Extended emission and star formation in I Zw 18

The results of sky limited emission-line (H-alpha and forbidden O III 5007A) and line-free (6450 A and 5100 A) continuum imagery of the super-metal-poor blue compact galaxy I Zw 18 are reported. The main body of star formation in the galaxy is surrounded by a 3 kpc diameter extended envelope in H-alpha. 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 x 10 to the 6th solar masses, or about 5 percent 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 deg 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 calculated that I Zw 18 has had previous, but disconnected, episodes of star formation, possibly from chance collisions of widely separated H I clouds.