A spectroscopic study of component C and the extended emission around I Zw 18

Long-slit Keck II,(1) 4 m Kitt Peak,(2) and 4.5 m MMT3 spectrophotometric data are used to investigate the stellar population and the evolutionary status of I Zw 18C, the faint C component of the nearby blue compact dwarf galaxy 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 reveal H gamma, H delta, and higher order hydrogen lines in absorption. Several techniques are used to constrain the age of the stellar population in I Zw 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 delta absorption lines are consistent with a 15 Myr instantaneous burst model. We find that a small extinction in the range A(V) = 0.20-0.65 mag is needed 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 than or similar to 100 Myr ago. We use all available observational constraints 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 high as similar to 15 Mpc. The deep spectra also reveal extended ionized gas emission 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 no observational evidence for extended stellar emission in the outermost regions at distances less than or similar to 15 from I Zw 18.