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

Long-slit Keck II, 4m Kitt Peak, and 4.5m MMT spectrophotometric data areused to investigate the stellar population and the evolutionary status of I Zw18C, 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 IZw 18C, implying that ionizing massive stars are present. High signal-to-noiseKeck 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 toconstrain the age of the stellar population in I Zw 18C. Ages derived from twodifferent methods, one based on the equivalent widths of the H$\alpha$,H$\beta$ emission lines and the other on H$\gamma$, H$\delta$ absorption linesare consistent with a 15 Myr instantaneous burst model. We find that a smallextinction in the range $A_V$ = 0.20 -- 0.65 mag is needed to fit the observedspectral energy distribution of I Zw 18C with that model. In the case ofconstant star formation, all observed properties are consistent with starsforming continuously between ~ 10 Myr and < 100 Myr ago. We use all availableobservational constraints for I Zw 18C, including those obtained from HubbleSpace Telescope color-magnitude diagrams, to argue that the distance to I Zw 18should be as high as ~ 15 Mpc. The deep spectra also reveal extended ionizedgas emission around I Zw 18. H$\alpha$ emission is detected as far as 30" fromit. To a B surface brightness limit of ~ 27 mag arcsec$^{-2}$ we find noobservational evidence for extended stellar emission in the outermost regions,at distances > 15" from I Zw 18.Comment: 38 pages, 11 Postscript figures, accepted for publication in the Astrophysical Journa