A spectroscopic study of component C and the extended emission around I Zw18

Long-slit Keck II,(1) 4 m Kitt Peak,(2) and 4.5 m MMT3 spectrophotometric d ata are used to investigate the stellar population and the evolutionary sta tus of I Zw 18C, the faint C component of the nearby blue compact dwarf gal axy I Zw 18. Hydrogen H alpha and H beta emission lines are detected in the spectra of I Zw 18C, which implies that ionizing massive stars are present . High signal-to-noise Keck II spectra of different regions in I Zw 18C rev eal H gamma, H delta, and higher order hydrogen lines in absorption. Severa l techniques are used to constrain the age of the stellar population in I Z w 18C. Ages derived from two different methods, one based on the equivalent widths of the H alpha, H beta emission lines and the other on H gamma, H d elta absorption lines are consistent with a 15 Myr instantaneous burst mode l. We find that a small extinction in the range A(V) = 0.20-0.65 mag is nee ded to fit the observed spectral energy distribution of I Zw 18C with that model. In the case of constant star formation, all observed properties are consistent with stars forming continuously between similar to 10 and less t han or similar to 100 Myr ago. We use all available observational constrain ts for I Zw 18C, including those obtained from Hubble Space Telescope color -magnitude diagrams, to argue that the distance to I Zw 18 should be as hig h as similar to 15 Mpc. The deep spectra also reveal extended ionized gas e mission around I Zw 18. H alpha emission is detected as far as 30" from it. To a B surface brightness limit of similar to 27 mag arcsec(-2), we find n o observational evidence for extended stellar emission in the outermost reg ions at distances less than or similar to 15" from I Zw 18.