A Spectroscopic Study of Component C and the Extended Emission around I Zw 18

Abstract

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 Hα and Hβ 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 Hγ, Hδ, 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 Hα, Hβ emission lines and the other on Hγ, Hδ 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 ~10 and <~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 ~15 Mpc. The deep spectra also reveal extended ionized gas emission around I Zw 18. Hα emission is detected as far as 30" from it. To a B surface brightness limit of ~27 mag arcsec^-2, we find no observational evidence for extended stellar emission in the outermost regions at distances >~15" from I Zw 18.