 1 . The H line proles exhibit an asym- metric structure, except for the NW main compact component. Atleastpartofthisasymmetrycouldresultfromaccretedand/or expelled surrounding gas from the main star-forming core(s) of thegalaxy.Contrarytoprevioussuggestionsthatthesouth-west and 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 rst 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 in- teraction with the Zwicky's "flare" if this latter is a neighbour extreme dwarf object.