EXTENDED EMISSION AND STAR FORMATION IN I-ZW-18

We report the results of sky limited emission-line (Hα and [O III] λ5007) and line-free (6450 Å and 5100 Å) continuum imagery of the super-metal-poor blue compact galaxy I Zw 18. The main body of star formation in the galaxy is surrounded by a 3 kpc diameter extended envelope in Hα. In addition, there exist arcs of emission emanating from both central H II regions and winding around to the west, as well as a broader ridge of emission extending northward from the main body. The total mass of ionized material is estimated to be 3 × 106 M⊙, or ∼5% of the mass of the observed H I clouds. Several extended stellar continuum regions exist to the northwest of the main body, the brightest three forming a line along PA = 310° out to a 3 kpc distance from the central region of the galaxy. The correlation of the morphology and location of stellar continuum and emission regions with the distribution of H I is discussed. It is concluded that I Zw 18 has had previous, but disconnected, episodes of star formation, possibly from chance collisions of widely separated H I clouds. The nucleosynthesis products from prior star forming episodes have not mixed with the clouds associated with the currently observed starburst.